Fixing old 723 variable power supply.  Should I replace the guts? - Page 1
Forum section: electronics / projects

author: David Hess
author_rank: Super Contributor
timestamp: March 16, 2025, 01:14:58 am
content: I have an old 723 based variable power supply that I built when I was about 13. Finding myself in need of another bench supply, I dug it up and fixed the enclosure. I may change the big input capacitors, but what about designing and building one of my super precision regulators, adapted to variable operation, to replace the insides, with noise, load, and line regulation under a microvolt?The transformer has 1 amp 14.5 volt center tap and 0.5 amp 43.5 volt center tap outputs.The printed circuit board came from the long gone R&D Electronics on Orangethorpe avenue in Fullerton, California, not far from Orvac Electronics.

author: KE5FX
author_rank: Super Contributor
timestamp: March 16, 2025, 01:38:42 am
content: Heh, reminds me of the 'classic' I just recapped:TwoLM723s andtwo2N3055s in that one.In other news, through-hole electrolytics cost more than eggs these days.

author: David Hess
author_rank: Super Contributor
timestamp: March 16, 2025, 02:03:23 am
content: Quote from: KE5FX on March 16, 2025, 01:38:42 amAnd axial ones are more expensive than that.The two 1100 microfarad 50 volt capacitors were trashed with 25 ohms of series resistance. I replaced each one with a pair of NOS (New Old Stock) 2200 microfarad 35 volt capacitors in series which are "only" 30 years old.

author: TomKatt
author_rank: Frequent Contributor
timestamp: March 16, 2025, 02:08:48 am
content: Quote from: David Hess on March 16, 2025, 02:03:23 amA few years ago I was working on my HP 180 analog scope that was built in the late 1960's... I figured that while I was in there I should probably replace the main power supply caps, which were big screw top 'computer grade' cans made by Sprague. I was shocked when they tested good and showed virtually no ripple. So I left them in the scope, which continues working well to this day. Just as well, because even a few years ago they were listed around $20 each and there are 6 in the scope - that's more than I paid for the scope to begin with lol.I've heard stories about some old caps built to last - but didn't really expect to run across any that did.

author: floobydust
author_rank: Super Contributor
timestamp: March 16, 2025, 02:20:08 am
content: Everyone needs a linear PSU. My favorite was a LM317 one I made when I was 14.1.5A is enough to not make too much smoke.I recently did a full redesign of it using discrete op-amps and it ended up being a lot of work. Still need to add UVLO so the output does not spike on power up/down. I wanted adjustable current-limiting.I would say it has not been worth it, aside from the fun designing and building it. I just don't need that much performance when powering things.I also have a few other linear PSU's, Topward I just bought and recapped and it's meh ok.That '723 board pretty vintage but it's missing a couple protection diodes so I'm surprised the 1967 2N3055 is alive.

author: TomKatt
author_rank: Frequent Contributor
timestamp: March 16, 2025, 02:37:05 am
content: Quote from: floobydust on March 16, 2025, 02:20:08 amBack in the day it seems like virtuallyeveryelectronics hobbyist built a linear power supply, decade box and things of that nature - they were things you needed and building them provided valuable experience. Now it seems like everyone starts with module boards like Lego blocks... Seems like things are generally more digital these days. I don't think that's good or bad, just different.I built a power supply similar to the OP, but my Siglent SPD1305X is like Buck Rogers in comparison lol. Though I still use the decade box I built in the 70's - it's hard to find a decent one today at an affordable price.

author: David Hess
author_rank: Super Contributor
timestamp: March 16, 2025, 02:40:57 am
content: Quote from: TomKatt on March 16, 2025, 02:08:48 amBig can capacitors have so much excess electrolyte that they rarely dry out, but what does happen is when the seal becomes leaky, oxygen gets pumped in with pressure changes and corrodes the aluminum strip which connects to the leads. My PS503 capacitors, shown below, failed this way. The replacement capacitors were selected based on diameter so that they fit in the original mounting clamps.Quote from: floobydust on March 16, 2025, 02:20:08 amI have a pair of Tektronix PS503A power supplies, but it would be handy to have another simple supply.I am leaning toward just designing and and building another super precision power supply with its own enclosure. There is no need to sacrifice an old working supply. I might add a simple variable 317/337 dual tracking supply to this one with the extra transformer winding.QuoteThat is not the original 2N3055. I replaced it at least once.

author: floobydust
author_rank: Super Contributor
timestamp: March 16, 2025, 02:49:19 am
content: If I'm doing analog circuit design, then I need quiet power. How many people even notice the difference between 5mV or 200mV ripple?People conflate a PSU with fancy display/keypad with high performance. Modern PSU's are frequently crappy.Have you put the Siglent in CC mode at 10mA with say 25VDC out and shorted the output? That's when you find the big fat 470uF output cap...Charge a gel-cell with it and see what happens if you switch it off/mains outage occurs. Or if the setpoint is low a few volts and you connect it to a 12V battery.Homebrew linear PSU's done right can take the abuse and shit. A faithful horse- dependable, repairable. Not something you have no schematic for. With a hurricane cooling fan lol.

author: CaptDon
author_rank: Super Contributor
timestamp: March 16, 2025, 03:08:16 am
content: I find the 723 series to be easily damaged and often short lived especially in those 13.8vdc 20 amp ham radio supplies. I have experienced brand new old stock that simply would not work at all in more modern power supplies. I guess there was an increase in current and possibly voltage capacity between 723 and 723A and so forth. So many prefixes also LM, uA, CA, SA and more. We had probably 100 from mixed manufacturers all at least 20 years old. I gave about 20 of them 'a ride home' to fix many makes and models of friends 'dead' power supplies. In almost every case the 723 was kaPut and the ones I put in generally died within a year. O.E.M. supplies probably pushed them to their limits. Many blew out after short interval mains 'power flickers'. I had better luck with the Philips ECG and NuTone NTE replacements. They seemed a bit more robust than the OEM parts.

author: floobydust
author_rank: Super Contributor
timestamp: March 16, 2025, 03:35:48 am
content: Many 723 power supply designs are minimalist and have weaknesses.When you look at the circuits, they are relying on a bit of luck.Example: missing resistor to limit 723 output current for base-drive to the pass transistor(s).So the IC gets damaged when a PSU is under heavy load etc. OP's seems to have no driver transistor(s)... A naked Darlington is not so tough.Another missing resistor for the current-limit transistor base. I could go on but the subtleties need to be looked after for a really good PSU.I had one project that could not work with 20mV ripple (at load) and when I dug in, it was just poor star-grounding moved a few nodes and got it down to 2mV.

author: KE5FX
author_rank: Super Contributor
timestamp: March 16, 2025, 03:45:39 am
content: These are all true, and they are among the reasons why I'm rather attached to this old Radio Shack job. They did their homework:Nothing flashy, but nothing is missing that needs to be there. It's one of the supplies I trust to run expensive OCXOs for months at a time.

author: coppercone2
author_rank: Super Contributor
timestamp: March 16, 2025, 04:02:31 am
content: I think it should be replaced unless you fix that ugly burned PCB up

author: ME
author_rank: Frequent Contributor
timestamp: March 16, 2025, 04:10:49 am
content: Quote from: KE5FX on March 16, 2025, 03:45:39 amCan you upload the full schematic diag ? , Looks like a good project.

author: floobydust
author_rank: Super Contributor
timestamp: March 16, 2025, 04:51:58 am
content: Micronta 22-121, I couldn't see it around 1980'sRadio Shack catalogs.It's way under-rated 0-15V 1A x 2 channels.https://elektrotanya.com/micronta_no.22-121_dual-dc-power-supply_sm.pdf/download.htmlI have the Project Kit dual PSU dumpster rescue uses a few 741's.

author: KE5FX
author_rank: Super Contributor
timestamp: March 16, 2025, 04:55:04 am
content: Quote from: ME on March 16, 2025, 04:10:49 amI snagged the service manualhere. I'll upload it to ko4bb.com in case that site stops working.

author: David Hess
author_rank: Super Contributor
timestamp: March 16, 2025, 06:14:12 am
content: Quote from: floobydust on March 16, 2025, 03:35:48 amI checked the board and there is no current limiting except for the 723's Vbe current limiting.Quote from: floobydust on March 16, 2025, 02:49:19 amI just noticed that there is no output capacitance on this design. Somehow it does not oscillate.

author: David Hess
author_rank: Super Contributor
timestamp: March 16, 2025, 10:26:54 am
content: The design is based on the positive voltage regulator with NPN pass transistor in the 723 datasheet, but with the reference divided down to 2.5 volts to lower the minimum output voltage.

author: DimitriP
author_rank: Super Contributor
timestamp: March 16, 2025, 10:33:05 am
content: Quote from: David Hess on March 16, 2025, 01:14:58 amI miss the old neighborhood:)Which "super precision regulator" did you have in mind ?I figured if it was good enough to run the HP10811 ..it's good 'nough for me

author: RoGeorge
author_rank: Super Contributor
timestamp: March 16, 2025, 11:21:05 am
content: Nice power supply!It is not clear from the pics if the entire enclosure is made of aluminium, if so, might worth changing the 723 with a better design.Though, I like 723 power supplies, too, particularly because they were usually put in boxes with passive cooling, so they were quiet. Same for the first HCs (Home Computers) from about the same time with 723 ICs. The computers were quiet, running hot but quietly.Annoying fans started (for me at least) with the apparition of the IBM-PC and with the invasion of desktops at home. That was somewhere around the 90's I think, and since then, I've never manage to get rid of all the fans and their noise. All the desktops and laptops and lab instruments nowadays are more-or-less noisy.

author: David Hess
author_rank: Super Contributor
timestamp: March 16, 2025, 11:33:05 am
content: Quote from: DimitriP on March 16, 2025, 10:33:05 amSomething like that shown below, but with a separate bias source for the reference and a negative bias supply to allow operation down to zero. The reference is divided by a poteniometer to make it variable so the feedback network impedance is fixed making frequency compensation better. Using a fixed regulator for the power pass element adds free thermal and short circuit protection, but I would likely add adjustable current limiting with a second operational amplifier.I have oodles of TO-3 fixed and variable regulators, as well as big power MOSFETs, bipolar, and Darlington transistors available for any power pass element configuration, but the transformer can only supply an amp at most unless I dig up another transformer. Using a fixed 7805 or 7808 has some advantage because it lowers the output range requirement of the operational amplifier.I could also design a fast response low output capacitance power supply using TO-220 ring emitter transistors.

author: David Hess
author_rank: Super Contributor
timestamp: March 16, 2025, 11:53:53 am
content: Quote from: RoGeorge on March 16, 2025, 11:21:05 amOnly the front panel is aluminum. The top and bottom are thin plywood, and the sides are particle board which is where the front panel screws pulled out and needed repairs. Keep in mind that my father and I made this when I was like 13.So far I have repaired the enclosure and replaced the big input capacitors. I will add a small output capacitor, but it works as intended now.QuoteLinear power supplies become impractical at high power. I remember CP/M systems with linear supplies where the power supply was a separate cabinet and required two people to move safely. I think all of them relied on the 723.QuoteWhere forced air cooling is required, I like to build a small linear fan control circuit to adjust speed based on temperature. Of course me being me, I use only the integrator part of a PID controller and tune the step response, so the fan varies speed while the temperature remains constant until overload.

author: floobydust
author_rank: Super Contributor
timestamp: March 16, 2025, 05:58:21 pm
content: Do you know why the resistors burned up? It could maybe be tap-switched.When I made my first bench PSU at around age 13 the shop teacher freaked out because I was on the drill press for quite a while, surrounded by electronic parts. It was a class where you made a sheet metal funnel or changed a bike tire lol and it was as if I was from another planet. I didn't have a dad or a drill at home. It worked out OK, mostly dumpster dive and Radio Shack parts, an LM317K and oil-can selenium rectifier from the army surplus store. It powered hundreds of breadboard projects for many years.You'll have to be clear on what features you want. Last iteration I started out by wanting adjustable current-limiting which sent me down a rabbit hole more complicated than I wanted. OP-07, LM4040, 2SC5200 kind of thing with an added transformer to make +/-12V for the IC's.Magnetics cost a small fortune nowadays. I must have a few $grand worth stockpiled over the years.After the pandemic, my rule is "use what you've got" instead of buying new parts.

author: David Hess
author_rank: Super Contributor
timestamp: March 16, 2025, 08:26:57 pm
content: Quote from: floobydust on March 16, 2025, 05:58:21 pmTwo resistors burned up at different times.The obvious one is in series with the voltage control potentiometer to limit the voltage range. My guess is that it failed when the output transistor shorted and the potentiometer was turned one way.The other burnt resistor is part of the Vbe current limit and just overheated.QuoteI was looking for suggestions about this particular power supply, but you are right; I should consider what would be most useful.If I was building a supply from scratch, I would want a low noise precision voltage and current tracking supply with low output capacitance and high isolation, but this is beyond the scope of this project. Achieving high isolation and low noise might require a high frequency sine inverter, and this is a future research project for me to test feasibility.QuoteI have quite a collection of big and small power transformers, including a pair of plate transformers which are too heavy to easily move.

author: coppercone2
author_rank: Super Contributor
timestamp: March 16, 2025, 08:37:00 pm
content: the low Cout is actually a good reason to keep the supply and just fix the burned thing that's irritating mescratch or nibble it out and coat with epoxy or sandwich in teflon and use epoxy with glass beads or fiber to patch itadding anything to the supply that effect its Cout is a detriment to its capability , build another supply for other capabilitiesfor real hacking that c is a big problemit could be improved though by putting the power resistors on big 'turret' standoffs, fixing the wires to PCB issue, leaving the SLCC capacitor alone, repasting the transistor with a modern thermal grease, checking to see if that through hole can can use a heat sink, washing the pcb, adding fuses to the chassis if it does not have any, replacing the banana holes with binding posts, adding a chassis binding post if its compatible with the design (along with a shorting jumper), etching a scale for the potentiometer and adding a bright mark to show the position (unless its a multi turn, then it could benefit from a vernier), putting a better panel meter and cleaning the chassis

author: KE5FX
author_rank: Super Contributor
timestamp: March 16, 2025, 09:02:27 pm
content: At which point he will have the Power Supply of Theseus.

author: coppercone2
author_rank: Super Contributor
timestamp: March 16, 2025, 09:03:30 pm
content: it is capable of slaying capacitance related problems

author: coppercone2
author_rank: Super Contributor
timestamp: March 17, 2025, 02:14:33 am
content: Another feature can be a output isolator switch so you can plug things in and adjust it before turning it on (possibly with a lowish value power resistor to drain the circuit it will connect to)sometimes when the power supply is too basic it feels like its a ghetto ass hydraulic system where it starts spilling fluid on you when you disconnect it (without like a check valve or something). Its nice to have a switch for thisA really good supply IMO would come with a variable load, so you can power it up on a load to get it warmed up the correct amount and then transfer it over to the circuit you want to run (if its precision work on stable devices), so an hour after you want to plug something in a green light comes up (like a txco)

author: David Hess
author_rank: Super Contributor
timestamp: March 17, 2025, 03:09:41 am
content: Quote from: coppercone2 on March 17, 2025, 02:14:33 amThe output switch on the Tektronix PS503A power supply shorts out the capacitor in an RC filter after the reference. So when enabled, the output ramps up and when disabled, the output is instantly removed and pulled down with a few milliamps.Some power supplies have a class-b or class-ab output stage so will actively discharge their output capacitance. Others may include a low current pull-down. The PS503A does the later, but with an added twist that the operational amplifier used for the error amplifier has an external offset applied which is always greater than its possible input offset voltage. This means that the voltage control loop will actually control the output to zero volts, and a couple millivolts negative. And no trimming of the operational amplifier is needed to allow it to regulate down to zero volts. I have seen some old designs where this was done by deliberately pulling the offset null adjustment one way to induce an offset, but in some operational amplifiers this can ruin input offset voltage drift.QuoteThe voltage reference dominates any warm up errors, and should be settling within milliseconds. After that the error amplifier is the next largest error source. Unless you are expecting microvolt stability, I would not worry about warming up unless you have an ovenized reference. Some old designs had an ovenized error amplifier to reduce input offset voltage drift.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 17, 2025, 03:48:38 am
content: Quote from: David Hess on March 16, 2025, 01:14:58 amWith just an analog meter, and a standard pot, I presume you jest there.The first upgrade I would do to any bench supply is to add decent V.I displays.Aliexpress these days has 5 digits ones like this V.A.Whttps://www.aliexpress.com/item/1005001830298410.htmlor this one for Ahttps://www.aliexpress.com/item/1005005855666549.htmlClaimsMeasurement accuracy: 0.01%（+/-2）*2Refresh rate: 3 times/secondDisplay mode:5 bit 0.36 inch LEDMaybe it uses NAU7802 or HX711 ADC or similar?Adjustable current limit, and Adj V to 0V, are also useful on bench supplies.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 17, 2025, 11:07:40 am
content: any 723 based psu's can be improved. mostly on the current limiting sectionthe "comp" capacitor is supposed to prevent any oscillations from occurring

author: RoGeorge
author_rank: Super Contributor
timestamp: March 17, 2025, 12:02:02 pm
content: Quote from: David Hess on March 16, 2025, 02:03:23 amIf you still have those 1100uF, it would be interesting to re-form them by progressively applying up to 60...70Vdc for a few days, or at least 50V. Then measure C and ESR again, to compare that with the values measured before forming, just out of curiosity (not as an attempt to keep using them).I've fooled around last year with some random electrolytics fabricated during 70's - 90's, and most of them were able to regrow the oxide layer after keeping them for a few days at 1-1.2 Vnominal. At some point, there were horrible spikes in the plotted current before reforming, then after only a few hours of oxide layer regrowing, the leakage dropped to 1uA or so, and not much noise:https://www.eevblog.com/forum/projects/reforming-long-unused-electrolytic-capacitors/msg5680727/#msg5680727I wonder if other brands of many decades old electrolytics can be reformed with similar success.

author: PGPG
author_rank: Frequent Contributor
timestamp: March 17, 2025, 02:12:08 pm
content: Quote from: David Hess on March 16, 2025, 01:14:58 amBeing about 13 I have started to do my power supply (in a plywood casing), but I had no access to any ICs so it was whole made by discrete elements.I had no regular radiator but had an aluminum block 10x10x1cm so 2N3055 is mounted to it.Knowing that it is not good in heat dissipation my concern was to allow for dissipate as small heat as possible (output current limit was regulated in a range 0...4A).Because of this I wounded at 100W trafo coils giving me 6+6+6+6+6+1+1+1+1+1V and using bana plugs I can select any AC voltage from 1V to 35V used for next supply stages.Few years later I have added separate small transformer to get negative voltage and replaced the regulating circuit with LM723.At home I don't need anything more from very rarely used supply

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 17, 2025, 02:58:25 pm
content: It’s safe to say that a significant amount of electronic hobbyists built their own linear PSU with a 723 and a 2N3055, I certainly did.If you rebuild the unit, in addition to the output enable switch mentioned earlier, a bright “OUTPUT ON” Led.But most importantly, replace the potentiometer with a multiturn type, if you can find one at a reasonable price. Otherwise use two pots in a “COARSE/FINE” arrangement.A further refinement, if you have a transformer with a dual winding secondary, to include a “HIGH/LOW” switch, to reduce the power dissipation while outputting low voltages.

author: David Hess
author_rank: Super Contributor
timestamp: March 17, 2025, 06:19:26 pm
content: Quote from: schmitt trigger on March 17, 2025, 02:58:25 pmThis one was a local kit as I described, with just a printed circuit board and the parts.After a few 7805/7815/7915 fixed voltage power supplies using the same transformer type, and when I knew enough electronics to be dangerous, I designed and built a reference and operational amplifier power supply with adjustable current limiting and 10 turn poteniometers. It worked, but was a disaster because of the needed frequency compensation to deal with the cascaded operational difference amplifier used for current sensing. After that, I went back to boosted current 317/337 designs for a while.

author: floobydust
author_rank: Super Contributor
timestamp: March 17, 2025, 08:03:11 pm
content: There's big jumps in what the single chip regs can do.For all the three-terminal regs the regulation and drift is not that great. A few % maybe. I think designing for really tight regulation is not practical unless you go 4-wire.The LM317 has no adjustable current-limiting and the 1-1.5A output limit can be a bit less than you need.Went to the LM350K and it's too high though at 3-4.5ACurrent-limit is not great on the 723 I think it's more intended for short-circuit protection.I need to go below 1.2V for single-battery circuits.I need 13.8V for charging batteries so the 7815 was out.This rules out most of the simple circuits out there. I think it leads to using several op-amps?The ubiquitous chinese kit 0-30V 2mA-3A (copy from 1978 Practical Electronics) does not perform well but is really in the zone of what is commonly needed, I think. If the PCB layout was fixed, ratings made conservative and a few other quirks fixed, I think it would be popular.They also have a Hiland LM317 one (pic) with added current-limit (hey great idea) but it's kinda hilarious. I was going to draw the schematic as it sets a new world record for the number of 1N4007 diodes (12) used lol.Both of these need that -ve rail which adds complexity.

author: David Hess
author_rank: Super Contributor
timestamp: March 17, 2025, 09:35:01 pm
content: Good load regulation with 3-terminal regulators requires careful layout. The top of the output divider should be attached as close as possible to the package, otherwise the error from the voltage drop between the package and top of the divider gets multiplied. Of course the load regulation will never be as good as a regulator with remote sense, but there is no need to make it worse with poor layout.With a current boosted design, a significant or majority of the output current may be diverted around the regulator, improving load regulation.Remote sense can be added with a single operational amplifier, improving load regulation considerably.I know a clever mind bending circuit for allowing a 3-terminal regulator to work down to zero volts without an additional reference, shown below. The operational amplifier operating as a different amplifier samples the internal reference and uses that to generate a negative reference for the adjustment divider. Of course it still requires a negative bias supply. I could do the same thing for my 723 power supply, and have a transformer winding to generate the negative bias supply, however I question whether the effort is worth it because the same operational amplifier plus a reference would make a better regulator whether the 723 was included or not.

author: David Hess
author_rank: Super Contributor
timestamp: March 17, 2025, 09:43:16 pm
content: Quote from: coromonadalix on March 17, 2025, 11:07:40 amI know from previous experience with this power supply that without any output capacitance, it will oscillate with some unbypassed loads. I think I will add a small output capacitor to improve the stability.One thing I do not like about these designs is that the output divider being within the feedback loop alters the frequency response, so the frequency compensation becomes a compromise. The Tektronix PS503A works this way also, but it was deliberate to provide remote operation, so at least its design has an excuse.Quote from: PCB.Wiz on March 17, 2025, 03:48:38 amI do not disagree with what you suggest, but it would be outside the scope of this project because I would instead just design and build another power supply from scratch. I will likely to that eventually, but not at the cost of only reusing an enclosure that I can replace in a couple hours.One of the things I would like to investigate is improving common mode isolation and noise. Modern power supplies seem to suffer even more from this problem, and many of my old designs did as well.

author: David Hess
author_rank: Super Contributor
timestamp: March 17, 2025, 10:26:51 pm
content: I put it all back together, measured the ripple, and non-unexpected found the problem shown below, which I believe is snap recovery in the rectifiers. So now I have another problem to fix.I have found this problem in lots of power supplies, including my own, and it is why I have a keen interest in a design with better isolation.

author: coppercone2
author_rank: Super Contributor
timestamp: March 17, 2025, 11:03:40 pm
content: that is why they put those ring of capacitors around the rectifier I think? Hp always has a few capacitors, not always four, sometimes 3 (muntzing?). I am almost certain once I thought I had to replace four but then there was only three.Something like 1 to 10nF.Sometimes they have a cap across the transformer (ceramic) that likes to make acoustic noise, sometimes they also have caps across the diodes. Their old RF things IIRC have 4 caps right around the rectifier, but on things like power supply I have seen that arrangement but the caps are like a inch away on the rectifier/capacitor PCB. I wondered why this was, because they could have put the capacitor right there, but instead it was farther away. I wonder if it is using the inductance of the trace or something. And I think it had a separate trace back to the transformer pin (like a star connection) rather then looping back across just the diode...? I mean it felt like they had to go out of their way to make it like that instead of just putting it across the diode. I need to try to remember which unit that is. Could it have been the HP current source 6177.. possibly the A version which got dispatched for being a nuisance(the remains are in a 'canopic jar'. )Maybe just start with 4x 1nF 500V SLCC caps around the rectifier and see if it goes away ?Those tiny ceramic capacitor on the rectifier/capacitor board are the ones that sing, they always give me a problem, making me replace them with foils. But do they do their job, I am not sure, because the SLCC capacitor "bandwidth' is hugealso it could be related to shortwave RFhttp://www.ll-leasing.com/pritch/shortwav.htm

author: KE5FX
author_rank: Super Contributor
timestamp: March 17, 2025, 11:23:29 pm
content: Another effect worth considering is described on page 410 of AoE X Chapters, in which the transformer's leakage inductance causes a voltage spike to appear when the diodes turn off.That was a revelation when I came across it, because I've often used the Signal Transformer split-bobbin parts that they talk about in various homebrew projects. I had seen the phenomenon happen (and been annoyed by it) several times without understanding what was going on.

author: coppercone2
author_rank: Super Contributor
timestamp: March 17, 2025, 11:33:36 pm
content: I wish I had a time machine so I can go back 60 years and plug the same box into the outlet to see if its the outlet or if they just did not care about the singing noisefyi related to this, putting random IEC filters does not really help eliminate the noise. The only thing that did it is replacing the caps with foil. I think the foil cap is just heavy and it dampens itself.

author: coppercone2
author_rank: Super Contributor
timestamp: March 17, 2025, 11:35:53 pm
content: Is the solution for the leakage inductance to put a capacitor across the transformer?So then you need a cap across the transformer and caps across the diodes to deal with both problems? Though I am thinking if you use 4 caps across a bridge, their basically across the supply (but with reduced capacitance because their series capacitors)... then you still might need a 5th capacitor if it needs a larger value then the one created by the caps across the diodes?I see some creative solutions to transformer problems, this guy is thinking to use dampened capacitorshttps://www.diyaudio.com/community/threads/rc-snubbers-for-diode-recovery-noise.190638/page-2it seems that it would be fun to try to find a bad transformer and see if these solutions could have a measurable effect on a scope.. I only saw that kind of arrangement specified for EMI filters for 'military specification' DC/DC converter solutions (R+C dampener)... they looked more complicated then the ones you commonly see for inlet filters.

author: KE5FX
author_rank: Super Contributor
timestamp: March 17, 2025, 11:44:55 pm
content: Exactly, they recommend a snubber cap in the 1 uF range directly across the secondary, with a few ohms in series to cut down on ringing.

author: coppercone2
author_rank: Super Contributor
timestamp: March 17, 2025, 11:48:00 pm
content: The 6227B has a cap across the transformer (with no R) and then cap's across the diodes, I think that is where I saw this.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 18, 2025, 02:40:35 am
content: Quote from: David Hess on March 17, 2025, 10:26:51 pmThat looks a bit long for recovery times?What load was that, and can you include the pre-regulator ripple waveform ?

author: floobydust
author_rank: Super Contributor
timestamp: March 18, 2025, 04:03:59 am
content: There's 4 diodes (bridge rectifier) on the top of the PCB and then more on the bottom side like a center-tapped rectifier so I'm confused why two?I use an RC snubber, or just a cap across a transformer's secondary to lower leakage inductance ringing (actually it just lowers the frequency), especially if slow recovery diodes are there causing the problem.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 18, 2025, 06:58:49 am
content: The shown waveform has too long drops to be the diode recovery. Diode recovery times for 1N400x is at some 3 µs. A capacitor of some 1 µF at the transformer secondary could help a little against voltage spikes and as a side effect it would take over part of the magnetizing current. So the AC loading is more a beneficial and not an issue.The ripple shown could be the voltage doping too far. Another possibility would be a poor layout so that the charging current spike couples to the output. One chould check this, by looking at the ripple at the filter capacitors too. The charging current effect comes a little later.

author: David Hess
author_rank: Super Contributor
timestamp: March 18, 2025, 09:44:28 pm
content: So it was not the reverse recovery, but there is a problem. Shown below are the ripple at the output and the input. The output ripple occurs during the time time that the input capacitors are charging, and not after reverse recovery. I am going to check the layout for return current from charging the input capacitors getting into the control loop.I added 20 milliamps of output load at 10 volts and it had no effect. The headroom is like 15 volts.Note that the power supply has a high output noise level not shown here. I think I need to filter the reference.

author: floobydust
author_rank: Super Contributor
timestamp: March 19, 2025, 03:10:46 am
content: I've seen that waveform - filter cap ripple current make a mess when the PCB layout/PSU wiring is not right. You need a star ground to the filter cap for the bridge rectifier, filter caps, PSU output and 723, output pot etc. It can be just a takeoff or trace routed wrong.

author: David Hess
author_rank: Super Contributor
timestamp: March 20, 2025, 10:01:16 pm
content: Apparently the 723 has horrid line rejection on its V+ pin, so I was setting up to power V+ separately and accidentally applied reverse voltage, damaging the only TO-99-10 723 that I have, so the improve ancient power supply project is over. I am sure not going to buy a replacement metal can 723 for $20.I will likely hack together an operational amplifier based reference and regulator using an LM329 reference and LM350 regulator as a pass element.

author: floobydust
author_rank: Super Contributor
timestamp: March 21, 2025, 03:20:05 am
content: It's an unfortunate happening but those rare IC's kind of make things fragile when replacements are expensive.My regret for one PSU was buying the chinese kit for $5 and modding it to get it working better. I know their pricing is a honey trap- this included PCB, potentiometers, all parts etc.It ends up taking more time (labour cost) to do this instead of just rolling out your own design in the first place. False economy when modding junk.I'd still want adjustable current-limiting though. LM350 can smoke a breadboard fairly good lol.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 21, 2025, 03:37:11 am
content: Quote from: David Hess on March 20, 2025, 10:01:16 pmya they got expensive, i have a few ''723 cans'' laying somewhere, with originals mc1466 and the dip case of 723 tooi do have an old 723 psu design based on a old pyramid psu case and x-former, maybe 20 years or slightly less, it's a mix of 723 and an external current limiter who sits before the power output transistors, minimum voltages losses, 6x hfe matched 2n3055 ...from around 10 ma (directly drive a led without limiting resistor), up to 10-12 amps, can't remember if i did a schematic out of it, survived many shorts and tough uses, and yes this design took a very long time before a final designi stopped theses designs looong time ago, and got real psu brands loll Kepco, Kikusui etc .. for serious stuff and precision / stabilityfor the classics op designs lm324 or any other designs, and ones with secondary x-former taps switching who are better, many mastech models are known, and can be found on the web 3005 3010 3020 etc ...you do have better kits with op's on ebay or elsewhere

author: David Hess
author_rank: Super Contributor
timestamp: March 21, 2025, 04:56:06 am
content: I doodled this out tonight.The current control loop is straightforward from this point. Since the reference cannot be bootstrapped from the variable output, a separate precision supply powers the operational amplifiers and bootstraps the reference, requiring a third operational amplifier. The output ground shown is a single point ground, so the output voltage is sensed at the terminals, with the current through the reference creating a small error in offset and temperature coefficient. Regulation for this configuration is typically better than microvolts with commensurate noise. Frequency compensation is simpler than shown.The only unknown is clamping of the voltage and current control loops, which is not strictly necessary. Nobody makes a precision operational amplifier which supports clamping that I know of, but maybe I can do it externally without too much trouble.Unfortunately this design requires a low voltage negative supply, but the transformer has a separate winding which can be used for this. A negative supply would be required to use a three terminal regulator down to zero volts anyway.The current control loop should deliver good precision however it sees the current through the output divider. I have to supply a reference for the current control loop referenced to the output voltage using operational amplifiers for current mirrors, or a separate reference.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 21, 2025, 08:17:47 am
content: If you go for a discreet Vref, tested some months ago this one here (with a jumbo red LED instead of a Zener, I didn't use any C, it was stable in my tests), and it worked uncanny well:As seen in this threadhttps://www.eevblog.com/forum/projects/self-biased-zener-reference-using-wilson-current-mirrors/msg5784305/#msg5784305That one was brought to attention by usermagicafter reverse engineering the decapped die of a LT1236LS8 inhttps://www.eevblog.com/forum/metrology/decapping-the-lt1236ls8/msg2938346/#msg2938346Later I've tried that 3 transistors Vref on a breadboard, and it worked outstandingly well. Also have found accidentally a strange LTspice result (that apparently was producing even better input rejection than the LT1236 topology) but that was only an LTspice simulation artifact, not observed in practice. However, you can find some comparison plots if interested in:https://www.eevblog.com/forum/chat/invention-of-the-day-the-rotaluger/msg5804015/#msg5804015The input rejection for the Vref topology used in LT1236 (3xBJT + 3xR + LED) was so good (cca -120dB in LTspice) that it was unmeasurable in practice with my mtx3283 DMM (max 100_000 counts).

author: floobydust
author_rank: Super Contributor
timestamp: March 21, 2025, 09:41:18 pm
content: I drew a schematic for the Hiland LM317 0-25V 0-1A PSU kit. Qty. 12 of 1N4004 seemed weird, and why yes it is. Overall, it made my head hurt. A funny approach to making a -ve rail. Too bad they wasted 1/2 of the LM358.It's just for comedy really.

author: coppercone2
author_rank: Super Contributor
timestamp: March 22, 2025, 09:42:25 am
content: I will buy a LM723H because this ended on a cliff hanger

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 22, 2025, 10:36:04 pm
content: Quote from: RoGeorge on March 21, 2025, 08:17:47 amThat's cute.It has some startup issues, and those depend on power ramp slew rate.Spice says at 1V/ms rise, it needs ~ 500k B-E to prevent startup.At 0.1V/ms, it still starts but now ~ 30M hi side leakage to 2.4M low side leakage can prevent startup.Those are quite low, a good PCB should be above that, but it does show very slow ramps can give issues.Even 100M of active bias can improve startup, and you can null the HOE effects of the PNPs with a resistor Q1.E to GND, ~ 118.6k gives parabola VCC rejection.Of course, LED + BJT have problems to thermally track and couple, and they will come from different suppliers.A Dual Zener is probably a better choice, for thermal tracking, or you can just buy a good reference.These days they are quite cheap.A TL4051A gives 0.1% and you can make a precision current source using that, with a PFET - then fixed R or a pot can set output voltages.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 23, 2025, 06:18:38 am
content: Yes, that discreet Vref reversed engineer bymagicfrom the die of an LT1236 is just the draft, the main idea.Sometimes might not start, indeed. I've powered it from a DP832 power supply, and most of the times it was starting. Mentioned that schematic for its huge Vin rejection (to me that was incredibly well, the biggest I've ever seen, though I'm not into Vref and metrology), after reading in this topic how 723's Vin rejection might not be very good.In practice, the 3 transistors Vref will need a start up circuit. Same for the thermal coefficient. In practice might need extra care. Didn't measure myself the thermal coefficient of that jumbo red LED (only used it because it was looking silly, and because it happened to be on the workbench at that moment). I guess a selected 5V BZX might get a much better TC, but I didn't try.Measured however the dynamic resistance of that jumbo red LED (because Ri directly affects the PSRR, the smaller the better), and found the jumbo red LED's dynamic Ri to be about the same order of magnitude as a Zener's Ri at a similar current (IIRC 40 ohms? don't recall exactly, but it was close with a Zener, did back then some comparative plots of Ri for the jumbo red LED, other colors of normal 5mm/3mm LEDs, a few Zener 3V3/6V8/etc. and a few of various direct polarized diodes/BJTs).Another funny thing about that Vref with the big LED (funny as in unexpected to me) was that the Vref was apparently not affected by the ambient light. Same when shining a casual torch light (4.5V AA + incandescent light bulb) above the LED, couldn't measure any Vref variation on my DMM (range 9.9999V full scale).

author: David Hess
author_rank: Super Contributor
timestamp: March 24, 2025, 01:00:38 am
content: If someone wants to send me a metal can 723, I will be happy to finish diagnosing the problem, including measuring the power supply rejection of the V+ input. In trade I offer NOS 1N270 diodes.Below is the design I am considering, including external clamping of the voltage and current error amplifiers. This would be a lot easier using LM301A operational amplifiers because their compensation pin can be used for clamping. I removed the emitter series diodes because with clamping, the base-emitter junctions of the PNPs can no longer be driven into reverse breakdown.Not shown is the reference bootstrap circuit which also produces 30 volts for the error amplifiers; it is the same type of regulator circuit but without the extras.This design could be easily converted to a floating one using +/-15 volts (or lower) while regulating a much higher voltage.The current limiter requires a small precision adjustable current with compliance down to the negative supply which is not shown. Generating this current with compliance down to ground is trivial, however it must have negative compliance to support current limiting down to zero volts.I worry that during a short, the fast common mode swing would cause the current limit error amplifier to behave poorly, however other power supplies using this method do not have any problems with it. With a floating design, no common mode swing would be seen.Update: I found this page discussing the shortcomings of the 723 useful, although much of it has been mentioned here already:https://electronicprojectsforfun.wordpress.com/power-supplies/a-collection-of-proper-design-practices-using-the-lm723-ic-regulator/