Another Noob PSU Design Needing Review, Please - Page 1
Forum section: electronics / beginners

author: t1d
author_rank: Super Contributor
timestamp: September 20, 2021, 12:07:38 pm
content: This is a draft of a circuit that I propose to build. It is based on an old National Instruments data sheet design. I found it on the web. The author has made some changes to it. Specifically, adding a negative rail, so that the output can reach a true zero volts.My design goal is up to 30VDC and up to 10A (I am using a more powerful BJT) for the output. Of course, the typical 30VDC/3A criteria are fine, if we make adjustments that end up with that output.I am sure that there are better more modern designs, but this will do for my purposes. I would very much appreciate your thoughts. I do no want to go with a switch-mode solution.Here is a specific question:The Power BJT is rather expensive. I have lots of MOSFETs on hand. Might I use a MOSFET, instead? If so, can it be dropped directly in place of the BJT? If it would need additional circuitry, what would that be?I eagerly await your replies. Thank you for your help!

author: t1d
author_rank: Super Contributor
timestamp: September 20, 2021, 12:17:58 pm
content: Here is the author's link:http://circuit-zone.com/index.php?electronic_project=835Here is the author's original design:

author: Kleinstein
author_rank: Super Contributor
timestamp: September 20, 2021, 12:42:49 pm
content: There are quite a few weak points with this design:the voltage setting uses a fracion of the negative of -5 V to compensate the 1.2 V min voltage of the LM317. So this part also has to be quite stable.The pots are used in a way that a bad contact would cause a higher output voltage, which is not very good. Chances are the OP could see too high a supply voltage with the higher voltage version.A tantalum cap at the output (especially in the 30 V version - as it would be hard to use a 3 fold higher voltage rating here) is prone to catastrophic failure.It is unlikely the regulator would be stable against oscillation for a low ESR capacitove load.The given transistor may be large enough for about 2-3 A not really more when stying inside the SOA.

author: t1d
author_rank: Super Contributor
timestamp: September 20, 2021, 01:40:12 pm
content: All good stuff, Klienstein. Through the years, you have always proven to be most helpful and reliable, to me. Thank you!Yes, I would need to verify that all of the components can handle the higher volts and amps created by my component substitutions.- The -5V supply would be from a LM7905, which I would think would be quite stable.- What would be the solution for the risk of a bad pot contact?- Yes, the op amp voltage needs to be verified.- Yes, the Tantalum cap voltage needs to be verified. If it can not be sourced at that high of a voltage, a typical work-around seems to be to use a common ceramic cap. Or, an e-cap at x10 rating, IIRC.- What is the solution to harden the circuit against low ESR capacitive load oscillation? Terminate with a 50 ohm resistor on the output? That seems problematic.- As said, if the transistor is only good up to 3 amps, that is fine. Final testing will sort that out. Would using a MOSFET instead be of any improvement? If so, what is needed to incorporate it?

author: iMo
author_rank: Super Contributor
timestamp: September 20, 2021, 01:57:04 pm
content: Also your voltages are at max or even outside the TL071 datasheet values.The absolute max power voltage is 36V (yours is 38V) and the abs max input voltage is Vcc- + 36V (yours is 35V).

author: Kleinstein
author_rank: Super Contributor
timestamp: September 20, 2021, 02:43:03 pm
content: The tantalum cap problem is relatively easy to resolve, just use a low ESR electrolytic cap, maybe in combination with a 100 nF or so ceramic.For the stability, one would have to test - the specs for the LM317 are not that detaild that one could calulate this upfront. Not all LM317 are the same too - so a Ti part can be different in the details from a ON part. Usually it help to have some capacitance with significant ESR at the output. It may be enough or not.The way the circuit is made, by using the pot's as variable resistors makes it hard to avoid problems with an open viper. I see no easy fix for this.A LM7905 for the -5 V is OK. That should be stable enough for the rest of the ciruit.For the power handling capability there is no big difference between BJTs and MOSFETs. Both don't like that much power. Especially at more than some 20 V one also needs to look at the SOA curve, bit just the simple power rating. The nominal max power is for very good cooling and one is usually better off with about 1/2 of that, so the heat senk can be at some 60 C instead of 25 C case temperature as assumed for the power rating. With so much power, the usual way around is either using a SMPS or using multiple transformer taps. Both are not simple, but at more than 100W this may be worth the effort.For higher power it usually needs multiple transistors in parallel with resistors at the emitter to ensure current sharing.For the input voltage is also depends on the source. With a transformer and filter cap there will usually be some extra ripple on top. One will than need more than 33 V (at least as wrst case) at the input to get 30 V out. This adds to the needs for the OP and SOA.

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 03:19:57 am
content: Thanks, folks, for your wonderful help.Most of the issues appear to be from trying to get too much power out of the unit. I will gladly downgrade the specs to 30VDC/3A. Otherwise, I think Kleinstein sees this problem...Quote from: Kleinstein on September 20, 2021, 02:43:03 pmI am not sure what Klienstein means by an "open viper." My guess is that he means some unforeseeable major problem? Maybe someone knows how to harden the circuit against such things?Thanks for all the good help. Keep it coming. You are teaching much!

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 04:08:47 am
content: It makes sense to try to use whatever components that I have on hand...On my thread regarding terminating the null set pins on a TL741, Terry Bites suggested bypassing op amps that use a null set function and just go with a precision op amp. That makes great sense.I have three precision op amps in stock... All Analog Devices brand: 1) ADA4075 2) ADA4004 3) AD8675. Numbers 2 and 3 list 36 volts in there brief descriptions. I will have to read the data sheet on all three, as I have not used them before and I am not familiar with them. EDIT: 4) AD8671I have lots of MOSFETs. Are there any particular model numbers that are commonly used in bench PSUs?Cheers!

author: Electro Fan
author_rank: Super Contributor
timestamp: September 21, 2021, 04:27:03 am
content: Quote from: t1d on September 21, 2021, 03:19:57 amMight have just meant “open wiper”?

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2021, 04:31:25 am
content: During normal operation, the LM317's ADJ pin is 1.2V below the OUT pin.With VR2 at max setting, half of the 1.2V will be applied across the opamp's input pins.A precession opamp is not needed.

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 05:21:38 am
content: Quote from: xavier60 on September 21, 2021, 04:31:25 amGood point!

author: floobydust
author_rank: Super Contributor
timestamp: September 21, 2021, 05:21:51 am
content: TL071exhibits phase-reversal and non-linear gain, you don't want those surprises in a power supply lol.

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2021, 06:18:33 am
content: Im not certain what a TL741 is. If equivalent to the LM741, it would be a reasonable choice. From my own observations, input common mode range is about 200mV short of its supply voltage.Current regulation doesn't imply short-circuit protection. There will be a brief and large current spike when the output is shorted.Difficult to know if it should be dealt with or not.Extra: I overlooked the fact that the -5V rail adds to the total opamp's supply voltage. This reduces the margin between supplying enough voltage and what's not safe. It would need to be close to 31V to the + supply pin.

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 06:33:03 am
content: In the potentiometer (or a trimmer) a "wiper" slides over the resistive surface.It could happen the wiper looses its contact with the surface while sliding on the surface, or because oxidation, or mechanical failure, etc.There are designs where the output voltage (or current limit) drops down to MIN voltage, or, there are designs where the output voltage jumps to MAX voltage in case the wiper's contact with the surface is lost ("open wiper").The better is the solution where output voltage drops to MIN voltage for obvious reasons.

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 06:51:38 am
content: High voltage opamps from ADITABLE..

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 07:35:56 am
content: Quote from: floobydust on September 21, 2021, 05:21:51 amThe history of this design is that it comes from an old National Instruments LM317 data sheet. The attached picture shows they originally used a LM301A op amp. IIRC, it is still included in the TI LM317 data sheet.I listed some op amps that I have in stock that look like they can handle 36 volts. With reducing the goal to 30V output, maybe one will work. But, I need to research them fully. I am still working on that.The first step is to learn how to determine their Safe Operation Area. I know what the term means, but not how it is determined for a specific component.Notice that the original did not bother with the 0 volt goal/-5V rail.

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 07:41:50 am
content: Quote from: xavier60 on September 21, 2021, 06:18:33 amAs I recall, the TL is supposed to be "comparable" to the LM, but the devil is in the details. And, as said, I am looking at the other op amps I have in stock. With just a quick look, the AD8675 looks like it might do. Data sheet below.

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 07:46:31 am
content: Quote from: imo on September 21, 2021, 06:33:03 amThat is a really good explanation! Do you have a link that shows the two approaches, or do you have any terms to search for them?

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 07:47:08 am
content: What does the D2 do on the pin8 of the LM301 in the original schematics?Where is the V+ of the LM301 in the original schematics wired to?Still I think operating an opamp at "absolute max voltage" specs is not a good idea..Open wiper: nope, I do not have a schematics comparison handy..

author: Kleinstein
author_rank: Super Contributor
timestamp: September 21, 2021, 07:59:34 am
content: Quote from: floobydust on September 21, 2021, 05:21:51 amThe phase reversal happens at the low end and is thus not a problem here, as there is a -5 V supply. So the OPs inputs should hardly go below some 3.8 V from the OPs neg. supply.The AD8675 would not work well, because the input common mode range does not include the positive supply. So it would need a relatively high supply (can still work with somthing like seprate filtering and voltage limit for the OPs supply). If ordering new, the TL071H or OPA991 could be an option (with 40 V maximum supply voltage).As shown, at the max setting the voltage across the shunt would be 1.2 V and thus relatively high, but to limit the heat there it may be advisable to change to Pot more to 50 K or less, so that the max would be more like 0.6 V or maybe even 0.3 V. The current limit is not high precision anyway. So from the precision the TL071 should be good enough, unless one uses a very low (< 100mV) drop on the shunt.

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 21, 2021, 08:00:05 am
content: not sure if D3 will ever work properly, but i haven't put the pen on paper to check if he can light correctly with current draw from adj pin, maybe it's safer to put that missing resistoranyway i don't like that configuration with led entering conduction linear-like, i like the on/off approach, either the current protection (limit) is ON and LED is ON, either it's off. with your schematic i'm not sure it works that way

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 08:00:59 am
content: Quote from: xavier60 on September 21, 2021, 06:18:33 amThat is a really good point that must be included in the supply consideration, too... Meaning the transformer, etc. I am sure that I have some transformer with two, or more, secondaries that would work to supply the 33V and -5V rails.As the negative rail only needs to output ~-1.25V, maybe its input could be reduced to -3.3V, or -2.5V. That would save a little headroom. Hmm... I need to see if I have any LM79xxs in that low of a voltage. I rather think not.

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2021, 08:01:14 am
content: Quote from: t1d on September 21, 2021, 07:41:50 amA suitable opamp needs to be able to operate with its inputs at close to + supply rail voltage.The "Input Voltage Range" spec doesn't look quite right.Edit: Already mentioned by Kleinstein.

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 08:07:07 am
content: I would recommend to put the new schematics into the LTspice.. All parts are available.Soon or later you will start to deal with questions around dynamic parameters (ie stability under various loads/settings, gains, phase margins, overshoots) and the simulation works pretty good as we know from past.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 21, 2021, 08:09:31 am
content: D2 at the LM301 should speed up the recovery from saturation and thus the reaction the current limit somewhat. The higher speed of the tl071 would about make up for this.If powered from a transformer, and this with some variations and hum on the main supply, it would be a good idea to have a seprate limit for the OPs supply. This could be a simple resistor +33 (or 35) V zener + emitter follower type circuit. So the OP would not see the variations and hum on top the main supply.A simulation is definitely a good idea. However it depends on the details of the LM317 model, so the higher speed details in the response may be a bit off. Sill it is better than the calculation one can do in the head.

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 08:20:28 am
content: Open wiper: some time back I recommended to wire an higher value resistor Rx to the wiper against "something" thus when the contact is lost the Rx will pull/push the input of the opamp such the output voltage jumps towards MIN voltage.

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 08:26:41 am
content: Quote from: imo on September 21, 2021, 08:07:07 amThat's a good suggestion, but LTspice is another h-u-g-e learning curve. I already have electrical engineering, KiCad, C Code, PIC MCUs/MPLab X, Lab Equipment Operations, etc. on my plate. So, simulation programs have had to wait. lol...

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2021, 08:27:19 am
content: Quote from: t1d on September 21, 2021, 07:35:56 amIt's not clear to me where LM301's V+ supply comes from. A suitable rail-to-rail, single-rail opamp could work being powered from the PSU's output.Extra: Actually only 2 important things are needed. the opamp needs to be able to safely run on 36V and the input common mode range needs to include V+.

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 08:32:49 am
content: Quote from: Kleinstein on September 21, 2021, 08:09:31 amI understand the what and the why of what you are suggesting, but I will have to work on the how of how to accomplish it. Meaning, designing the needed circuitry.

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 08:33:58 am
content: Quote from: t1d on September 21, 2021, 08:26:41 amSure, you have to master many tools to become an experienced MakerBut the simulation with LTspice has to have the Priority, as messing with PCBs, MCUs has no sense when the circuit you are designing does not work, imhoWith mastering the LTspice your time spent with soldering jobs and endless debugging will drop down significantly, indeed..

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 08:34:55 am
content: Quote from: imo on September 21, 2021, 08:20:28 amDo you have the link where you discussed this trick?

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 08:38:08 am
content: Quote from: t1d on September 21, 2021, 08:34:55 amFor example here in this thread. There is also schematics with the resistor somewhere..https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 08:40:57 am
content: Quote from: xavier60 on September 21, 2021, 08:27:19 amAhh... And, I see that the op amp negative supply is tied to a mystery -6 volt rail. I had missed that...

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 08:41:36 am
content: Quote from: imo on September 21, 2021, 08:38:08 amThanks!

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 09:00:49 am
content: Here for example.. The R2 and R10 1Meg resistors (at CC and CV potentiometers) will not affect the setting linearity "too much" and in case of an open wiper the resistors pull the opmap's inputs to gnd.Your design is different, so you have to think twice where to wire the cold sides of those Rwipers to..

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2021, 09:25:30 am
content: The TL071 is suitable "Common-mode input voltage range includes VCC+"It could be given some Zener protection, but put the dropper resistor in series with the V- supply.

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 10:33:06 am
content: While I am learning about the needed op amp, maybe you good folks could give consideration to my options for the power transistor... My options are to use what I have, or buy something else. I would rather use what I have, of course.Here's what I have in stock:MJ4502G Power P-Transistor - 100V/30A/200W - Qty(1)IRF9540NPBF P-MFET - 100V/23A/140W - Qty(2). Curiously, the data sheet description says this component is typically used for 50 watt applications?SPP18P06PH P-MFET - 60V/18.7A/81.1W - Qty(2).Data Sheets below.It appears that two of the available P-MFETs would be needed to manage the watts with sufficient room to spare. It is (I think) within my skills to design the extra circuitry, if using two MFETs is our consensus... Other than there seems to be some debate as to whether the small balancing resistors go in the Drain, or the Source, of the MFETs.To be honest, it is clear to me that it would require a full college course to learn all the ins and outs surrounding the choice of the op amp. Kleinstein has been kind enough to rule out the ADAD8675 and to suggest the TL071H or OPA991, if having to purchase it. But, I wonder if any of the other op amps that I listed as having on hand might do?

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 10:37:13 am
content: Quote from: imo on September 21, 2021, 09:00:49 amThank you so much for the extra effort to post this. It is very helpful!

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2021, 10:47:17 am
content: Just in case it was missed. Powering the opamp from the PSU's own output will reduce the voltage stress to the opamp.

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 10:49:17 am
content: The power loss (the heat created in Watts) at your power transistor in your PSU design does not depend on the type of your power transistor, but on the equation (aprox):P = (Vin - Vout) * Iout [V, A]In your case Vin = 33V and Vout = 0..30V, and Iout = 0..3AFor example with Vout = 1.5V and Iout = 3A your P = 94.5 Watt.Therefore you have to use a transistor(s) which can deal with a) current, b) voltage difference Vin-Vout, c) power loss in the entire operating area of your PSU.The max power ratings of your transistors (like 100W or 200W) are valid only when you keep the temperature of the transistor's case at 25degC. Could you do it? That is the big question..

author: Kleinstein
author_rank: Super Contributor
timestamp: September 21, 2021, 11:10:14 am
content: If the OP is powered from an extra supply, it does not need to work all the way to it's upper supply at the input.The OPs listed are pretty similar low noise BJTs and they work up to some 2-3 V to the positive rail only. With the positive supply at some 33 V they would work. They may have a lower limit to the supply and thus powering from the output does not work.Current sharing with MOSFETs would be with a resistor at the source. It needs relatively large resistors or some additional banancing circuit. As a downside the MOSFET would only start to turn on with more voltage at the gate. So it needs an input voltage that is some 2-4 V higher than with the current circuit.So the P BJT is already a reasonable good choice.I don't know why the NS datasheet claims 5 A for the shown circuit - with 35 V at the input the transistor SOA is only good for some 3 A.To get 30 V DC out, one would need some 33 V as the minimum at the input. With some ripply this would be more like 36 to 40 V needed at the input.

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2021, 11:15:10 am
content: I should have mention that the the opamp would be powered from between the PSU's output and the -6V rail. The TL071 should work.

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 11:25:28 am
content: TL071's absolute max power voltage rating is 36V.. (TI'sdatasheet RevS).PS: I would behaving my TL071 powered from 36V..

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2021, 11:30:24 am
content: This data sheet says 40V. Anyways, designs that stretch things to their limits should be avoided.https://www.ti.com/product/TL072?utm_source=google&utm_medium=cpc&utm_campaign=asc-null-null-GPN_EN-cpc-pf-google-sa&utm_content=TL072&ds_k=TL072+Datasheet&DCM=yes&gclid=CjwKCAjwhaaKBhBcEiwA8acsHKgsVQwyEYIaCTSuLtymSBBGl8DBvwAMnrsrspMm3qQ7u_7eziXOLRoCeRgQAvD_BwE&gclsrc=aw.ds

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 11:36:06 am
content: I added above the link for 2021 DS of the TL071.TL071 36VTL071H 42V

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2021, 11:42:18 am
content: The opamp's VCC- pin only needs to be say 10V below its VCC+ pin to drive the LED and have full control of the LM317.But achieving this could get complicated. The can of worms is opening.

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 11:55:05 am
content: In my reply #14 there is a link to ADI's high voltage opamps.For example the in Metrology section pretty popular 2057 - its high voltage version LTC2057HV is 60V(slow for this app, imho).ADI lists 103 opamps with Vcc abs max > 40V (15 opamps from 50V up to 220V)TI lists 127 opamps with Vcc abs max >40V (17 opamps from 50V up to 180V)..

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2021, 12:11:21 pm
content: The LTC2057HV is a bit expensive but no where near so as most of the prices I just seen for high voltage opamps.

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 03:46:00 pm
content: Any brave worms collector here?Below the LTSpice file..

author: rstofer
author_rank: Super Contributor
timestamp: September 21, 2021, 04:29:54 pm
content: How is the pass transistor going to survive an output of 1V at 10A (specified in the original post). It can't really handle 320W, it is only rated for 200W and that's probably when the moon is low on the horizon.If you back off on the 10A requirement, maybe...As drawn in the original sketch, there is a missing joint in the current adjust circuit.

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 21, 2021, 11:41:58 pm
content: I'd put a dc/dc buck module, then add a linear regulator after it to clean the ripple.These linear designs have the efficiency of a rock!

author: David Hess
author_rank: Super Contributor
timestamp: September 22, 2021, 12:42:02 am
content: Quote from: imo on September 21, 2021, 07:47:08 amD2 uses the 301A's compensation pin to clamp its output making recovery time faster. The 301A is almost uniquely suited for this sort of circuit because it supports clamping.Quote from: Kleinstein on September 21, 2021, 08:09:31 amExternal compensation through C5 limits speed, so a faster operational amplifier does not improve recovery time.Quote from: xavier60 on September 21, 2021, 08:27:19 amThe 301A has an input common mode range which includes its positive supply, and the better ones can operate up to 44 volts.Quote from: rstofer on September 21, 2021, 04:29:54 pmAnd the pass transistor is not protected from an instantaneous short in this circuit anyway. The configuration shown below allows the regulator's current limit to protect the pass transistor or transistors by better controlling the current gain.Parallel transistors are still a good idea at this power level but the configuration below handles that as well.

author: t1d
author_rank: Super Contributor
timestamp: September 22, 2021, 04:48:04 am
content: Below is a list of the Op Amps that I have on hand. Maybe someone will see one that would work. That failing, I will order a 301A, because Kleinstein seems to think it okay.Here is a thread I started on the easy way to create the inventory.https://www.eevblog.com/forum/projects/an-easy-method-of-making-a-inventory-of-your-components/msg3703468/#msg3703468I will be replying to some of you recent posts, but I have to take a break and eat, presently.

author: t1d
author_rank: Super Contributor
timestamp: September 22, 2021, 05:52:50 am
content: The can-style LM301 is quite expensive. The On brand LM201AV appears to be more robust and way less expensive. $0.81USD. Data sheet below.

author: t1d
author_rank: Super Contributor
timestamp: September 22, 2021, 05:57:39 am
content: Quote from: rstofer on September 21, 2021, 04:29:54 pmGreat catch! Corrected.

author: t1d
author_rank: Super Contributor
timestamp: September 22, 2021, 06:05:02 am
content: Quote from: imo on September 21, 2021, 03:46:00 pmThank you, imo, for this amazing contribution of effort!

author: t1d
author_rank: Super Contributor
timestamp: September 22, 2021, 07:05:58 am
content: Quote from: imo on September 21, 2021, 09:00:49 amAhh... This is a wonderful teaching point... Let's see if I understand it correctly... If the wiper of RV2, or RV3, losses contact, then the op amp will swing to its full output, which drives the BJT to its full output, which creates a meltdown. By adding high value resistors, when the wiper contact is lost, we shunt the controlling voltage through the protection resistor, which would cause the op amp and, therefore, the BJT to go low. The cost of adding this protection resistor is the small amount of current that it leaks continuously.If adding the protection is desired, then the question becomes where to inject and terminate the protection resistor. This is where my knowledge is lacking...But, I must admit that, for a stationary bench device, I wonder how great of a probability this type of failure would be. The consideration of it is well worth the education therefrom. However, will it actually be included in the final build? I am not sure.It does occur to me that the issue could be resolved for the offset trimmer by testing the unit with a trimmer and, once the trimmer value is known, to replace it with individual resistors. Of course, there is the factors of drift to consider, with a fixed resistor approach.I will think on it.

author: t1d
author_rank: Super Contributor
timestamp: September 22, 2021, 07:09:51 am
content: Quote from: rstofer on September 21, 2021, 04:29:54 pmYes, the design goals were changed to 30V/3A, in the run of the text.

author: iMo
author_rank: Super Contributor
timestamp: September 22, 2021, 07:19:13 am
content: Before you spend a lot of effort and $, prototype the PSU with what you have handy and with lower voltages and currents. The stuff did not work in my simulation with the values as in your schematics (see above the LTSpice file with your PSU).

author: t1d
author_rank: Super Contributor
timestamp: September 22, 2021, 08:17:34 am
content: Quote from: imo on September 22, 2021, 07:19:13 amYep, that is always a good idea.Quote from: imo on September 22, 2021, 07:19:13 amAs mentioned earlier, I know nothing about LTspice. So, I would not know how to run the file that you were so extremely gracious to create and share. I say that with regret and embarrassment, because you worked so hard on it. Hopefully, it will help others that do know how to use it to give me tips, clues and instructions. My extreme apologies.

author: t1d
author_rank: Super Contributor
timestamp: September 22, 2021, 08:32:02 am
content: Quote from: David Hess on September 22, 2021, 12:42:02 amA few posts back, I added the data sheet for the LM201AH. I am hopeful that it will be suitable. I do need your instructions as to what to do with the two Balance pins (#1 and #5) and the Compensation pin (#8).Quote from: David Hess on September 22, 2021, 12:42:02 amI very much want to include these improvements, but I lack the knowledge to be able to adapt my proposed circuit. See the next post.Quote from: David Hess on September 22, 2021, 12:42:02 amI plan to use dual IRF9540 MOSFETs, See the next post.

author: t1d
author_rank: Super Contributor
timestamp: September 22, 2021, 08:43:08 am
content: At this point, I think it would be good to summarize where we are and create a new schematic that incorporates the decisions that we have made. Once we have the new schematic in hand, we can continue to tweak it. It would be easier to use a new schematic to add David Hess's improvements for better controlling the current gain. So, I will be working on the schematic and I hope to have it ready soon.I am having great fun working on this with all of you. You are teaching me lots. Thank you so much.

author: t1d
author_rank: Super Contributor
timestamp: September 22, 2021, 12:42:20 pm
content: Here is a summation of where we are in the project, the updated schematic and the questions resulting therefrom.Summation- The design goals have been adjusted down to 30V/3A, due to power management considerations.- A transformer with separate secondaries for the 33V and -5V rails will be used to lessen the total voltage spread. I am not sure that I know what changes this requires in the circuit. I think all it requires is tying the two grounds of the two secondaries.- The Tant cap was changed to a 10uF e-cap; low ESR and a 100nF ceramic cap was added.- The op amp considerations seem to be pointing toward the LM101/201/301 series of op amps. The LM201AH has been used as a place holder in the schematic. I terminated the two Balance pins and the Compensation pin, pending instructions.- I have incorporated two MOSFETs for the BJT and added dual Schottky diodes, for balancing, because that is what I have on hand.- I added fuses on the supply inputs.- David Hess’s current limiting design changes are pending his instructions.- imo’s voltage pot and trimmer design changes are pending his instructions.What follows… I went back through the posts and tried to pick up any issues that might still need consideration...@ Kleinstein“As shown, at the max setting the voltage across the shunt would be 1.2 V and thus relatively high, but to limit the heat there it may be advisable to change to Pot more to 50 K or less, so that the max would be more like 0.6 V or maybe even 0.3 V. The current limit is not high precision anyway. So from the precision the TL071 should be good enough, unless one uses a very low (< 100mV) drop on the shunt.”Are you saying that I need to change the value of the Current Limiting pot? If so, to what value. Remember, the op amp choice is now narrowing to the LM201AH/LM301.@ parieanuo“not sure if D3 will ever work properly, but i haven't put the pen on paper to check if he can light correctly with current draw from adj pin, maybe it's safer to put that missing resistoranyway i don't like that configuration with led entering conduction linear-like, i like the on/off approach, either the current protection (limit) is ON and LED is ON, either it's off. with your schematic i'm not sure it works that way”Did you want to suggest a solution? A comparator, possibly?@ Kleinstein“If powered from a transformer, and this with some variations and hum on the main supply, it would be a good idea to have a seprate limit for the OPs supply. This could be a simple resistor +33 (or 35) V zener + emitter follower type circuit. So the OP would not see the variations and hum on top the main supply.”A TL431 Adjustable Zenner has been added to the circuit. I will need help with determining the supporting component values; please and thank you. Should one of the resistors in the divider that sets the Zenner voltage be a trimmer pot?@ Kleinstein“If the OP is powered from an extra supply, it does not need to work all the way to it's upper supply at the input.The OPs listed are pretty similar low noise BJTs and they work up to some 2-3 V to the positive rail only. With the positive supply at some 33 V they would work. They may have a lower limit to the supply and thus powering from the output does not work.”Two question arise for me from your comments:1) Excepting “Over-The-Top” op amps, I thought that op amps must be powered at least with the full output they are to drive (via the MOSFETs) plus any voltage drops.2) Are you suggesting a third transformer secondary to power the op amp separately. If so, I think that is doable.Consideration of the Back Voltage Diode; D2D2 is for relieving back voltage. In addition to common 1N4007 @ 1000/1A rectifying diodes, I have two additional types of Schottkies, (in addition to the two MBRD10150CT dual Schottkies that I have already assigned to balance the input voltage to the MOSFETs.) They are- 1N5818-G @ 30V/1A- 1N5822-T @ 40V/3AI am considering what to use for D2. I know that back flow voltage spikes can be very high in voltage amplitude. And, IIRC, that the amperage of those spikes is likely to be small. But, we have the 3 amps of the supply that we must prevent slipping by the MOSFETs. So, I need help in allocating the diodes. I guess D2 could be four 1N4007s in parallel. What’s the trick, here?I would be glad to provide my KiCad files to anyone that would like them to work with. But, I think I will have to use DropBox to get them to you. The files are too big to post on this forum, IIRC. I will be sure to make them freely available, once we finalize the project.Thanks for all of your great help. We are making fantastic progress!

author: t1d
author_rank: Super Contributor
timestamp: September 22, 2021, 06:50:53 pm
content: Okay, imo, let’s see if I can meet your objectives, for protecting the circuit from a pot wiper failure. But, keep in mind that this is outside of my present knowledge, so this is just my attempt.The op amp’s inverting pin is supplied -5V through the zero voltage trim pot and then the voltage adjustment pot. Let’s connect a 1M resistor from the -5V rail to the low side of the voltage adjustment pot. If either of the pots’ wipers fail, power passes through the complete resistor track of the voltage adjustment pot and continues on to the inverting pin. The power would remain uninterrupted.In the normal (non-failed) working state of the voltage adjustment pot, the power flowing through the new 1M resistor would flow back to ground. The downside to this arrangement is that the low side of the voltage adjustment pot would always be offset by the voltage flowing through the 1M resistor. I am not sure that I know all of the ramifications of this voltage offset. So, what say you?

author: t1d
author_rank: Super Contributor
timestamp: September 22, 2021, 07:17:27 pm
content: Look at the implementation of the LM301 op amp in the original data sheet version of this circuit. It shows how to connect the two Balance pins and the Compensation pin. Notice the connections and components for pins #1 and #8. If those are correct for our use, I can add them to our schematic. Pin #5 is not shown, so I take it that it is supposed to just be not connected?

author: David Hess
author_rank: Super Contributor
timestamp: September 22, 2021, 10:30:26 pm
content: Quote from: t1d on September 22, 2021, 08:32:02 amThe LM301A, LM201A, and LM101A are all the same part with different temperature and supply voltage ranges. They are functionally interchangeable. The A suffix distinguishes them from the original and short lived LM301, LM201, and LM101 which can be ignored. Any place an LM301 is called for, use an LM301A. (1)The 75 picofarad capacitor between pins 1 and 8 implements the external frequency compensation which other operational amplifier have internally. It *must* be included if an LM301A is used.Check the datasheet for how to implement offset null, but I think you can leave it out. The current regulation of this power supply circuit is not particularly good because it depends on a constant ratio of current through the regulator and pass transistor, which is optimistic at best. The pass transistor suggestion I made improves this situation.Add the clamping diode or not as you prefer. You do not *have* to clamp the operational amplifier for the circuit to work, but it does give some performance benefit when switching to constant current mode which other operational amplifiers will have difficulty duplicating performance wise without help.(1) The A series of these operational amplifiers use an improved internal biasing arrangement which leads to a lower and more consistent input bias and offset currents. The A parts completely replaced the non-A parts so the non-A parts can be ignored.

author: t1d
author_rank: Super Contributor
timestamp: September 24, 2021, 06:48:27 am
content: Quote from: David Hess on September 22, 2021, 10:30:26 pmReally good info; thank you. I have ordered the two LM201AHs from Mouser.Quote from: David Hess on September 22, 2021, 10:30:26 pmI will add it in.Quote from: David Hess on September 22, 2021, 10:30:26 pmAgreed.Quote from: David Hess on September 22, 2021, 10:30:26 pmAs I mentioned in post numbers 60 and 62, I would very much like to include the pass transistor, but I lack the knowledge to incorporate it, even with the schematic that you were so kind to provide. What would be wonderful is either a drawing, or instructions as to what components to add and where they go, and an explanation of what they do and how they do it. As always, please and thank you.Quote from: David Hess on September 22, 2021, 10:30:26 pmI will add it.

author: t1d
author_rank: Super Contributor
timestamp: September 24, 2021, 07:37:45 am
content: Per the original circuit design, the Clamping Diode and the supporting Cap have been added. Pin5/Compensation has been terminated as floating. Please confirm that I have it all correctly.On the PCB, should the cap be closer to pin 8, or pin 1? It appears to be pin 1.

author: t1d
author_rank: Super Contributor
timestamp: September 24, 2021, 07:51:34 am
content: Shunt Resistor Consideration...I have a very nice 0.1R/16W/Precision/Aluminum Case resistor. Will it do for the 0.2R/5W resistor that is specified? What effect would using it have. I think it only changes the math calculation, when calculating the current? x10, instead of x20?EDIT: I do have the 0.2R/7W on order.

author: t1d
author_rank: Super Contributor
timestamp: September 29, 2021, 03:51:28 pm
content: Bump... I still need some help with post #62 and forward... Please and thank you.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 29, 2021, 04:21:39 pm
content: For the compensation cap it does not really matter if closer to pin 1 or 8 as the OP is relatively slow. Pin 1 seems to be the more sensitive pin ( lower internal impedance), so I would put the cap closer to pin 1 to minimize the capacitance at that pin.A smaller shunt resistor increased the current. As a compensation one can change the resistors at the OPs input.As shown in the DS circuit the voltage limit there is quite high. The more normal shunt to use for 5 A would be even less than 0.1 Ohms.The DS circuit is anyway only good for some 2.5 A with 35 V in. So they should change the current setting pot.The 1 M resistor added for the voltage setting port would not save much if the wiper goes open - the voltage would still go up quite a lot. The main point would be getting a really good pot, so it would rarely fail. I see no easy fix for the problem in this circuit. Modern lab supplies use a different way to set the voltage, but this does not work with the LM317.