Power supply design (2) - Page 1
Forum section: electronics / beginners

author: paul18fr
author_rank: Contributor
timestamp: August 29, 2014, 12:47:53 pm
content: In parallel to a current topic (https://www.eevblog.com/forum/beginners/power-supply-design-35761/), I've found this (old) project:http://www.electronics-lab.com/forum/index.php?topic=39481.msg1017450http://diyfan.blogspot.fr/2012/02/adjustable-lab-power-supply.htmlJust for my information, it's it interesting ?Paul

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2014, 04:07:43 pm
content: Is it interesting in what way? It will certainly work although apparently there is an issue with startup surges.I might include fold-back current limiting and thermally protect the output transistors.The offset and gain adjustments on U2 seem out of place when it is not a precision design.I like that it uses singled ended current sensing instead of an instrumentation amplifier.

author: paul18fr
author_rank: Contributor
timestamp: August 29, 2014, 08:32:28 pm
content: Quote from: David Hess on August 29, 2014, 04:07:43 pmI'm currently looking for a Power supply (2nd hand one), when I found it on internet ; I've been thinking that, with a help of a skill people (that I'm not), it might be interesting to build it, economically but also in order to learn things.The transformer is the most expensive part (http://www.digikey.com/product-detail/en/VPS28-4600/237-1281-ND/666167) ; the other ones seem to be more "classical" ...... another delirium among many othersPaul

author: liquibyte
author_rank: Frequent Contributor
timestamp: August 29, 2014, 09:04:28 pm
content: I've built two of these. I have a third that was redesigned to use surface mount components that redwire over at their forums did. The startup and shutdown spikes are an issue and I'm still working on figuring out a way to mitigate them based on suggestions I've gotten but haven't had much time for it lately so I'm not quite there yet. I've tried asking about this here but the only real response I got was that the design was shit and I should just buy off the shelf. Works fairly well for my needs, and other than the spikes, does what it was designed to do.I have a few extra boards that I had made that are available and do work if you're interested in trying out the design I did based on that schematic, just PM me.

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2014, 09:20:45 pm
content: Quote from: liquibyte on August 29, 2014, 09:04:28 pmOddly enough we recently had a discussion over on Tektronix list about a circuit that may have been included in their TM500 series PS-503 power supplies by a thoughtful designer to prevent output glitches during start up and shut down. Take a look at Q15 and Q115 on the PS-503 schematic.From the theory section in the PS-503 manual:Q15 and associated components make up a shut-down equalizer circuit. When the TM-500 series power module power switch is turned off, or power fails, the shut-down equalizer circuit will cause the supply (+ or -) that has the lightest load to reduce its output voltage to prevent spiking of the lightly loaded supply as the filter capacitor is discharged.

author: mikerj
author_rank: Super Contributor
timestamp: August 29, 2014, 09:48:41 pm
content: That looks like the design thatQuasar electronicssell as a kit. I built one many years ago (probably 15 years) and it seemed to work ok though I don't think I ever looked at the output transients at the time!

author: liquibyte
author_rank: Frequent Contributor
timestamp: August 29, 2014, 10:07:11 pm
content: Quote from: David Hess on August 29, 2014, 09:20:45 pmI'm assumingthisis the manual you're referring to, or at least similar enough in the schematic to use as a basis for study? I immediately spotted Q15-Q115 in the schematic but the quote is from the "Power Line Regulation" section 5-2 starting on page 27 at the bottom. The way it's worded makes it sounds like it's a feature for when the supply is in dual tracking mode. Either way, it's a solution that bears looking into, much appreciated.

author: liquibyte
author_rank: Frequent Contributor
timestamp: August 29, 2014, 10:21:49 pm
content: Quote from: mikerj on August 29, 2014, 09:48:41 pmYep, I justlooked it upand the schematic inthismanual is identical to theoriginal projectthat was reworked to solve a lot of the issues the first builders had with underrated components.

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2014, 11:16:51 pm
content: Quote from: liquibyte on August 29, 2014, 10:07:11 pmThat is the one. Their PS-501 power supply is similar but only has one side and lacks the Q15/Q115 circuit or I would have suggested it instead.The designs for the PS-503 and the one you asked about use similar voltage and current control loops or-ed together with diodes. The PS-503 clamp circuit works on that same point to pull the output down preventing glitches which is why I mentioned it.There is no relationship to dual tracking mode. The positive and negative supplies share a common ground and may be operated independently or with tracking. The Q15/Q115 clamp circuit operates on both no matter how they are configured.

author: liquibyte
author_rank: Frequent Contributor
timestamp: August 30, 2014, 12:06:29 am
content: Quote from: David Hess on August 29, 2014, 11:16:51 pmI thought it might have something to do with being in dual tracking mode because Q15 is a 2N2222A and Q115 is a 2N2907A and their tied together in an odd way (at least to me) between the positive and negative supplies. I'm having a hard time seeing where this could tie into the above posted circuit. U45 is in the voltage loop and U55 is in the current loop but their configuration seems to differ a bit from the above schematic. Given that U45 outputs to Q85 irrespective of U55 which seems to correspond to Q2 I'd assume a test could be done by tying a variation of this into pin 6 of U2 out to ground.

author: liquibyte
author_rank: Frequent Contributor
timestamp: August 30, 2014, 02:32:14 am
content: I just realized that Q1 in the original schematic serves this very function. Now I'm wondering why it was ever removed.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2014, 02:34:15 am
content: Quote from: liquibyte on August 30, 2014, 12:06:29 amThe PS-503 has both because the positive and negative regulators are separate despite the common ground and tracking feature so they each need to be protected against output glitches. What that share in common is that one biases the other so if power drops on one, the error amplifier on the other loses its + or - 5 volt bias supply.I think what is needed to fix the glitch issue is an under voltage lockout which disables the output stage by pulling the base of Q2 to ground when the input voltage is low and that would apply especially to the negative supply voltage for the operational amplifiers.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2014, 02:36:48 am
content: Quote from: liquibyte on August 30, 2014, 02:32:14 amHaha, Q1 not only does what I suggested but in the same way that it is done in the PS-503 schematic. Maybe whoever removed it did not know what it did. It is kind of hard on the operational amplifier.

author: liquibyte
author_rank: Frequent Contributor
timestamp: August 30, 2014, 03:00:59 am
content: The original op amps used were TL081's, the new version uses TLE2141's. I have a third supply sitting here that I was testing out for another member from the other forum and I think I'll hack Q1 back in and see how it performs. I really appreciate you pointing me to the PS-503, that made the light bulb go off when I started looking at it. If it works, I'll probably re-design the circuit once more and put things to rights.

author: paul18fr
author_rank: Contributor
timestamp: August 30, 2014, 06:48:54 am
content: With the schematic & the B.O.M, there is an additional pdf file with comments ....

author: Kevin.D
author_rank: Frequent Contributor
timestamp: August 30, 2014, 12:34:34 pm
content: Hi there liquibyte.Heres a few helpfull things I hope .You have no reverse Vbe protection in the first schematic for your pass transistors .D10 served that purpose on the later (original) schematic you posted,theres also other way's to protect against this .When you attach a battery , or any Vsource on the output and it's Voltage is higher than your supply Vout setting then feedback error amp tries to pull the output voltage down by reducing base voltage .This leaves the pass transistor's with it's Veb reversed and recieving full Battery voltage you applied.BJT's Veb reverse Breakdown at ~7v and permanently damaged (it reduces HFE).In a darlington pair like your using it's usually the smaller drive transistor (Q2) that will be the one that's damaged since the large one is protected by the speed up resistor (typically a few 100R from base to emitter) which is usually present .(R16 in your schema).When using pots or anything else that are common fail points ,It's best to try to connect them in such a way that they 'fail to safe', that is the V Supply output Drops when these devices fail .You could do that here but sometimes you cant always do this directly but then you can usually put 'pull to safe' resistors at various strategic points .eg . Like a large value resistor across your C4 which then if the V control pot wiper lost connection (common failure with cheap pots) this then pulls the non inv input down and so supply V output falls..Without it the output would rise since your opamp sources current (input bias current can flow out of opamp inputs ,it depends on opamp input stage type) out of it's non inv input and charges C4 so supply output rises to max .I wonder why ther'es no minum load on the output ?,R12 is to big and doesnt count. usually emitter follower type Vregs benefit alot or even require a minumum current to maintain a decent regulation performace when sourcing low output currents <10mA . Thinking about it this is what a bench supply probably spends most of it's life doing with todays low power electronics devices. Would be interesting to see how it's performing at low output load currents .Do you have a sim available .?Good luck .

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2014, 02:20:58 pm
content: Quote from: Kevin.D on August 30, 2014, 12:34:34 pmIntegrated converters whether they use an emitter follower or not usually rely on their feedback or output pin to sink their quiescent current for biasing which leads to their minimum output load current requirement. For instance a 7805 uses its common pin for this so generally does not care about minimum output loading while a 317 uses its output pin so must operate with a minimum load.Like most bench supplies, the minimum load requirement for this design only matters if collector leakage of the pass transistors is high which is possible but usually not a problem.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2014, 04:44:35 pm
content: Quote from: liquibyte on August 30, 2014, 03:00:59 amI would not have made the link to and mentioned the PS-503 except that we were discussing its design over at TekScopes@yahoogroups.com last week.The glitch problem may be related to a common (Haha! Pun!) issue with operational amplifiers. Many will undergo phase reversal of their output when their input common mode range is violated. Various early JFET operational amplifiers including the TL081 series were known for this but many bipolar and CMOS amplifiers suffer from it as well.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: August 30, 2014, 06:15:52 pm
content: Quote from: David Hess on August 30, 2014, 02:20:58 pmHi there David .For me it's more of a problem of output impedance increasing at such low output currents and causing the load pole to move down in frequency .You can see it clearly in loop gain /phase plots of followers under light load ,but then when you do a transient response performance test you still get really good performance , even though now the loop phase margin has reduce to only a couple of degrees. Its good proof that source/emitter followers make such good inherant V regulators that even with a crappy loop at a 3 deg phase margin you still get great transient load/line performance . (if I get time over the weekend I will post a few plots showing exactly what I mean).Here In this slow static type supply it doesnt seem to be a big prob . but i bet would cause you trouble though if you ever wanted a dynamic V supply ,or in some other senario I havent yet thought about .

author: Kevin.D
author_rank: Frequent Contributor
timestamp: August 30, 2014, 06:40:33 pm
content: Just had another quick look at this schematic shows another more serious prob with the way the current sense amp U3 is controlling the pass transitor via U2 .This is not going to be stable since these two amps alone without any other poles will cause a close to a 180 phase shift (each contributing nearly 90 each in the way they are configured).I tried simulating a similar setup and it was totally unstable in cc mode most of the time spice wouldnt even sim it .if your CC stays stable under most load then i'm a monkeys uncle.The way to do it would be to let U3 control Q2 directly .(so the anode of D9 goes to the base of Q2 .And you put a 3K ish resistor on the output of U2 ,enabling U3 to directly pull Q2 base drive down.Alternative would be to make U3 very low fixed gain( flat response so no phase shift)but I prefer the former way.If I feel like it this week ,I will post the quick spice analysis sometime .Regardsp.s Correction ,when I said both opamps contribute 90 deg ,I was mistaken, the second opamp U2 only behaves like a follower to U3's input so contributes no gain or phase shift .I was to sloppy with my quick generic sim ,,but that still looks iffy loop . It's got no phase margin and it's teetering right on the brink and starts oscillating under slight changes .Perhaps I or someone will do a more thourough sim this weekend.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2014, 09:23:43 pm
content: Quote from: Kevin.D on August 30, 2014, 06:15:52 pmI understand what you mean but have never found it to be a problem unless the frequency compensation was marginal to begin with. Like you point out, emitter/source followers are really good in this respect but I have seen common emitter/source outputs also work well with no output load.Some bench supplies include a constant current preload which allows the output to sink current if necessary so they operate effectively in class AB.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2014, 09:31:56 pm
content: Quote from: Kevin.D on August 30, 2014, 06:40:33 pmThe PS-503 example we discussed has both control loops connected directly to the output element and most designs I have seen work the same way.When the current control loop acts through the voltage control loop, it is slower but extra frequency compensation may not be necessary. Often the output transistor and output capacitor limit the frequency response producing dominant pole compensation. Put too fast of a transistor in there or let the output capacitor wear out and there can be problems however.Designs that use a separate instrumentation amplifier inside the current control loop suffer from a similar problem with extra delay possibly causing frequency response problems which is why I prefer fast singled ended current sensing.

author: liquibyte
author_rank: Frequent Contributor
timestamp: August 31, 2014, 03:19:58 am
content: You guys are starting to lose me in a big way.I've tried a test with a version that seems to be suffering an inversion problem that the original apparently had as well but that I thought might go away with the spiking problem if Q1 were added back in.To wedge this back in, I combined both versions into one pic so I could compare and I also added the component values to see the differences there as well. One thing that stood out to me almost instantly is that U2 on the revised version is no longer tied to U3 below ground so I don't know if this would be an issue or not. I never made the original project so I can't comment on its performance other than having read the issues others have had with it. The only issue that I can report, having tested the current version under full load at 3A at all voltage levels, is that on startup and shutdown there is a transient surge that would basically destroy any components not rated for it were they to be hooked up while that occurred.I hacked together a 2N2222A (I don't have a BC548) with a 10K and 1.5K divider across its base like in the original. I didn't put R15 or D10 back in and didn't move U2's ground and I don't know if that's important or not. I did hook up the other side of R14 to the point between R3 and D7b. What did happen was that I could only get around 200-250mV on the output regardless of where the current and voltage pots were set so I'm assuming that the transistor turned on and stayed that way and shunted everything back to ground while on instead of just during startup and shutdown.One thing to note on this is that I am using a slightly modified version of this PS to do tests with but it does suffer from the same issues as the ones I built based on the revised schematic. I'm attaching that schematic as well but please don't give me grief over it because all I did was do an image export of the .sch I got and didn't do the work myself, I'm way more anal about my schematics. If I can solve this issue with transients, I'm going to redesign both my boards and the surface mount version that redwire did.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: August 31, 2014, 10:06:02 am
content: The only thing I can see to cause that glitch is if U3 is pulling non inv input of U2 below U2's gnd at startup and leaving C4 with neg charge on it compounding the problem ( U2 output will invert if any of it's it's input go .3V below it's own -Vsupply ). disconnect a leg of D9 ,does your startup glitch go away ?.If so then you need to reconnect U2 -Vsupply to the neg supply to solve this issue . If that was the cause here then the original wouldn't have suffered the problem this problem .RegardsLater Addition :- I think I see what might have happened on the orginal when U2's -V was tied to the neg rail . The derived -neg rail has such a high source impedance so that during the initial turn on current surge the neg rail ends up being higher than the normal gnd rail .Since U2 has it's + input tied to normal gnd via C4 then it is now below u2's -neg rail and hence the polarity reversal of U2 output .This is probably slightly different to what is happening to you now on your revised schematic (earlier reply) when U2 supply is not on the -neg rail . So by reconnecting U2 back to neg rail like I said earlier after the D10 test wouldn't actually help unless you where also re-adding Q1 (which shut's the output off when the -neg rail rises) .If you wanted to keep your revised schema and leave out Q1 and keep U2 where it is now on the normal gnd (which is probably best), then a solution you can try is putting a schottky diode from U2 non inv input to -out gnd node (anode to -out).This is the kinda thing that happens when you start having multiple gnd's one of them's high impedance and they all move relatively to one another .You have each of your 3 opamps powered from different gnd points and it can make unforseen problems.Good luck .

author: David Hess
author_rank: Super Contributor
timestamp: August 31, 2014, 03:57:58 pm
content: Newer operational amplifiers may be immune to phase reversal by design. For a while that was a selling point for replacing older amplifiers.

author: liquibyte
author_rank: Frequent Contributor
timestamp: August 31, 2014, 04:31:26 pm
content: We've solved it. The new 2141's don't suffer from this, you are correct. Audioguru over at e-lab was working with me on this so pin 4 stays at the revised ground point with R10 but adding in Q1 again with a voltage divider across its base of 12K and 160R to the negative ground solved the transient issue. I was getting 7.6V until he suggested I may have the collector and emitter reversed. Apparently the datasheet I referred to for the pinouts was different than the 2222A I was using. Radio Shack parts, go figure. I don't think I've been this happy to have figured something out in my life. I've also learned that I hate working with surface mount components.

author: David Hess
author_rank: Super Contributor
timestamp: August 31, 2014, 04:52:33 pm
content: Quote from: liquibyte on August 31, 2014, 04:31:26 pmThat is right. I did not examine the offset voltage correction circuit carefully because it is not needed; I would have left it out. On the TLE2141, it is suppose to connect to the negative supply.From the TLE2141 datasheet:Inputs can operate between VCC? ? 0.3 to VCC+ ? 1.8 V without inducing phase reversalIt is sometimes a good idea to include external glitch suppression even if the operational amplifier does not suffer from phase reversal because the operational amplifier may be changed in the future by someone unaware of the issue.QuoteJust wait until you reverse the collect and emitter of a bipolar transistor and the circuit worksbetter.

author: liquibyte
author_rank: Frequent Contributor
timestamp: August 31, 2014, 05:19:24 pm
content: Quote from: David Hess on August 31, 2014, 04:52:33 pmI can't even begin to imagine a scenario where this would be apparent to me in any way.I'm good at following directions and I'm desperate to learn the intricacies of how all this works but I still need to go to the various calculators online to help work through circuits and even then some things a lot of you guys take for granted stump me. One of these days it'll click I'm sure but for now I still struggle at times, especially when trying to understand op amps. I solder better than anyone I've ever met though, so there's that.Edit: I've attached a modified version of the parts list and schematic for posterity. From what I've seen so far, the power supply now performs admirably for what it's worth.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: August 31, 2014, 06:52:10 pm
content: Quote from: David Hess on August 31, 2014, 04:52:33 pmHe did have it connected correctly in his revised circuit with the glitches .I think also the schottky ( maybe a schottky Vf drop might be just a slight bit to much for the -0.3 this opamp gives as max) may have solved it for him without Q1 .Dont forget D10 and that R15 (or some scheme) for your Q2 Veb(max)protection liquib .

author: liquibyte
author_rank: Frequent Contributor
timestamp: August 31, 2014, 07:20:41 pm
content: Quote from: Kevin.D on August 31, 2014, 06:52:10 pmThe guy that did the revision seemed to think that these parts really didn't add anything to the circuit.QuoteI've posted over there about it but I can't imagine that it was in the original circuit for no reason, someone thought it important enough to include.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: August 31, 2014, 07:55:18 pm
content: Quote from: liquibyte on August 31, 2014, 07:20:41 pmHeres a little experiment for you .Go get a known good bjt comparable to Q2 and measure it's HFE then pass a ~10ma reverse current through the Veb junction for a just a few seconds then remeasure it's gain afterwards .(Vbe breakdown at ~ 7V- so in your supply you would get about ~ 7mA through 1k R16 and Q2 junction if you connect a ~13.8V battery to your supply output and turn your V setting down ) .I think the gain stops reducing when it reduces to a certain point but how low it goes is probably variable between devices and depends on reverse current.A little extra math shows that once more than 7mA is flowing through R16(1k) and Q2 then you will have enough drop between the emitters and bases of Q5 and Q4 for them to also start breaking down (so a voltage of >~ 15V applied to the output will be enough to damage all you pass bjt's ).

author: David Hess
author_rank: Super Contributor
timestamp: August 31, 2014, 08:28:44 pm
content: Quote from: Kevin.D on August 31, 2014, 07:55:18 pmHfe degradation because of base-emitter reverse breakdown is well known (although maybe not here) but only a significant problem at low collector currents like you would find in a differential input stage which is why precision and low input bias current bipolar operational amplifiers have protection against excessive differential input voltages. Even a little bit of reverse breakdown will ruin them. It can be ignored with transistors operating at high collector currents which will have low gain anyway.In a power supply, I would worry more about reverse breakdown destroying the output transistors if the input supply shorts and the output capacitance discharges through the pass transistors. Integrated regulators sometimes die from that also.

author: David Hess
author_rank: Super Contributor
timestamp: August 31, 2014, 08:58:30 pm
content: Quote from: liquibyte on August 31, 2014, 05:19:24 pmIt is not widely advertised but bipolar transistors will work with their collector and emitter reversed. They have low gain (maybe even be below 1) and low breakdown voltage when operated this way but also have a very low saturation voltage.They used to manufacture bipolar transistors with a more symmetrical emitter and collector for special applications like choppers and switches but JFETs and MOSFETs took over that role. I still occasionally see a circuit design that takes advantage of a reversed bipolar transistor as a low saturation voltage reset switch. The advantage over a JFET or MOSFET is that it can be turned on with only 0.6 volts so it is handy in low voltage circuitry.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 01, 2014, 02:50:25 pm
content: On the suggestion of another member I tested moving the collector to the input of U2. Still getting a voltage spike but not near as bad as it was. I'm starting to think the way to mitigate this may be some sort of isolation scheme such as a relay but I could also be way off too. The scope I have is old but it does agree with what I'm seeing with my meter which is also not very good. One of these days I intend on getting proper equipment but for now this is what I can offer in terms of data. What follows is what I posted over there.I've tested with the collector of Q1 at pin 3 and pin 6 of U2. Given my meters error rate (and that of my eyes), the results are roughly the same. There is definitely a voltage rise happening on power on. Once I get above 1V the auto resolution of the meter probably hides the rise I get on power off but starting at 800mV and under I can see a definitive rise to around 1.25V - 1.5V as the caps drain out over many seconds. I wouldn't want to keep something that required very low power levels hooked up on power off because all control seems to be lost.To test consistently, I waited to power on once my meter settled to under 200mV as the caps drained and then turned on. I did this several times to make sure it was consistent and to compensate for my brain not registering the number the first go around.Pin 3 Pin 630V 36.75 36.625V 33.06 3220V 29 2815V 23.56 2310V 16.23 16.695V 7.95 8.563.3V 5.49 5.62V 3.6 3.91.5V 3.2 3.41.2V 3 2.81V 2.7 2.6.8V 2.2 2.2.5V 1.6 1.5.2V .9 .3.1V .36 .30V Too small and quick to see (39.2mV == 0)

author: David Hess
author_rank: Super Contributor
timestamp: September 01, 2014, 04:58:37 pm
content: I think you are looking at the wrong thing or at least making the wrong comparison. The control group would be the design using the original TL081 and no Q1.And if Q1 was wired to pull pin 3 of a TL081 down to ground, it could cause the very problem that it was added to solve.The TLE2141 should make Q1 superfluous anyway because it is not suppose to suffer from phase reversal.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 01, 2014, 05:47:44 pm
content: I don't understand op amps enough to know what phase reversal really is. Q1 definitely has had an effect on the transient spikes which is what we're trying to mitigate along with some ringing that was observed. I'm just beginning to understand some of this and, most assuredly, don't understand all of it.So far, there's only been two people that have been testing this design to try and work out the bugs, the third guy has never built one but made all the changes from the TL081's to the TLE2141's etc. The other guy just got a DSO and has been working on the noise and ringing issue and sent me one of his boards so I could look at the issue along side the transient problem after replacing U1 with a voltage regulator.For me, the transients are the thing my scope can see and what I noticed most, the noise is non-existent or has been dealt with in the design with the regulator in place of U1 although the ones I built aren't bad.When you posted the model of the PS you were discussing, I got to looking at the circuit compared to this one and the original and made the leap to Q1's removal contributing somewhat to the issue. I haven't tried this with the version that has a U1 in it yet, I have to build a third board up to do that. Having said all that, the guy that hasn't built one is the one that suggested the pin 3 swap to avoid having to put D10 and R15 back in, which I haven't done yet. I haven't tested this loaded either, just unloaded at the output and have been very careful to avoid shorts and such.At this point I kind of wish that I'd just bought a supply. I could have gotten something decent to use and done this one later. As I said before, it's still been fun and I am learning, albeit slowly. I'm going to take one of my spare boards and build it up and start testing it as well to see if I get the same results as redwire's version. I suppose that I should make a version of the original as well just to be thorough. It was my understanding that the original suffered some serious issues due to underrated components etc.

author: David Hess
author_rank: Super Contributor
timestamp: September 01, 2014, 06:26:16 pm
content: Quote from: liquibyte on September 01, 2014, 05:47:44 pmPhase reversal means that when the input voltage range is exceeded, the output reverses. In extreme cases, this can cause the operational amplifier to latch up and even fail. In modern operational amplifiers which suffer this problem, it usually comes about when either the inverting or non-inverting input is pulled too close to ground or below ground causing the output to shift to the positive rail.Page 11 of the LT1055 datasheet has a pretty good explanation:http://www.linear.com/docs/2885The ubiquitous LM324, LM358, and the TL071 and TL081 series all suffer from this problem.QuoteYou would not have learned as much though. Maybe we need a simplified modern lab power supply design that has been thoroughly vetted for problems.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 01, 2014, 08:56:30 pm
content: Quote from: David Hess on September 01, 2014, 06:26:16 pmThanks for the explaination. The reason for not pulling pin 3 below ground through Q1 though. That actually makes more sense now. I'm going to find reading material now.Quote from: David Hess on September 01, 2014, 06:26:16 pmI have begun to see patterns in circuits even when they're arranged differently so I guess that's a good thing. I just wish my math were better.I hope to have a better scope within a year or so but to be honest it took me a couple of years to get around to a temp controlled iron which I just got on Friday. I can solder up a mean board with a firestick though. I'm not sure if it'll help though. I seem to do fine without even using one but they are handy for helping to mitigate noise issues.I picked this supply out of the nthversions for its simplicity figuring it had already been thoroughly vetted given its age. I was wrong on that note but am now determined to see it through to completion. To be honest, the versions I built vs. the one I've been using to test Q1 are more stable on shutdown though they do suffer the same spikes as the one with Q1 on power on but don't have it. There's something odd going on with the surface mount version with the voltage regulator in place of U1. I have a dual version that I built and I'm going to pull one of the boards out of the case and hack in a Q1 and test side by side and see how things behave.

author: David Hess
author_rank: Super Contributor
timestamp: September 02, 2014, 01:17:02 am
content: Quote from: liquibyte on September 01, 2014, 08:56:30 pmIt does not necessarily need to be pulled below the negative supply. With some operational amplifiers, it just needs to be pulled below (or above) the common mode input voltage range and that may be a volt or more away from either supply voltage.The ancient LM709 was the one that would latch up internally under these conditions and self destruct although their may have been others. This could happen if the power supplies were not sequenced correctly so when the LM741 and LM301 came out advertising "no latch-up when the common mode range is exceeded" to replace it, there was much rejoicing.Operational amplifiers which suffer from phase inversion may latch up depending on their external circuit configuration.

author: paul18fr
author_rank: Contributor
timestamp: September 02, 2014, 11:12:51 am
content: Dear All contributorsI'm quite interested by this "project" and I hope that a consensus will be found to freeze a design; for hobbyists (that my point of view at least), it's interesting, rewarding and motivating to build some materials (PSU, basic function generators and so on).I cannot hide the economical aspect as wellPaul

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 02, 2014, 02:30:32 pm
content: Quote from: David Hess on September 01, 2014, 06:26:16 pmWhile I am trying to learn, these guys are light years ahead of me on this subject so I'll defer to them.I can add that I did add Q1 with the voltage divider across the base into one of my supplies and that it is working very well except that I'd recommend not having anything sensitive hooked up to it on power on. There is still a P1 voltage dependent spike happening when the power is switched on.There is some question as to RV1 even being needed as well. I can state that on my version it does nothing measurable even though it's supposed to zero the voltage and I'll probably remove it going forward based on the testingthisguy did with his version which also has Q1 I might add.I can tell you this does work, I have two of them in one case and use them. I never turn them on hooked up to a circuit unless I'm absolutely sure the components are rated above what the spike produces. At 30V, you'll get a 36-37V spike. At 12V, you'll get a 19-20V spike. At 3.3V, you'll get a 5-6V spike. If you can live with this, build yourself one. The nice part is that the components are reusable if you decide to go with something else. The transformer, pots, and pass transistors doubly so.

author: David Hess
author_rank: Super Contributor
timestamp: September 02, 2014, 07:33:31 pm
content: Quote from: liquibyte on September 02, 2014, 02:30:32 pmIf the voltage spike is above the set voltage, then something is wrong. I would find such behavior in a power supply unacceptable.QuoteRV1 works through the operational amplifiers offset adjustment pins. In most power supplies, the effect from the error amplifier's offset voltage is insignificant. Further, if the offset voltage adjustment is used to removeothersources of offset, it will likely degrade the error amplifier's offset voltage drift although again, in most power supplies that will be an insignificant effect.Who cares if the 0 to 30 volt power supply can only go down to 15 millivolts instead of 0 millivolts?QuoteThat is outrageous. I would consider the design broken if it produced a start-up or shut-down output spike like that.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 02, 2014, 07:38:37 pm
content: I took the liberty of posting the parts and costs etc. below sourced from Digikey. I chose fairly good quality parts so you could probably end up a fair bit cheaper but don't forget to factor in shipping as it adds up quick across multiple orders.Code:ValueQtyPart NumberEachTotal0.47 16W=1(A102127-ND)$3.06$3.062.2K 2W=1(RSMF2JT2K20CT-ND)$0.43$0.4382 2W=1(RSMF2JT82R0CT-ND)$0.43$0.430.33 2W=2(RSF2JTR330CT-ND)$0.56$1.121.5K 1W=1(RSF1JT1K50CT-ND)$0.35$0.10220 1/2W=1(CF12JT220RCT-ND)$0.14$0.1468 1/4W=1(CF14JT68R0CT-ND)$0.10$0.10150 1/4W=1(RNF14FTD150RCT-ND)$0.15$0.151K 1/4W=3(CF14JT1K00CT-ND)$0.10$0.302.2K 1/4W=2(CF14JT2K20CT-ND)$0.10$0.2010K 1/4W=5(CF14JT10K0CT-ND)$0.10$0.5027K 1/4W=2(CF14JT27K0CT-ND)$0.10$0.2033K 1/4W=1(CF14JT33K0CT-ND)$0.10$0.1056K 1/4W=1(CF14JT56K0CT-ND)$0.10$0.10Total=$7.185K trimmer=1(3296W-502LF-ND)$2.41$2.4120K trimmer=1(3296W-203LF-ND)$2.41$2.41100K trimmer=1(3296W-104LF-ND)$2.41$2.4110K 10 turn pot=2(987-1523-ND)$11.99$23.98Total=$31.2115000uF 63V=1(399-5658-ND)$16.04$16.0447uF 50V=2(ESW476M100AH2AA)$0.48$0.9610uF 50V Film=1(399-5999-ND)$2.70$2.70220nF film=1(399-6037-ND)$0.30$0.30100nF film=2(399-5861-ND)$0.22$0.44330pF ceramic=1(399-4173-ND)$0.40$0.40100pF ceramic=2(399-9707-ND)$0.44$0.88Total=$21.726-10A 50V=1(GBPC15005FS-ND)$2.86$2.861N4148=6(1N4148TACT-ND)$0.10$0.601N4001=1(1N4001FSCT-ND)$0.18$0.18BZX79C5V6=1(BZX79C5V6-ND)$0.13$0.13BZX85C10=1(BZX85C10-ND)$0.22$0.22LED=1(365-1189-ND)$0.14$0.14Total=$4.132N4401=1(2N4401-ND)$0.22$0.22BC557=1(BC557BGOS-ND)$0.38$0.38BD139=1(BD13910S-ND)$0.46$0.462N3055=2(2N3055GOS-ND)$2.34$4.68TLE2141=3(296-10456-5-ND)$1.81$5.43Total=$11.1728V 4.6A 130VA=1(237-1281-ND)$27.34$27.34Grand Total=$102.75$102.75, HOLY CRAP!!!Let's look at this though. Source your own transformer, 2N3055's, big filter cap, trimmers and pots, bridge rectifier, and 0.47R resistor.Transformers can be had many places and used isn't usually an issue as long as it can supply 28V @ 4A. You can get 2N3055's elsewhere cheaper if you look, just avoid the counterfeit ones. I'd recommend not skimping on the filter cap but the value I got was 15,000uF, the design only calls for 12,000uF so you could probably save something there. The trimmers and pots I used are all 10 turn versions and rather expensive. You can get away with single turn versions if you'd like for much, much cheaper. I used 10 turn trimmers so that the PS can be calibrated more accurately and 10 turn pots for better resolution. You can get away with a much cheaper bridge rectifier, I over engineered here by a lot. You can also get a non-aluminum resistor for this, the design calls for 0.47R @ 10W. I couldn't find a 10W aluminum resistor at 0.47R so I went up. I'm glad I did though, it gets hot under load.You ready for this? $17.56 if you take out everything I just said you can source elsewhere. You can get even cheaper if you order the more common values in bigger quantities but cheaper is relative here because you're still spending money. This cost is for only the quantity of components required.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 02, 2014, 07:54:08 pm
content: Quote from: David Hess on September 02, 2014, 07:33:31 pmThat's the thing I'm trying to fix.Quote from: David Hess on September 02, 2014, 07:33:31 pmIt's around 35 millivolts actually, not that it ever bothered me. RV1, according to audioguru over there, was supposed to be able to take the output to true zero. It doesn't.Quote from: David Hess on September 02, 2014, 07:33:31 pmI agree, it's unacceptable as it stands. I'm attempting to learn enough to fix it though. To be honest, I'm not even sure where it's coming from. I have a hunch that it's got something to do with the transformer inrush but I always thought that the filter caps helped there. I could be way off though, I'm a newbie. What I don't get is that people have been talking about this design since 2003 and no one has noticed this enough to fix it. They've blown up plenty of TL081's though, which is why it was redesigned with the 2141's from what I understand.I'd like to sim the circut but I suck at spice because I get absolutely lost. That and I could never find a model for the 2141.

author: David Hess
author_rank: Super Contributor
timestamp: September 02, 2014, 09:55:59 pm
content: Quote from: liquibyte on September 02, 2014, 07:54:08 pmWhich just means that there is additional offset outside of the amplifier creating the other problem I just described. I have never measured the offset on any of my PS-503s ...On the one PS-503 that I have plugged in at the moment, its offset is -6.1 millivolts on one output and -7.6 millivolts on the other, meaning that the output ranges extend a few millivolts on the other side of zero, however the design deliberately arranges for the outputs to always go slightly negative given the worst case input offset voltages of the error amplifiers. It would be easy to trim them to zero (without the operational amplifier offset pins!) but offhand I cannot think of a reason this would be important.I checked for glitches at power-up and power-down both with the power switch and the standby switch and in no event could the output rise about the current voltage setting and there were no visible glitches as such. There was a tiny millivolt level fast glitch at turn on which I suspect is common mode noise since the PS503 outputs are floated with respect to chassis ground.If there was no load on the output and the output capacitor was not discharged, then the voltage sometimes dropped before rising again when the power switch was cycled which I suspect is caused by the action of those two protection transistors we discussed earlier.QuoteThe only way the output can rise in this design is if the output of the operational amplifier rises for whatever reason or if there is leakage through the pass elements.One thing I would try which is part of the PS-503 design is to add a low current preload from the output to the low voltage negative bias supply. A 6.8k resistor would be about right.QuoteSimulating startup of an operational amplifier strikes me as pretty difficult to do. Some operational amplifier models will definitely not handle it correctly if only because they do not always model common mode input range violations.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 03, 2014, 01:43:31 am
content: Quote from: David Hess on September 02, 2014, 09:55:59 pmYou just lost me here. I've tried several times to formulate the right question but you really lost me at output. My mind wants to think of output as pin 6 on one of the op amps but I can also think of it as output of the power supply. Honestly, my mind also wants to think "low voltage negative bias supply" means U3 and I think that corresponds to U55 but I'm not seeing the equivalent pre-load in the service manual. I think if I just start hooking a 6.8K resistor to various outputs and different pins of U3 I'm going to blow something up or at least damage it past the point of working right and make tracking down the problem even worse.

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2014, 08:42:36 am
content: Quote from: liquibyte on September 03, 2014, 01:43:31 amI mean the PS-503 has a preload on the output of the power supply although you have to look hard to find it. The preload resistors are R85 and R192 and attached directly to the emitters and collectors of the output transistors and go to the opposite bias supply.On yours, the resistor would connect directly from the output of the power supply to the -5 volt bias supply which powers the operational amplifiers.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 03, 2014, 04:38:56 pm
content: Quote from: David Hess on September 03, 2014, 08:42:36 amI thought those might be it but wasn't sure.Quote from: David Hess on September 03, 2014, 08:42:36 amStill not doing anything to mitigate the spikes. I've also tried with a 1K to load it more because I'm measuring -1.5V and not -5V but still not making a dent in my readings. I've been testing at 10V to make seeing things and the mental math easier, both between what my meter is seeing and what my scope is seeing. I'm getting a consistent 7V spike at that 10V. I'm starting to think that this circuit is just plain failure and not a whole lot can be done about it. It's a shame really because other than the transient on power up, the rest of it works rather well.

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2014, 09:56:24 pm
content: What does the output of U2 do at startup?What does the non-inverting input of U2 do at startup?QuoteI am confused about this. With the output set to 10 volts you get a 17 volt spike or it spikes at 7 volts before rising to 10 volts?

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 03, 2014, 10:35:07 pm
content: Quote from: David Hess on September 03, 2014, 09:56:24 pmWith the output set at 10 volts, pin 3 sits at 3.74 volts. When I measure during power on at pin 3 the voltage rises to 6.3 volts before falling back to 3.74 volts. The same thing happens at the inverting input.Pin 6 outputs 10.65 volts. On power on I get 17.65 volts.Quote from: David Hess on September 03, 2014, 09:56:24 pmWith the output set to 10 volts, I get a spike up to 17 volts before falling back to 10 volts as it stabilizes. Since I don't have a DSO, I can only eye it but it looks like this happens over about a half of a second. It's hard to eye it but I have the scope set at 50mS and it looks like it spans between 5 and 7 cm before it settles. Keep in mind that this scope is an old Conar 255 that I haven't calibrated but it does seem to agree with my meter more or less.

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2014, 11:29:20 pm
content: Quote from: liquibyte on September 03, 2014, 10:35:07 pmThat answers that then. The output from U1 is actually the problem.The inverting inputshouldfollow the non-inverting input if the operational amplifier is acting as an operational amplifier.My WAG (wild ass guess) is that U1 operating open loop during startup before D8 conducts results in overshoot caused by windup of its internal compensation capacitor. If U1 had a negative offset voltage, then I think it could latch off but we need not worry about that since it is not happening at the moment. In a production design I would want to test for that.Try adding a capacitor across D8. I am not sure what the value should be but a .1 microfarad ceramic or film would be a good place to start. This will force U1 to operate closed loop during startup and lower noise from the reference as a side effect.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 04, 2014, 12:18:12 am
content: Quote from: David Hess on September 03, 2014, 11:29:20 pmI've added a 0.1uF ceramic across D8 but I'm still getting about 16.7 volts on startup before it settles back to 10 volts. I have a jar of many values of ceramics from 1pF to 100nF that I got offEbayfor doing just this kind of thing.The funny thing is that the version that I have that has a vreg in place of U1 has the exact same issue and it doesn't have a D8 either.I've even tried loading the output from 2.5K to 3K (6 & 5 15K parallel resistors) to see if it helped but it didn't. It does help load enough so that RV1 can get the offset of U2 down to around 15mV though. I'm not sure if the difference between 35mV and 15mV is worth the trouble considering the transient voltage problem. If it helped in that regard, I'd definitely keep it.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2014, 03:10:50 am
content: You could try a larger capacitor across D8.Try removing C4.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 04, 2014, 04:23:20 am
content: Quote from: David Hess on September 04, 2014, 03:10:50 amI tried several from around 0.1uF up to 2.2uF with no luck. Removed C4 but nothing changed.I've tried measuring across C1 to see if something is going on from there and I get 61 volts initially settling to 43 volts. Could this be what's causing this? Would there be a way to limit this initial inrush to 44 volts tops by using a TVS or something?

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2014, 10:35:35 am
content: Quote from: liquibyte on September 04, 2014, 04:23:20 amThis is a problem and not only because of what it might cause the operational amplifiers to do. The supply voltage surge could cause this problem although I have no specific experience with TI's Excalibur series of operational amplifiers to know for sure. Their lower voltage CMOS operational amplifiers would just die with that kind of surge.The absolute maximum supply voltage for the TLE2141 is 44 volts. That rating isnotfor continuous operation and in practice the supply voltage needs to be significantly lower for reliable operation.You do not want to use a TVS or shunt to try and stop that kind of surge. A better option would be to use a zener shunt regulator to control and limit the operational amplifier supply voltage to about 36 volts.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 04, 2014, 03:47:06 pm
content: I was afraid this would eventually come up. I had concerns back when I started this project that the op amps were being kept too close to their upper limit and that it might have an effect on performance. What kinds of bad effects do keeping op amps towards their upper supply limit cause? I was told that everything was within tolerance and not to worry. Something still kept bugging me about it but since many people had built these and they supposedly worked, I built mine. I guess not many people have really tested the design until now.What if I were to use an op amp that had a bit higher limits, say ±48 volt such asthisone orthisone? The characteristics are similar and it would put me a few more volts above my input voltage giving a tad more headroom if I can contain the initial surge from the transformer. Pinout is identical. The first one has closer specs I believe but the latter has a higher supply voltage range. I'd really like to solve this but I'm not entirely sure it can be done without a significant redesign.I have a feeling that I'm going to be looking for something different to build soon. I wanted to build this so that I could get the ±15 volts to start learning more about op amps and I find it ironic that I don't have a supply that can to help solve this issue. Can you recommend something to be on the lookout for on Ebay, dual tracking perhaps or at least cheap enough to get two and use in series?

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2014, 07:13:11 pm
content: Quote from: liquibyte on September 04, 2014, 03:47:06 pmThey become generally unreliable and more likely to fail. Often they immediately fail if you exceed the maximum rating by just a couple volts.QuoteThe design assumes that the unregulated supply is not spiking above 44 (or 36) volts when power is applied.QuoteNeither of those are single supply amplifiers so they will require about a -5 volt supply to operate correctly in this circuit. The opa445 is fast and would probably require frequency compensation.QuoteThe input voltage surge problem would be easy enough to fix with a 33 or 36 volt zener shunt regulator on the positive supply to the operational amplifiers. This would be similar to the shunt regulator used to generate the negative supply.Old Power Designs, HP, and Tektronix power supplies are pretty good. Full documentation makes them relatively easy to repair and maintain.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 04, 2014, 08:15:49 pm
content: Quote from: David Hess on September 04, 2014, 07:13:11 pmI did notice that even though it's rated at a 44 or ±22 volt max, datasheets often suggest much lower. I couldn't find anything in this one about that and had assumed with the assurances that things would be ok even towards the upper limitQuote from: David Hess on September 04, 2014, 07:13:11 pmThat's what has me so worried about suggesting this supply to anyone to build and use. Theory differs from reality most days that I'm aware of.Quote from: David Hess on September 04, 2014, 07:13:11 pmNoted. I didn't think about the single supply feature on the 2141.Quote from: David Hess on September 04, 2014, 07:13:11 pmIn your opinion, what sort of wattage should I be looking out for? D13 is rated at 1W but I'm thinking I'd need 3W or better but I could be wrong.Quote from: David Hess on September 04, 2014, 07:13:11 pmI had looked for some Power Designs stuff but Ebay got eevblog'ed and everything went through the roof when folks here started buying them. I'll set up a few watches and see if I can come up with a deal or two.I do appreciate all your help with this. I was starting to get a little lost and was pulling my hair out. I think I'll try and put in a zener as you suggest and see if things can be stabilized.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2014, 08:59:17 pm
content: Quote from: liquibyte on September 04, 2014, 08:15:49 pmAbsolute maximum ratings on datasheets are just that. They do not represent operating conditions conducive to reliability.QuoteSingle supply operation is not necessary if a negative bias supply is used. As a general rule, single supply operational amplifiers compromise other characteristics so it is better to use dual supply operational amplifiers if feasible.Nothing in a general purpose power supply requires anything better than a 301 or 741 type of operational amplifier. I like using OP-27/LT1007 style amplifiers in precision low noise power supplies.Quote from: David Hess on September 04, 2014, 07:13:11 pmIn your opinion, what sort of wattage should I be looking out for? D13 is rated at 1W but I'm thinking I'd need 3W or better but I could be wrong.I would use a 5 watt 36 volt 1N5365 and a 1 watt 180 ohm series resistor.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 04, 2014, 10:46:38 pm
content: Quote from: David Hess on September 04, 2014, 08:59:17 pmI think they went with the 2141 as a better fit with the redesign considering it's supposed to do 0-30V @ 0-3A. One of the things that I've heard is that a 30V supply isn't really necessary but I went with this one to have some overhead. After struggling with this and having learned a few things, I'm going to get some reading material and try to design my own and see how it works out. I figure 15V should be enough and the two you mentioned above should be a good starting point.Quote from: David Hess on September 04, 2014, 08:59:17 pmI was actually looking at the 1N5365 @ Digikey earlier but wasn't sure of the wattage rating so I didn't add it to a cart. One thing I don't understand, what would the resistor be for? D13 doesn't seem to need one and it doesn't even get warm.

author: David Hess
author_rank: Super Contributor
timestamp: September 05, 2014, 01:20:50 am
content: Quote from: liquibyte on September 04, 2014, 10:46:38 pmI get by with my PS-503 which is effectively two 20 volt supplies in series. It provides either 0 to 20 volts plus and minus which is just right for powering analog circuits or 0 to 40 volts.QuoteI am not sure why they included D13 but it is only dropping the voltage by 10 volts and is not regulating the supply to U3.The zener shunt regulator would be configured the way R3 and D7 are with the series resistor dropping the voltage. 44 volts - 36 volts = 8 volts across the resistor. 8 volts / 180 ohms = 44.5 milliamps. 8 volts * 44.5 milliamps = 356 milliwatts in the resistor and 36 volts * 44.5 milliamps = 1.6 watts in the zener diode.A 3 watt 36 volt 1N5938 could be used as well.The current in the zener diode is actually lower than 44.5 milliamps by any current that U1 and U2 are drawing.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 05, 2014, 03:12:27 am
content: Quote from: David Hess on September 05, 2014, 01:20:50 amThe way it was explained to me is that since the positive rail is sitting so close to the limit of the op amp and that because they pull the negative rail 1.3V below ground that a voltage drop was required to keep from overloading the op amp.Quote from: David Hess on September 05, 2014, 01:20:50 amMakes sense. I've got a few other things I'm putting together for another order so I'll add those to the cart. I'm ordering some OP-27's too, I'd like to try my hand at a dual supply from scratch. I've got a question about that by the way. When you build two separate supplies and hook them together in series to get positive/negative, do you have to rate the components in the design higher than if you just used it as a single supply? If so, is there a general rule such as 2x or 2.5x. For instance I build a supply that uses 50V caps, would I have to use 100V caps or even higher? I know there are instances where diodes across the output in a certain configuration is required.

author: David Hess
author_rank: Super Contributor
timestamp: September 05, 2014, 05:59:04 am
content: QuoteThere is nothing wrong with TL081s (The TL071 is or used to be a TL081 graded for low noise.), 741s, and 301As. The lowest cost precision amplifier is probably the OP-07. OP-27s are more expensive and their extra speed may require a more complex design so I would get something working with a cheaper amplifier first. Their added performance is not going to make a difference in a general purpose power supply.QuoteThis just depends on the implementation. All of the dual output designs I am familiar with are either separate supplies or a pair of supplies that share a common ground. In both cases each supply or side just sees its own voltage.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 05, 2014, 03:48:43 pm
content: Quote from: David Hess on September 05, 2014, 05:59:04 amI get lost looking at the damned things and the datasheets don't help. I did compare the pricing though and for 10 OP-27's you can get 10 each of the others. I did that instead. I already have a few LM358's that I got from somewhere as well.Quote from: David Hess on September 05, 2014, 05:59:04 amWhat I was looking to do when I started this project was play around with op amp circuits and wanted a positive/negative supply to use. When I built these, I used quality components and made sure I didn't skimp on anything just in case. I was worried about the caps so I upped the recommended voltage from 50 to 63 on all of them for margin.If I wire the resistor and zener between the rectifier and filter caps that would put it before the 10V zener as well. There's 33.5 volts into pin 7 on U3 right now with 35.1 volts across pin 7 and pin 4 which would be reduced to around 26 volts and 27.6 volts respectively. Should I cut the trace to that and run a wire to before the 36V zener or would the voltage drop matter all that much being that it's the current limiting part of the circuit?I plan to eventually redo the boards because I've learned a few things since I did these, they were my first attempt at designing one and they are a pain in the ass to get in and out.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 06, 2014, 06:59:54 pm
content: Parts will be arriving on Monday. I'll post here with the results when I'm done testing. I'm still wondering if a soft start circuit might not be a bad idea though.I think I need to find a better alternative to the terminal blocks I used for power to board connections. I used these because I don't have a crimper yet and I thought it would make things easier to get in and out. Turns out I was wrong there.

author: paul18fr
author_rank: Contributor
timestamp: September 06, 2014, 08:34:02 pm
content: my question is probably quite naive, but I'm wondering if numerical simulations can help of course I've no experience in electonics ? ... (in mechanical engineering I commonly use specific sovers)I found some free ones :-Qucswhich has a GUI :http://qucs.sourceforge.net...- SPICEhttp://bwrcs.eecs.berkeley.edu/Classes/IcBook/SPICE/- CircuitLabhttps://www.circuitlab.com- and probably many others ....Paul

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 06, 2014, 09:08:39 pm
content: Quote from: paul18fr on September 06, 2014, 08:34:02 pmDon't forgetLTspice IVandTINA-TI.I suck at spice simulation. I'm not sure if it's even possible to model transformer inrush let alone what David Hess said about op amps. I guess you could have a go at it. I'm sure with your background you'd get much closer than this 8th grade graduate could. Not that I can't do math, I just haven't gotten that far into things yet.Quote

author: David Hess
author_rank: Super Contributor
timestamp: September 06, 2014, 09:38:41 pm
content: QuoteCut the trace and use the resistor and zener as a shunt regulator for at least U1 and U2. I would alter the design to use it for U3 as well and get rid of D13. The bottom of the zener can go to the negative supply so the voltage across the operational amplifiers is restricted to 36 volts total assuming that the negative supply can handle the current.There are better ways to do all of this but adding the 36 volt zener is the most simple.Note that some operational amplifiers will run at 36 volts with a 44 volt absolute maximum voltage and some will run at 30 volts with a 36 volt absolute maximum voltage. Do not use the ones with a 36 volt absolute maximum voltage at 36 volts!QuoteI have done that before.

author: David Hess
author_rank: Super Contributor
timestamp: September 06, 2014, 09:48:27 pm
content: Quote from: paul18fr on September 06, 2014, 08:34:02 pmSPICE tends to have problems in situations like these because the models are not usually designed to operate outside of narrow bounds.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 06, 2014, 11:36:12 pm
content: Quote from: David Hess on September 06, 2014, 09:38:41 pmJust to clarify because I was thinking in terms of the 10V zener configuration as in circuit A in the picture I did. You mean for me to do it like circuit B right? If so, I was thinking B1 but then you said that B2 is a possiblility if I remove the 10V zener.Quote from: David Hess on September 06, 2014, 09:38:41 pmI always follow the datasheet which is why I questioned the 44 volt level on the other board. I thought that it was too close to the limits of the op amps and given any untoward occurance, failure would be inevitable. I ordered several of each of the ones you had mentioned and the first thing I did was save the datasheet to my op amp datasheet folder. I also went and got some AN's and other reading material on the subject.Quote from: David Hess on September 06, 2014, 09:38:41 pmI have a better idea of the constraints of the case and plan on using a different scheme for the terminal blocks. I haven't decided the best way to do it yet but I'll come up with something.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2014, 05:21:16 pm
content: Quote from: liquibyte on September 06, 2014, 11:36:12 pmB2 may be better in general but if you use the zener regulated 36 volt supply to power U3 (and maybe the Q3 circuit) as well, then the 10 volt zener circuit will not be needed. They are separate issues.I would use the B2 connection and increase the negative supply to -2.5 to -5.0 volts but if you use 44 volt operational amplifiers instead of 36 volt ones, then you can do this and use the B1 connection instead so the maximum output voltage will be a little higher.What would be clever is to change the Q3 circuit by using the 10 volt zener to level shift the output of U3 so it can drive Q3 even with Q3 supplied by the higher voltage. Then the current limit LED will light during current limiting or high input voltages. Or that could just be confusing. /shrug

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 08, 2014, 10:02:04 am
content: Quote from: David Hess on September 07, 2014, 05:21:16 pmI'd have to do too much with the boards as they are to change the negative supply so I'm going to go with desoldering the cathode of D13 and running a wire from it to before the 180R resistor. The rest of it is just a matter of soldering the zener across the bottom of R1 and soldering the 180R to the positive side of the filter cap where it comes into the terminal block. C1 and the bridge rectifier are off board, R1 is on the board.Quote from: David Hess on September 07, 2014, 05:21:16 pmI looked up what you were talking about and didn't even know that was a thing or at least what is going on is what it's called. I'm not seeing how to do it as it applies to U3 and Q3, so your last statement applies for the moment.I think what I need to do is visit more service manuals on BAMA and the like to see how certain problems have been solved amongst various power supplies over time. Given that these are proven designs, or at least were at the time, I think reverse engineering would be a more logical approach to understanding than magic smoke experiments. I've been rather lucky in that regard. I've only burnt a few transistors and resistors so far, and once an exploding cap. To be honest, I grabbed the wrong voltage but it did scare the crap out of me when I powered on. I used to work for a place that built motor braking solutions and it was nothing compared to when the labs there blew stuff up, it was always fun when an IGBT let go.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 08, 2014, 05:27:21 pm
content: My goodies came in this morning and I've managed to get everything in there like I posted in the pic in my above post.The good news is that 61 volt spike is gone. The bad news is that I'm still getting a fairly decent spike around 36 volts with the PS recalibrated at 30V 3A which is tp be expected given a 36V zener I suppose. At this point I don't know if this is inherent to linear supplies and inrush or if the circuit just needs some rework to catch this and shunt this based on P1's setting somehow. I'm too new to this to know off the top of my head what needs done but I have a feeling that a solution might be in the PS-503 service manual schematic. I might have to build one of these just to probe around in it. Some of the parts are unknown such as some the transistors and all of the op amps. The manual doesn't say much about these other than their part designation so I looked up the part number and get back C741C so I'm assuming some sort of oldish 741 op amp.Either way, it's working much better than it was and even though it's still spiking, at least it's not as bad as it was. One of these days I'll manage enough money to get a real scope and perhaps help tame this thing if I haven't already. I have another meter in mind and I think that I probably need to get an analog or two just for when things like this are going on. I do appreciate your help with all of this. I'll keep updating for posterity if I figure anything out.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 08, 2014, 11:30:56 pm
content: The really bad news is that this is a 3A power supply so the modifications are not going to work. If I load this down at 3 amps, the resistor is going to smoke.

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2014, 11:56:22 pm
content: Quote from: liquibyte on September 08, 2014, 05:27:21 pmThe output should never rise above the set point when power is applied or removed. I am inclined to believe that there is more than one problem but the lack of any regulation or protection on the supply to the operational amplifiers I consider a design flaw.QuoteThe PS-503 design is pretty dense and unconventional in some respects even if you ignore the complexity added by being a dual output design with tracking. There are changes I would make like having the output amplifiers operate with a fixed gain and varying the output from the reference because it would make the frequency response more predictable.The PS-503 operational amplifiers are just 36 volt 741 types and not critical. Besides qualifying their sources, Tektronix graded incoming 741s and the ones with high bias current, high offset current, high offset voltage, and high offset voltage drift are the -00 ones used in non-critical designs. They actually graded them for 40 volt operation as well instead of buying the 741/A/M version.One of my PS-503s has a pair of 301A operational amplifiers for U55 and U155 but they and one of the 741s are in sockets so I think someone who knew what they were doing repaired it in the past. They added the compensation capacitors needed by the 301As to the back of the printed circuit board.QuoteSome multimeters have a pretty fast peak detect mode.Any storage oscilloscope is handy in cases like these. DSOs are particularly useful for diagnosing startup problems and even an old and low performance unit can handle something like this. I very rarely need to use anything better than a Tektronix 2230 which is the oldest DSO that I can honestly recommend.I would never recommend a USB DSO unless it had a unique and useful feature like the ones from CircuitGear which support low frequency vector network analysis. That is particularly useful for designing high performance power supplies.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 09, 2014, 01:08:03 am
content: Quote from: David Hess on September 08, 2014, 11:56:22 pmThat's kind of what I was thinking. The design has never really worked from what I can tell. Reading the lengthy thread(s) on the design gives people that have had success building the thing but I don't think anyone has really tested it until very recently. One of the members over there just recently got a DSO and tried to solve the transient issue with the addition of the voltage regulator but I think he only tested within a certain range and reported it solved. It wasn't. I'm about ready to give up on this one for awhile and see if I can get something better going with a bit more research.Any word on a vetted design that would be relatively simple that I could perhaps use the two 28V 4.3A transformers with? I was shooting for 30V @ 3A but I also just got, in addition to the 301A's, OP07's and TL071's, a single primary, dual secondary transformer that can do 36V @ 1A series, 18V @ 2A parallel or 2X18V @ 1A independently. I was thinking 0-18V @ 1A positive and negative until I can figure out this other issue, I just need a proven design that won't set me back too much.Quote from: David Hess on September 08, 2014, 11:56:22 pmYeah, looking at it kind of intimidates me a little. I'm sure everything they did had a purpose at the time and I'm sure there are modern designs that run circles around it but I think simple would be better for me until I learn more. What I really want to learn is the analog side of things. When I was a kid and used to take things apart, the electronics always intrigued me but I never really did anything with it. Up until a few years ago I even had a few of those big assed aluminum variable capacitors. Digital wasn't here yet and I'd like to start with much less than a microcontroller.Quote from: David Hess on September 08, 2014, 11:56:22 pmThe scope I have is ancient even by ancient standards. Behold, theConar 255. I've been keeping an eye out on Ebay but things go fast there and the resellers are a big problem. I figure if I'm going to spend over $200 I might as well wait and keep saving to get a DS1074Z. Every now and then I check craigslist but there never seems to be anything there which is either odd or not considering I live fairly close to Boston.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 10, 2014, 04:23:58 am
content: I think I've found a solution that may help. I've seen the circuit in action on a video and it works but I simplified it a bit for my needs.

author: David Hess
author_rank: Super Contributor
timestamp: September 10, 2014, 03:28:06 pm
content: Quote from: liquibyte on September 09, 2014, 01:08:03 amUsually these problems are caused by improper startup and shutdown sequencing but linear power supplies are generally simple enough not to require complex solutions.QuoteThe only kit power supply I have is 723 based and 20+ years old. Since then I have usually designed and built my own as needed.I did a quick search online and did not find an designs or kits I would recommend. Variable constant voltage constant current designs tend to be as complex as the comparable Tektronix or HP implementations.QuoteThe Tektronix and HP/Harrison designers certainly knew what they were doing and were thorough. HP bought Harrison Laboratory Company in 1962.Any performance limitations are more from application limits then an old design. Modern designs intended for the same applications are not going to perform any better at least in a practical sense.The PS-503 has simple zener shunt and series regulators to protect the control circuits from high voltages. It has other design features which look like they were intended to prevent startup and shutdown glitches but the manual only discusses one of them.The similar HP/Harrison 6236 which is a more straightforward design has its own protection circuits. From its manual:4-11 The turn-on/turn-off control circuit prevents outputtransients when the supply is turned on or off. It does this bydelaying the application of certain bias and reference voltagesat turn-on and removing them shortly after turn-off.QuoteEven complex power supplies are simple enough that they are a great place to start learning electronics.QuoteThatisvery old. Even the lowest end of the inexpensive used analog oscilloscopes from Tektronix and others would be better but you need some type of storage to see glitches easily.QuoteThe 2230 that I have I picked up on Ebay for about $100 but I would not recommend one for a beginner unless you have the option to return it; they are somewhat tricky to repair and maintain and are getting old. The 2232 is a lot better but tends to go for $300.I have not used one but the CircuitGear USB DSOs I mentioned are interesting given their low cost. They are not particularly fast but they are certainly fast enough for power supply design and look like they were intended for a student environment.The least expensive Rigol I would consider is the DS1074Z. I would absolutely avoid at least their B, CA, D, and E series oscilloscopes because they lack peak detection as I understand the term according to Rigol. The documentation for their Z and 2000A series oscilloscopes is not clear on the matter either and naturally nobody who has reviewed them has tested for what exactly they do support so I am dubious about them as well which just leads me to conclude that Rigol makes junk.How can they design a modern DSO which supports peak detection or envelope detection but not both andthenscrew up the documentation repeatedly by confusing the two? I complained about this to them years ago trying to get answers; at first they did not understand the difference and then they lied about it.QuoteI am near St. Louis and see good analog and digital storage oscilloscopes show up on Craigslist about once or twice a year. I picked up a Tektronix 547 that way but missed a Tektronix 2232. I saw something like a Hitachi VC-6145 once and a Leader analog/digital CRT DSO but I prefer more common instruments with good service documentation.Boston should have a larger market then St. Louis for used test equipment because of the technical industries in the area. You might want to check out the MIT swap meet.Quote from: liquibyte on September 10, 2014, 04:23:58 amThat just looks like a soft start circuit. Power supplies generally lack soft start circuits except for their current limit and small output capacitor.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 11, 2014, 12:01:02 am
content: Quote from: David Hess on September 10, 2014, 03:28:06 pmWhat's kicking my butt about it is that the transformers are only 180VA and the caps are 15000uF. I've read that inrush is an issue with toroids or transformers around 500VA. I would think that the circuit could hold the voltage and current based on the pot settings.Quote from: David Hess on September 10, 2014, 03:28:06 pmI was impressed enough with the service manual on the 6236B that I printed it out and went looking on Ebay. I think I may be able to live with what I have until I get a better scope and then try and pick up a servicable (by me at least) design or two.Quote from: David Hess on September 10, 2014, 03:28:06 pmI've heard good and bad things about Rigol but I don't really know enough to make an informed decision. I think the least expensive I'd go for is the DS1074Z too, at least from what I've read. As far as Tek scopes go, from what I understand, you can get quite a bit of unobtainium down that road.Quote from: David Hess on September 10, 2014, 03:28:06 pmThe MIT swap meet is coming up on the 21st. I just talked with the wife and she's even going to let me go by myself. For what I have I don't mind too much. I can at least see something which is more than I've had in the past. Maybe I'll be able to find a few things there that would make life a lot easier. What kinds of things would you recommend I be on the lookout for scope and power supply wise? These are the two things on my wants list but I doubt I'll have more than $100 total. I won't count on it but I'm going to go and see what I can score.Quote from: David Hess on September 10, 2014, 03:28:06 pmIt was my understanding that this would allow the filter caps to charge at a slower rate and therefore take the spike out of the equation, or enough of it to make a huge difference before full power is applied to the power supplies. I'm also assuming that these are rather prevalent in audio amps of a certain power. I actually have two 10 ohm 100W aluminum resistors that I was going to use and then adjust the time before the circuit kicks the relay to right at the point where the spike disappears.

author: David Hess
author_rank: Super Contributor
timestamp: September 11, 2014, 01:02:24 am
content: Quote from: liquibyte on September 11, 2014, 12:01:02 amIt does not make a sense to me either.QuoteCustom parts are not usually a big problem because parts donors are common.QuoteThere are not many options for old Tektronix DSOs worth getting and I would not recommend any of them to a beginner unless they are known to work. Any old HP power supply should be repairable assuming there is service documentation available for it.QuoteThe problem should not be occurring whether there is a surge on the input or not.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 11, 2014, 02:36:56 am
content: I know it's one thing to help in a forum and another in real life but I could send you an unpopulated board or two if you'd like to have a look. I can populate it with the more common stuff but I figure that wouldn't matter much in the grand scheme of things. I'm lost with this not behaving properly and I'd like someone that knows what they're doing look at this on a scope. All of the parts are common stuff except for the output cap I used and maybe the 2141's. It's ok if you don't want to but I figured I'd ask. If I could figure this out, I think it would make a pretty good beginner project although it didn't work out for me as it is.

author: David Hess
author_rank: Super Contributor
timestamp: September 12, 2014, 10:29:22 pm
content: Before we consider going that far, I have some questions. Where is the input bridge rectifier and input capacitor C1 in that photo? What value did you use for C1? Did you calculate it?One thing which can cause a voltage surge across the input capacitor is if it is too small although I have never run across that problem myself.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 12, 2014, 11:12:13 pm
content: Quote from: David Hess on September 12, 2014, 10:29:22 pmI've got therectifierandfilter capoff board with thetransformer. I also went a little unusual with theBD139's being off board with the2N3055's. Links are the datasheets if you need to look at them. See the pic for the arrangement of everything. It's messy as far as the wiring goes but I've been slowly cleaning it up and organizing a bit better as I've gotten things where they're going to end up eventually. For instance, the BD139's are not upside down anymore though they are still on the beefy heatsinks you see in the image.C1 is 15,000uF because Digikey didn't have a 12,000uF in stock at the time so, if anything, it's larger than was originally calculated. I didn't calculate the value, I just used the values off the parts list for the schematic with the exception of the filter cap being larger, the other caps upped in voltage to 100V, the transformer being 4.6A instead of 4.3A, and the rectifier being 15A instead of 6-10A. The output cap C7 is a polyester film 10uF @ 63V because it was recommended to be able to adjust down to true zero.

author: David Hess
author_rank: Super Contributor
timestamp: September 13, 2014, 08:09:20 pm
content: The layout makes me wonder if leaking magnetic flux from the transformers is getting into the wiring going to the output transistors or something else. Leaking magnetic flux could go along with the surge at turn on across the input capacitor but at 12,000 or 15,000 microfarads, it should be large enough to prevent that problem.But this would not explain the surge problem reported from others with this design so if there is an issue, it is in addition to a problem with the circuit.Having long wiring runs to the pass transistors may cause problems. Low value resistors, like 4.7 to 47 ohms depending on the base current, in series with the bases at the transistor sockets could help. This could especially be a problem with the BD139 because they are pretty fast.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 13, 2014, 10:27:29 pm
content: Quote from: David Hess on September 13, 2014, 08:09:20 pmI don't think it could actually be either issue. I've got a separate board being powered from the second transformer with an on board rectifier, the filter caps and the pass transistor sitting right next to it about three feet away from the transformer and I'm getting the exact same results. I think I need to start probing around the circuit to see if I can find anything odd going on, I'm just not sure what to be looking for. Like I said, the wife's letting me go to that MIT thing so maybe I'll come back with a better scope.

author: David Hess
author_rank: Super Contributor
timestamp: September 14, 2014, 03:03:59 pm
content: Could the measurement of the surge voltage across the input capacitor and the output at turn-on be in error? An input surge that large does not make any sense unless the input capacitance is low.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 14, 2014, 04:43:23 pm
content: Quote from: David Hess on September 14, 2014, 03:03:59 pmAt the cap bank, the stabilized output is 43.7 volts and at the power supply output 30 volts (calibrated). If I allow the caps to drain to a low value, say <5 volts, at turn on I get a consistent 61.2 volts surge at the caps and 37 volts at output (output drained to under 200 millivolts). So we have a 17.5 volt spike at the caps and a 7 volt spike at the output. I also tested the output of the transformers independently with four 1N4007 diodes in a full bridge configuration and two 2200uF caps in parallel and I get a 63 volt spike, so this phenomenon is evident outside the actual power supply circuit. The caps must be drained to see this however.I can only see this briefly on the meter but the scope is more evident in its display. I've also measured the AC right at the connections where it comes into the board and I see no surge of any kind with either the meter or the scope which to be honest may not be fast enough to detect anything. What's happening is doing so at and after the filter cap(s) and I don't have the best equipment to be testing this stuff but they do agree as to the results so I don't think what is happening is an error. I don't have a second meter to test with and I obviously don't have another scope or even a second channel so until I get something better I'm stuck. Everything I've read says that with a 130VA transformer this shouldn't be an issue, or be negligible at best. I guess I'm just special.

author: sunnyhighway
author_rank: Frequent Contributor
timestamp: September 15, 2014, 03:23:49 pm
content: When I see a problem I tend to break the problem down in the smallest things possible and in the process allow for oversimplifying.Then I look for the simplest solution. Procedural solutions are allowed too. Then look for simplest resolution for the disadvantage(s) of the simplest solution.Problem: When turned on, a higher voltage than the set voltage is briefly outputted to the terminals, possibly killing or damaging the connected circuit.Possible solutions:#1: Start the PSU always with 0V at the output terminals. Wait. Slowly increment the output voltage.#2: Start the PSU always with the output terminals disconnected. Wait. Connect the circuit to the output terminals.Both solutions wil work as a procedural solution. But the disadvantage is that procedures tend to be forgotten or skipped.Final solution:Implement one (or both) of the procedural solutions using electronics or mechanics in such way it cannot be forgotten.The mechanical solution for automating procedure #2 could for example be a switch which (when off) covers one or both of the output terminals.I'll leave other possible solutions as an exercise for the reader.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 21, 2014, 04:30:52 pm
content: Quote from: David Hess on September 10, 2014, 03:28:06 pmI want to thank you for pointing out the MIT thing. I scored a perfectly working 465B with probes and manual for $65. I may never fix the issues with this design but I think I'll be better equipped to see what's going on.

author: David Hess
author_rank: Super Contributor
timestamp: September 22, 2014, 11:01:49 am
content: The Tektronix 465B and that whole series are really nice oscilloscopes. The somewhat related 464, 466, and 468 have storage in one form or another which would help in tracking down a voltage surge problem but the 465B is more reliable and certainly better than what you were using previously.

author: paul18fr
author_rank: Contributor
timestamp: September 22, 2014, 12:04:47 pm
content: HiMy following remark may not be relevant, but can we imagine than the initial spikes come from the transformer ?Would a toroidal transformer (instead of a classical one) fix the problem ? (I'm thinking about it after some readings)Paul

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 22, 2014, 03:19:22 pm
content: Quote from: David Hess on September 22, 2014, 11:01:49 amThere was a 7000 series there with a full set of plugins but I didn't have the $300-400 to negotiate on price with (looked very much likethis). There were also others and in each case I asked about price and the $300-400 range seemed to be ubiquitous. I was limited to about $160 and think I really lucked out considering. The guy that was selling it had it on the ground kind of hidden away from his table but I knew what kind of things I was after and spotted it almost immediately. I asked him if it worked and he said it shows a nice bright trace. I asked him how much and he said $65 and I saidsold. I knew if it showed a trace but was uber cheap I might have to fix something so I jumped at it. I got it home and hooked it up and there was absolutely nothing wrong with it. As a matter of fact, it's already come in handy due to the dual channels and better features than what I had. The power supply that I had boards made for is smoother than the one with the voltage regulator which is showing a distinct oscillation at the output. Either way, I feel like a kid a Christmas.There are the usual scuff marks from use but no dents. All of the knobs are there and unbent or cracked or broken or missing pieces and act like they're new. The feet on the back where the cord wraps around are all broken but that's literally the only fault I can find. I tried a cursory search and couldn't find any for sale using the Tek part number so I'll have to keep my eyes open for replacements. I'll be saving my lunch money over time and probably get a 1074Z eventually so I have something that can do storage.Quote from: paul18fr on September 22, 2014, 12:04:47 pmIf anything, I would think a toroidal transformer might make things a tad worse due to the nature of the way they work.Inrushis happening with the filter caps charging and is always going to be an issue with this supply but there are things we can do about it. The concern that I have is that the power supply circuit itself seems to lose all control over the voltage at these conditions and the designer(s) probably haven't addressed these situations in the design from the start. I don't know enough yet to say one way or the other what the eventual solution will be but I am working on it. Money is not something I have a lot of so it may take more time than if someone that does this for a living was working on it. I think that in that case though the design would have been scrapped in favor of something else. Have no fear though, I'm fully vested and refuse to give up and I feel that if a solution exists I'll figure it out.

author: David Hess
author_rank: Super Contributor
timestamp: September 22, 2014, 06:04:27 pm
content: Quote from: liquibyte on September 22, 2014, 03:19:22 pmThe 7000 series are generally older and more difficult to maintain so it is probably just as well. The only significant advantage they would provide over a 465B is 4 channel support and the ability to use specialized vertical inputs.It would not be my first choice but the 7854 like you linked can operate as a digital storage oscilloscope but not in the way most people would expect because it is not real time capable. With the right timebase plugin, a 7B87, it can capture single shot events and has pretrigger capability which is exactly what you need to look at what is going on to cause the surge voltage when power is applied. It can only capture one channel at a time though which would be a significant disadvantage in this case.QuoteWater with a little bit if dish soap and a rag is a good surface cleaner. The front panel controls are easy enough to remove so you can get under them.The feet are a known issue with these oscilloscopes. I know at least one person has used a 3D printer to produce very workable replacements and was selling them as sets.Quote from: paul18fr on September 22, 2014, 12:04:47 pmThe voltage surge at the input is one problem but that by itself should not cause the regulator to behave the way it is so there are at least two problems.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 22, 2014, 09:11:37 pm
content: Quote from: David Hess on September 22, 2014, 06:04:27 pmI'm very happy to have stumbled across what I did, the quality of the signal is so much better. I have no doubt that even if I had a DSO, I'd probably be using this for day to day use. It came with two 10X probes but I also have a set I got on Ebay that have the 1X - 10X switch. I've compensated both sets of probes so I now know how to do that. All I can say is what a marvellous invention.I've been doing a bit of reading and I think I may be able to use Ch.1 out to capture the event on my computer. I'll need to get a connector and cable made up for it and it's not ideal but is better than nothing.Quote from: David Hess on September 22, 2014, 06:04:27 pmThe case was a bit dirty but I did clean it with 401 countertop cleaner and it brightened up nicely. I'm going to hold off on the front panel because, to be honest, it's fairly clean. I'll wait to do that part until I get a chance to open it up and assess the innards.Quote from: David Hess on September 22, 2014, 06:04:27 pmThere was an idea by redwire over at the other forum that the circuit in the attached image might help. I don't have the TIP141 to test this out with and I have to wait to place another order at Digikey until the 3rd of next month. I've got a relay sitting there too, just in case.

author: David Hess
author_rank: Super Contributor
timestamp: September 22, 2014, 11:07:54 pm
content: Quote from: liquibyte on September 22, 2014, 09:11:37 pmBrightness and clarity of the trace is often under appreciated. Higher bandwidth analog oscilloscopes generally have higher acceleration voltages making them brighter and clearer even on low bandwidth signals. The extra brightness is also convenient when looking at low repetition rate or single shot signals.On a DSO this is irreverent except insofar as a CRT with high acceleration voltage allows for a higher resolution display which that 7854 takes advantage of by displaying a full 10 bits horizontally and vertically. It has a 10 bit digitizer as well.QuoteHere is a video of Jim Williams talking about his Minuteman guidance computer and repairing a 465B owned by Tom Osborne of HP calculator fame:When I went DSO hunting a couple years ago and before the Z series were available from Rigol, I decided to hunt down a Tektronix 2230 which is newer than the 465B by at least 3 years. It is a combination analog and digital storage oscilloscope and the descendant of the Tektronix 468. Its major advantage over the non-Z Rigols other than cost is that it supports real peak detection which largely alleviates aliasing concerns at slow sweep speeds which was a major consideration.QuoteThat could work and is pretty close to what the external vertical output was intended for.QuoteI have heard reports before of 401 cleaner removing some types of labeling from instruments. Of course soap and water will remove decals if not used carefully.QuoteThat could work but it should not be needed if the supply voltage to the operational amplifiers is regulated.There are a couple of possibilities but I think we narrowed it down to just one:1. The supply voltage to the amplifiers was exceeding their absolute maximum during the surge. We fixed this with a simple zener shunt regulator which should have been present anyway.2. The input voltage to the pass elements might be high enough to couple through via breakdown. I do not think this is happening because BD139 is an 80 volt transistor and the pass transistors are 2N3055s. Maybe the 2N3055s are defective or counterfeit.3. The output U2 showed the same glitch. That needs to be tracked down by looking at if the same glitch is showing up earlier in the signal chain at the non-inverting input of U2. If it is *not* there, then maybe the output is being dragged up which can be tested by disconnecting it from Q2 and shorting it to the other end of R12 which removes the pass transistors from the circuit.