0-30V, 0-3A PSU - Audioguru's version - Page 1
Forum section: electronics / beginners

author: soldar
author_rank: Super Contributor
timestamp: March 01, 2019, 09:33:40 am
content: This a followup tohttps://www.eevblog.com/forum/beginners/diy-0-30v-0-3a-again/It seems that project was not completed successfully.I am attaching from that thread what I believe is the final design.Is that considered to be the correct and final design and confirmed to work correctly?If so, is it possible to buy the PCB?In this thread I would like to come to a final, confirmed working, design, including specs, schematic, list of parts and PCB design.I am analyzing it. I see the op-amp on the right does the voltage regulation and it looks quite simple and straightforward.The op-amp and 5.6 volt zener in the center provide the voltage reference. I think it would be quite possible to simplify that and get rid of the op-amp and provide a simpler voltage reference. It might not be so exactly precise but I think it would be more than enough for most applications.The op-amp on the left does the current limitation control and Q3 lights the LED when current limit is active.I do not see why the 10V zener that lowers the supply voltage to U3 is needed. Maybe it can be omitted?Maybe I would add an "ON" LED, just to show voltage at the main capacitor.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 01, 2019, 02:22:53 pm
content: The circuit is an improved version from a rather common kit, that has quite some mistakes inside.It looks quite reasonable, though one might still want the transistor from the original to protect against an spike on turn on / turn off. With the auxiliary negative supply this may happen under some conditions during turn on.Another point might be a diode prevent to much reverse voltage to the output stage base / emitter current (add a diode like 1N4148 from output to right OP).The 10 V zener is there to reduce the supply to the first OPs. Depending on the OP used, this may not be needed or 10 V Zener could be still too little. To high a supply to the OPs was the main problem with the original circuit from the kit. A reduced voltage looks like a good idea.For the reference part there are a few options. The OP circuit can work in a way to keep the extra current through the shunt constant. So it's not such a bad solution. The OP could share the same reduced supply as the current regulator - so no special higher voltage OP needed here.By using only 1 raw voltage and with a relatively high drop, the circuit is not very suitable for high power. With some extra filtering one could at least improve a little on voltage lost: So something like 2 V lower AC could be sufficient. I am still not sure the circuit as shown would get to 30 V under all conditions (e.g. low mains and full load).

author: kripton2035
author_rank: Super Contributor
timestamp: March 01, 2019, 02:30:28 pm
content: seems you can buy this kit @ banggood and change the componentshttps://www.banggood.com/0-30V-2mA-3A-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-p-958308.html

author: Kalvin
author_rank: Super Contributor
timestamp: March 01, 2019, 02:55:51 pm
content: Probably one could increase the available power by using an optional tracking pre-regulator feeding the power stage. The tracking pre-regulator will increase the output noise somewhat, but one can get higher output current with less heat. If the pre-regulator would be bypassed by a switch when a lighter load is connected, one could achieve low noise output at lower currents. There should be some cheap and suitable dc-dc buck-converters available at eBay, which can be modified so that it will track the power supply output voltage by 2 - 3 Volts or so. This modification would extend the usability of this simple power supply without serious heat issues.

author: Kalvin
author_rank: Super Contributor
timestamp: March 01, 2019, 03:07:36 pm
content: I would also consider adding a unidirectional 30V TVS diode across the output, which would protect the power transistors from typical voltage surges occurring with inductive loads and connecting/disconnecting the load. Adding a polymer fuse to output should not hurt either.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 01, 2019, 04:20:07 pm
content: Quote from: Kalvin on March 01, 2019, 03:07:36 pmA polymer fuse at the output ( between the emitter resistors and the feedback) does add some resistance and thus effect the response a little towards a little slower response.AFAIK the board from the kit does not include the 10 V zener to reduce the supply to the not so critical OPs. So it would need better OP for all 3. The difficulty here is getting DIP8 OPs that can stand a high voltage (e.g. 40..44 V), without paying a premium price (like for OP27). Plain old 741 could work - though a little slow (especially slew rate for current limit - so quite some delay before CC mode sets in).The board I know also includes a layout blunder, that causes quite some ripple from the rectifier when in the CC mode (poor sense point for the negative side of the shunt). This can be fixed with a cut trace and bodge wire.

author: Kalvin
author_rank: Super Contributor
timestamp: March 01, 2019, 04:32:02 pm
content: Quote from: Kleinstein on March 01, 2019, 04:20:07 pmOne can use an op amp in SO-8 with suitable breakout PCB adapter board or a piece of a strip board. Of course the op amp will cost a few dollars, which will effectively double the original price of the kit

author: kripton2035
author_rank: Super Contributor
timestamp: March 01, 2019, 04:58:11 pm
content: LTC6090 @ around 8€ each ...

author: perieanuo
author_rank: Frequent Contributor
timestamp: March 01, 2019, 05:16:57 pm
content: Quote from: kripton2035 on March 01, 2019, 04:58:11 pmWhat's wrong with TL082 replacement?Envoyé de mon iPad en utilisant Tapatalk

author: Kleinstein
author_rank: Super Contributor
timestamp: March 01, 2019, 05:38:43 pm
content: There are good replacements like the mentioned TLE2141 or MC34071 - though not that easy to get in DIP8 form. An OPA604 could also work - though a little on the fast side and high supply current which is not that attractive with the negative supply. Only for 40 V, but otherwise the TLE2021 could be an option too.The easy way is to just reduce the maximum voltage a little.Another option would be to add a resistor and zener to limit peak voltages on the OPs supply. So the OP would not see the peaks.Just changing the OPs on the kit board only fixes some of the weak points. There is still only 1 power transistor, too small a filter cap, the high supply to the other 2 OPs that don't need it, the poor shunt layout, a poor choice for the negative supply of the reference OP. One could directly support an OP is SO8 case that is easier to get.There is already at least 1 thread in this forum about this supply and suggested changes. So there might already be an improved layout around.So it might be worth to make a new, revised board and not use the kit.Currently PCBs seem to be not that expensive when ordered in China.This circuit should fit on an 8x10 cm² size - especially with the power transistors externally.

author: rdl
author_rank: Super Contributor
timestamp: March 01, 2019, 05:43:36 pm
content: Why not just reduce the output to 20 volts? I doubt many people ever really need more than that.

author: Kalvin
author_rank: Super Contributor
timestamp: March 01, 2019, 05:47:25 pm
content: Quote from: perieanuo on March 01, 2019, 05:16:57 pmCheck the maximum operating voltage seen by the op amp: For 28V AC RMS rectified voltage, the peak rectified voltage will be 28V * 1.4142 = 39.5V. In practice one needs to provide few volts more across the power transistor, so in order to get stable 30V output voltage the RMS voltage needs to be 35V or thereabouts. Now, the peak rectified voltage seen by the op amp will be 35V * 1.4142 = 49.5V. Add to this the -1.3V negative power supply voltage and you will get 50.8V across the op amp, which will exceed the maximum allowed power supply voltage of TL082.Like others have suggested, reducing the transformer voltage ie. reducing the rectified peak voltage ie. reducing the maximum output voltage, one can use those cheaper op amps.

author: Audioguru
author_rank: Super Contributor
timestamp: March 01, 2019, 06:41:01 pm
content: The original Greek kit and the Chinese copies used TL081 opamps that have a maximum allowed total supply of 36V and need a negative supply that is at least 4V then its maximum allowed positive supply is only +32V. Its max output voltage would be +29V if the driver and output transistors have minimum hFE and the output current is 3A. I removed R15 in series with the output of the voltage regulating opamp and the driver transistor that reduced the voltage 2.5V max to +26.5V. The maximum worst case voltage loss of the driver and output transistors is 3V so the worst case maximum output voltage of this old circuit would be only +23.5V at 3A. You would want less supply voltage for reliability.To make the +32VDC supply plus including the 2V drop from the bridge rectifier you need 34V which is produced by a 24VAC transformer.Texas Instruments still make the 44V TLE2141 opamp in a DIL-8 package. Digikey and others have thousands in stock. I recommended it years ago, why use a tiny surface mount opamp that would probably get too hot? The new 140V tiny opamp does not spec its maximum output current when the supply is less than +/- 70V.Besides the Greek and Chinese kits not producing 30VDC at 3A and not being reliable, they might not be made anymore.

author: floobydust
author_rank: Super Contributor
timestamp: March 01, 2019, 06:43:56 pm
content: The original 0-30V PSU schematic has D10 and R15 which are needed.You can put a 1N4004 diode across Q4 E-C to protect against output overvoltages (instead of TVS) and it also stops the PSU failing when charging batteries.Lately, the kits are coming with wrong resistor R7 as 47R not 0R47 5W.

author: soldar
author_rank: Super Contributor
timestamp: March 01, 2019, 07:02:14 pm
content: Even though it is nominally 30 V I do not need it to go so high and I am happy to call it 24 or 25 volt PSU. I already have a 24 V transformer, rectifier bridge, filter cap, power transistors and some other components and I would only need to make the control circuit. I want to make it as simple as possible but also design something reliable that can be trusted to work. nothing fancy but definitely reliable and protected.Quote from: Kleinstein on March 01, 2019, 02:22:53 pmKleinstein, thanks for your input. Could you expand on this? I guess it is a matter of disabling the output for a second or two when first connecting the unit so that all voltages have time to settle? This sounds necessary.Quote from: Kleinstein on March 01, 2019, 02:22:53 pmGenerally I do not have the problem of connecting voltage sources to the output of the PSU and I would consider a reverse biased diode in parallel with the output to be sufficient protection. Again, can you expand on this?Quote from: Kleinstein on March 01, 2019, 02:22:53 pmWell, the specified op-amps have a max supply of 44 V and I am planning on using a 24 Vac transformer which would not produce anywhere near that so away goes that zener.Quote from: Kleinstein on March 01, 2019, 02:22:53 pmThis seems to me to be more than is really needed for this type os simple PSU. A simple zener or a zener and a transistor should be good enough. I will study this more carefully.Quote from: Kleinstein on March 01, 2019, 02:22:53 pmI agree. I am going to call it 25 volt and the power transistors and other heat-generating resistors will go on the back, outside the box. I'm thinking "outside the box" here.

author: soldar
author_rank: Super Contributor
timestamp: March 01, 2019, 07:08:28 pm
content: Quote from: floobydust on March 01, 2019, 06:43:56 pmYes, easy enough. OTOH, putting that resistor there means a voltage drop so I would like to supply the op-amp with a couple extra volts supply. Maybe, do on the positive rail something similar to what is done on the negative rail to get a couple extra volts. I will think about this. It may be worth it. I'd like to read opinions on this.

author: soldar
author_rank: Super Contributor
timestamp: March 01, 2019, 07:11:29 pm
content: Quote from: kripton2035 on March 01, 2019, 02:30:28 pmIt sounds like kits for sale are not to be trusted and I think by the time we are finished re-designing this it might look very different.

author: soldar
author_rank: Super Contributor
timestamp: March 01, 2019, 07:22:46 pm
content: Quote from: Audioguru on March 01, 2019, 06:41:01 pmAudioguru, thanks for your input. What is your opinion on restoring R15 and getting a couple extra volts to supply the op-amp as I mentioned earlier?Quote from: Audioguru on March 01, 2019, 06:41:01 pmI am not too concerned with the final voltage I can get. If it goes up to 25 Volts I consider it enough. I am making this with used junk parts and I have a couple of 12 Vac transformers from halogen lights. Now they are being replaced with LED lights I have a whole pile of these transformers.I am not interested in buying the kits as they sound like crap but I would be happy to devote some time to finish with a good and reliable design which others can build. I do not want it to go to so many volts or amps. Foremost I want it to be simple and reliable.

author: rstofer
author_rank: Super Contributor
timestamp: March 01, 2019, 07:26:42 pm
content: How much does Q4 dissipate when Vout = 1V and Iout = 3A?

author: soldar
author_rank: Super Contributor
timestamp: March 01, 2019, 07:33:07 pm
content: Quote from: Kleinstein on March 01, 2019, 05:38:43 pmOK, let us call this project a 25 V PSU. As I have mentioned I already have 24 V transformers so the voltage at the filter cap will not go above, say, 32 V. I realize that maybe with high current demand the output voltage may not be able to go all the way to the max, That is something I can live with. So something like 25 volts at 1 amp, 24 volts at 2 amps, 22 volts at 3 amps is something I can live with.It sounds like the suggested Op-amps go to 44 volts so that should be plenty.I'd like to come up with a totally revised design we can all agree is reliable and well done. If it can use cheap and easy to find components I would prefer that over getting a few more volts out.

author: soldar
author_rank: Super Contributor
timestamp: March 01, 2019, 07:37:58 pm
content: Quote from: rstofer on March 01, 2019, 07:26:42 pmThe answer is "it depends". It depends on the collector voltage, which depends on transformer and filter cap. It depends on the output voltage being output. Since the current is divided between two transistors each one is dissipating half but the total still has to be dissipated.Very bad case: collector voltage 33 volts, output voltage 1 volt, current 3 Amp, total power dissipated: 96 W. better have some good heat-sinks!

author: soldar
author_rank: Super Contributor
timestamp: March 01, 2019, 07:41:16 pm
content: Quote from: rdl on March 01, 2019, 05:43:36 pmWell, I would say 24 or 25 volts is good enough for general work.Really, going much above 20 volts would require switching input voltages or generating a ton of heat. I would say 25 volts is about the limit with a single supply voltage and is generally enough for everyday work.

author: soldar
author_rank: Super Contributor
timestamp: March 01, 2019, 07:43:15 pm
content: Quote from: Kalvin on March 01, 2019, 02:55:51 pmNo, that is totally out of the question. I am going for "really simple, cheap, straightforward and reliable".

author: soldar
author_rank: Super Contributor
timestamp: March 01, 2019, 07:59:44 pm
content: Quote from: Kleinstein on March 01, 2019, 04:20:07 pmSo, assuming a 24 volt transformer, so about 35 volt max input voltage, what are the options for common, cheap, through-hole op-amps?

author: rstofer
author_rank: Super Contributor
timestamp: March 01, 2019, 08:22:02 pm
content: Quote from: soldar on March 01, 2019, 07:37:58 pmI must have missed that, the schematic in Reply #13 seems to show just one pass transistor. The original schematic in the OP shows two pass transistors. Either way, it's a lot of heat and that's why these projects turn south.There is a reason why transformers for bench PSUs have multiple taps and relays.

author: rstofer
author_rank: Super Contributor
timestamp: March 01, 2019, 08:27:14 pm
content: Quote from: soldar on March 01, 2019, 07:59:44 pmGiven that you will be designing a new board, why limit things to through-hole? Assuming that SMD components can meet the output current and dissipation requirements.As to high dollar op amps, what difference does it make for a one-off power supply? $1 versus $10 is pretty insignificant when compared to the total project cost. OTOH, if the goal is to commercialize the design, BOM costs matter.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 01, 2019, 09:09:54 pm
content: For a one off unit cheap OPs are not that important, but even if not looking for the price the choice of easily available DIP8 OPs with at least a 40-44 V supply is not that large. Ordering from Mouser or similar could be a hassle from some countries.Many audio OPs like NE5534, OPA604 would need more current from the negative supply. So possible but not not really attractive.The right version of the 741 would be an option - though slow. The OP27 could also work - though high in supply current. The mentioned MC34071 can be tricky to get in DIP. So an SO8 - DIP Adapter may be needed.The TLE2021 might also be an option - though also difficult to get in dip.Starting with 24 V AC, the positive supply may reach some 35 V worst case. In addition one needs a negative supply for many OPs - especially the TL081 in the kit really needs some -3 V headroom on the negative side for the input common mode. Even with an extra diode to limit it, this could need some -4 V. Single supply OPs could work with less negative supply. So if single supply one could get away with a slightly lower supply rating.With 24 V AC the circuit would likely not give up to 30 V out - worst case it's more like 26 V depending on the OP. So even with a 44 V rated OP one would be near the edge with 28 V AC that may be needed to really get 30 V out.With a slightly modified circuit one could a a few small improvements, already discussed in other threads:1) limit the worst case voltage with something like a Zener. So less headroom needed for high mains2) Add extra filtering for the OP, so that it would see less drop due to ripple. This can reduce the dropout voltage and thus alow a lower transformer voltage. It could also safe a little on the filter cap - which is still too small for 3 A.3) reduce the voltage for the other 2 OPs (they only need some 15 V)this can also speed up the onset of CC mode4) add back the protection against a peak on turn on / off5) maybe add some fixed fast current limit as a fallback for transients6) fix the shunt layout7) Limit on how far negative the set voltage can be drawn done by the CC mode regulator: just a diode to ground as a hard clamp.Even the modified circuit would like an OP that can stand some 35 or maybe 40 V depending on the common mode and output range.

author: soldar
author_rank: Super Contributor
timestamp: March 01, 2019, 09:37:40 pm
content: Quote from: rstofer on March 01, 2019, 08:27:14 pmThrough-hole is much easier for beginners, repairs, etc.You are right that for a one-of build the cost of the op-amp is trivial but I am just thinking of making it a good design in every way and that is one aspect.I am looking at a commercially-bought bench PSU which I reverse engineered when it failed and it has a single LM723 which includes voltage reference, current limit, etc. This unit switches transformer windings to limit transistor dissipation but I am thinking I could copy the schematic removing the transformer switching part.Another thought is that secondary voltages which require very little current can be obtained using transformers from old wall warts. I have several crates of these.

author: floobydust
author_rank: Super Contributor
timestamp: March 02, 2019, 04:33:48 am
content: These 0-30V 0-3A PSU threads always seem to fizzle out because the requirements are never nailed down.People want it all - low cost, reliable, high performance, easy build.But this is unrealistic starting with a $5 kit that is based at the absolute max. ratings of many parts, as a chinese design prone to smoke.So right off the bat, people have to add parts and cost just to get the PSU reliable and meeting its original specs, capable of running more than a few seconds at rated output.Audioguru did fix things by adding an additional pass transistor and lowering Vcc for the op-amps.I would add power-up/down overshoot & backfeed protection and limit Q2's dissipation to make it dependable, like the odd Mastech and Manson models.For high performance I use an auxiliary transformer and regulated supply for the op-amps.To keep things cheap, change to rail-rail op-amps or one's with PNP diff amp. and consider a Sziklai NPN power transistor configuration.

author: not1xor1
author_rank: Frequent Contributor
timestamp: March 02, 2019, 06:06:24 am
content: Quote from: soldar on March 01, 2019, 09:33:40 amI think that the only possible comment for this PSU design is:I wonder how many ms can it survive Bob Pease's file test

author: soldar
author_rank: Super Contributor
timestamp: March 02, 2019, 08:47:43 am
content: Quote from: floobydust on March 02, 2019, 04:33:48 amYes, I have come to realize it's the "design by committee" phenomenon. In the end each one has to take the initial design and tweak it for his own needs and desires.I am attaching the schematic as I was redoing it. No great changes, just re-organizing it to make it simpler to analyze and understand. The more I think about it the more I think I won't go ahead with the project. The center op-amp which provides the reference voltage can easily be eliminated and replaced with a few discrete components. The right hand op-amp which provides the voltage control can also very easily be replaced with a few discrete components. The current sensing resistor dissipates a bit too much heat when it can be replaced by a much lower value and sensing with a current mirror as I have done in the past. Still, it is fun analyzing how it works.Many people seem to build a circuit they do not understand, just to save some money, and then go crazy when it does not work, sometimes by bad design but most often by mistakes made when assembling it. For me most of the fun is analyzing and improving circuits.

author: soldar
author_rank: Super Contributor
timestamp: March 02, 2019, 04:00:20 pm
content: I am looking at a bench adjustable power supply which I built some twenty years ago or more. It is very similar in concept but only uses discrete transistors. Three transistors and few other components do the voltage regulation and reference voltage which takes two op-amps in the circuit we are considering. Now, maybe some characteristics are better with op-amps but you cannot compete with the simplicity of the discrete transistor schematic.

author: floobydust
author_rank: Super Contributor
timestamp: March 02, 2019, 07:30:23 pm
content: It's a good building block for voltage control. The pass transistor config (double or triple Darlington/Sziklai) is what I think is best.But... discrete transistor solutions fizzle out doing the adjustable current-limit feature. It's always difficult to measure mV without drift and linearity problems, more so on the high side (if you use the sense (emitter) resistors). I haven't seen any good variable current-limit solutions with discretes or even the LM723 PSU's.

author: soldar
author_rank: Super Contributor
timestamp: March 02, 2019, 08:17:10 pm
content: Quote from: floobydust on March 02, 2019, 07:30:23 pmhmmm... I have a commercially built 30V - 5 A variable power supply which uses LM723 and the current limiting works fine. I also have several self-designed and built units with discrete components and they also work fine. Where do you find the problem?

author: floobydust
author_rank: Super Contributor
timestamp: March 02, 2019, 10:06:04 pm
content: LM723 has a current limiter for short-circuit protection, it's not adjustable with a potentiometer because it's a simple transistor E-B junction. Changing sense-resistors with a rotary switch is something my 1978 Radio Shack lab dual bench supply does lol.This is where the antiqueMC1466Ldoes better, but it needs a dedicated floating rail. It's almost discrete - two diff amps and some glue.If you have seen a way for PSU adjustable current-limiting without a negative rail or dedicated floating rail, let me know.On the chinese kit, I hate the -ve rail and the need for the V ctrl op-amp to run at the top rail while the I ctrl op-amp runs on the low-side. I can find away around it all, but haven't built it to prove it works.So far, the only low cost idea is a low-side sense resistor and op-amp with PNP diff amp inputs, like LM358 (too slow) or MC34071 looks ok, for inputs that can go to ground. This is staying with cheap op-amps, not the ones worth twice the entire kit price.

author: soldar
author_rank: Super Contributor
timestamp: March 02, 2019, 10:47:00 pm
content: Quote from: floobydust on March 02, 2019, 10:06:04 pmI see what you mean. Well, yes, it is a limitation and I have used different work-arounds. As you say, a separate floating rail is one way. That is what commercial Chinese PS does.What I usually do is have a fixed resistor and a pot in parallel so it allows a certain range of adjustment but, yes, it is not the best. I have never thought of switching resistors but it is not a bad idea. The pot in parallel allows continuous adjustment.What I have used with good results is a current mirror configuration. That requires a very low resistor value to produce a significant signal so that the heat dissipation in the resistor is kept low.

author: soldar
author_rank: Super Contributor
timestamp: March 03, 2019, 08:00:04 am
content: Quote from: floobydust on March 02, 2019, 10:06:04 pmCome to think about it, switching resistors seems like a very bad idea because you are left with no resistor while switching.Now that I think about it, many years ago I did something like this but it only had two ranges, high and low, so the switch only connected and disconnected a second resistor in parallel. The current indicating galvanometer instrument was connected to the same resistor and so the power supply had two ranges, 2A and 0.2A. In the low 200 mA range the instrument always indicated just under 10 mA, even with no load connected, because of some internal current in the PSU.So I am coming to the conclusion that the basic voltage regulation can be done with discrete components or with an LM723 or other op-amp and the current regulation can be done with discrete components but it means using several transistors and so this part may be done with an op-amp to make it with fewer parts. The op-amp for the reference voltage I think can be eliminated.The advantage of using an LM723 is that it includes the voltage reference.Regarding using secondary floating voltages, I understand the desire to avoid them in commercial products where cost is a consideration but for one-of home-brews I have several crates filled with wall warts that I would be happy to put to some use.

author: not1xor1
author_rank: Frequent Contributor
timestamp: March 03, 2019, 08:26:15 am
content: Quote from: soldar on March 03, 2019, 08:00:04 amis it so hard to conceive a resistor permanently connected and just switch other resistor in parallel to that?One might even use low Rdson MOSFETs for switching a binary network of resistors.In any case that kind of current limit depends both on Vbe drift, temperature and the output voltage so a few milliohms of drift would be the least of the problems.

author: iMo
author_rank: Super Contributor
timestamp: March 03, 2019, 10:34:43 am
content: This is something which works nice at least in the simulation. Experienced users may try.The R5 is actually an 1k pot/trimmer where you can set the current limit from 20mA to full in pretty linear way.The zener and 1k resistor is not related to the current limit circuitry (you may remove it).The Q3 feeds a constant current (around 800uA) into the base of the internal CL transistor. That current is being "balanced" against the current set by the R5, sensing the 0.56ohm shunt resistor.The 1n4148 with its series resistor have been selected such the scale of the R5 pot is somehow "linear".EDIT: this is from my simulation in adifferent thread:Code:R5 (w/ 100ohm in series) I_shunt_CL (0.22ohm)1000 0.82mA900 22mA800 157mA700 315mA600 503mA500 732mA400 1.02A300 1.37A200 1.75A100 2.14A0 2.53A

author: Kleinstein
author_rank: Super Contributor
timestamp: March 03, 2019, 10:43:08 am
content: Switching the current limiting shunt is not that simple, but can be done. Just a 2 nd resistor in parallel would add the switch / MOSFET resistance to the lower shunt - this can be a problem with a 0.1 Ohms or similar shunt. Given that in a 723 type current limit the accuracy is poor anyway, a MOSFET would be acceptable.The better way would be to have the 2 shunts in series and have a fixed path to read the voltage. The switch would change the path of the current, going one way or the other. Ideally one would need a make before brake type switch. If not, a diode could help.In the original circuit the OP for the reference can be eliminated, but a cheap OP (LM358 class) is not a bad solution to keep the current through the reference constant. This can help as the current through the reference would also end up to go through the shunt. The OPs for the reference and current regulation don't need a high supply and can be relatively simple types - down to the LM358 level, though the current control may like a more precise and maybe slightly faster one. Its already way to much text about possibly saving a 20 cent OP.

author: rstofer
author_rank: Super Contributor
timestamp: March 03, 2019, 05:17:28 pm
content: Quote from: not1xor1 on March 03, 2019, 08:26:15 amIt might be easier to use two or more very small resistors in series and then just short them out to change ranges. No make-before-break required. Of course, the switch will need an adequate current rating.

author: Audioguru
author_rank: Super Contributor
timestamp: March 03, 2019, 05:59:45 pm
content: I think if you are going to make the old Greek kit or a Chinese copy then the proper opamp that is still available should be used and its input offset adjustment should be correct like this:I also got it wrong, this correction is as per the datasheet:

author: jaycee
author_rank: Regular Contributor
timestamp: March 03, 2019, 06:33:23 pm
content: Having recently built a 0-18V 0-1A bench supply, and plans to build a 0-30V 0-3A, here's my five cents on the matter.First off, powering the opamps from the same supply as the raw supply voltage is fraught with issues. The best way to alleviate these problems is dont do that - most linear bench supplies have a separate supply voltages for the opamp, with the midpoint tap connected to the main regulator's output. This is a bit of a headf*** at first, but it works extremely well - i believe it was HP/Harrison Labs who came up with this "Regulator within a regulator" topology.This may sound complicated because you need the extra transformer tappings, but you can always use a second small transformer, it doesnt need to be powerful, so some tiny 5VA thing from an old digital bedside clock works very well. In fact that is exactly what I used when I was prototyping my supply... and I could have built the end unit like that too, I just happened to find a suitable transformer core that I could re-wind with custom secondary taps.Tap switching is well worthwhile doing to vastly cut regulator dissipation. Use a 2x18V transformer for 0-30V (remember it sags vastly under load so you need quite a bit of overhead), connect the windings so you have 0-18-36, and you can easily arrange a circuit which can switch between the 18V and 36V AC taps.I've attached the main schematic of my power supply. It works well, doesnt oscillate or do stupid things under load, and uses only bog standard TL082 opamps. I haven't shown the front panel board, but it is only metering and display. The controls are 10K linear pots wired to CN2 with their top wired to VREF, the bottom wired to AGND, and the wipers to V_FINE, V_COARSE, and I_CONT respectively.

author: jaycee
author_rank: Regular Contributor
timestamp: March 03, 2019, 06:41:23 pm
content: Here's a few pictures I took during prototyping it. As you can see, there's a tiny transformer with 9-0-9 secondaries. This provides enough for +/- 5V rails, and the raw rectified voltage is good enough to switch a relay with.

author: floobydust
author_rank: Super Contributor
timestamp: March 03, 2019, 08:56:53 pm
content: Using separate (floating) rails for the op-amps is best, it gets rid of almost all of the problems but adds cost and parts: additional small power transformer, fuse, bridge, filter caps, voltage regulators etc. The extra $10 might not be desirable, on a $5 kit.It also makes new issues:When you switch power on/off, the op-amps' rails and reference may or may not collapse/rise before the main DC voltage. Power-off under a heavy load, the main DC falls before the opamps & reference, but under a light load the main DC can stay up after the op-amps' rails first dropped off. Power on, you have to scope the rails rise and make sure the op-amps' rails come up ahead of main DC.Either way, there is an unpredictable output overcurrent/overvoltage spike to look after.TL0xx series JFET input op-amps exhibit phase-reversal at low common-mode voltages, below 3-4V above the negative supply.These $0.10 op-amps do a "surprise" which can smoke your load with overvoltage or overcurrent when you weren't looking.So I'm not sure about the mod lowering the -ve rail from -5.1V to -1.4V and deleting the UVLO, not introducing a new problem.edit: not relevant with mod's different op-ampTLE2141

author: jaycee
author_rank: Regular Contributor
timestamp: March 03, 2019, 09:11:53 pm
content: I dont have any issues with undervoltage on my build, but... I do have the OUTPUT_ON connected to a microcontroller on the front panel, which will drive the output off when power sinks too low such that it's brownout detector puts the processor into reset. It would be trivial to make OUTPUT_ON controlled by an "AC loss" detect circuit too.Phase reversal also shouldnt be a problem here, as it's unlikely the opamps will ever see anything near their rails.. however I used TL082 because I had them... I suspect any opamp would work just fine, though the compensation might need adjustment. I think I actually prototyped it with a 4558 first as I had it to hand.The cost of the additional small power transformer (not a cost at all if you can wind your own transformer) + associated bridge diodes, filter caps and regulators are small when the end result is a much better power supply.

author: Audioguru
author_rank: Super Contributor
timestamp: March 04, 2019, 12:11:02 am
content: Quote from: floobydust on March 03, 2019, 08:56:53 pmThe datasheet for the TL081 with phase reversal shows its input common mode range is 5V above its negative supply, the newer TLE2141 without any phase reversal has its range all the way down to and including its negative supply.One TL081CP at Digikey costs $0.53US today but in China a copy or reject might cost $0.10US. The TLE2141CP must be ordered at a minimum of 400 at Digikey but they sell one TLE2141IP at $2.09US today.One TLE2141 opamp in the 0V-30V circuit needs a -1V negative supply only because its output must go a -0.7V doped voltage drop less than the circuit's 0V when current limiting a shorted circuit output. Then its positive supply is safe to +42.7V.

author: floobydust
author_rank: Super Contributor
timestamp: March 04, 2019, 01:48:11 am
content: fixed - I missed the change toTLE2141in OP for your mods.Trying to improve this $5 chinese kit- without doubling its costs, is a good challenge. There is no other kit out there for beginners, aside from LM317 kits but no adjustable current-limiting feature in them.This PSU kit uses no op-amp IC's, just four TL431'shttps://www.eevblog.com/forum/beginners/cheap-chinese-adjustable-lab-power-supply-kit-on-ebay-(-3ag1-xiaolin-)/msg1412511/#msg1412511Try to do adjustable current-limiting with a TL431, for fun as a design puzzle.Voltlog #155found it has stability problems, CC oscillates.

author: Audioguru
author_rank: Super Contributor
timestamp: March 04, 2019, 01:53:56 am
content: "Cheap" usually means "poor quality".

author: floobydust
author_rank: Super Contributor
timestamp: March 04, 2019, 08:28:20 am
content: I didn't realize this chinese kit has been out there for 10 years!1000's of postings across the Internet trying to improve it, or suggest throwing it in the garbage. It's some form of turd that keeps on smelling.I find it sad one weak design can proliferate and the rest of the world cannot be organized well enough to fix it. Communism++For OP, perhaps check CC mode. It looks like the -1.4V rail adds hum to CC mode which brought about a change to LM337T:Paul's DIY electronics blogis at revision 6 but 5 looked more complete, sch attached.I found a higher rev (original mods) schematic with the shutdown transistor left in.late edit: I found theoriginal 0-30V 2mA-2A PSU circuit, Practical Electronics Nov. 1978.Project on pg. 41 but big 12MB pdf:https://americanradiohistory.com/Archive-Practical-Electronics/70s/Practical-Electronics-1978-10.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2019, 09:29:38 am
content: The OPs for the current regulation and reference do not need to be powered from a high supply - the modified versions often include the series zener or a 7824 regulator. So it is only the OP for the voltage control, that needs to have a high voltage swing (e.g. TLE2141,MC34081,OPA604,...).It is funny to have this old kit still around, with so many points that should be fixed or made better with not that much effort.Using a separate floating supply for the regulator changes the circuit completely, going from a classical regulator with a low output impedance power stage to a low drop type regulator with a current setting output stage. The floating regulators has advantages (less voltage lost, usually better current control and flexible in the voltage range), but it also has a more complicated compensation for the voltage control and still often slower, unless tuned very well.

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2019, 12:32:32 pm
content: Quote from: floobydust on March 02, 2019, 10:06:04 pmA negative supply isn't needed, if single supply op-amps are used. Here's a proof of concept, based on a Howland current source, with voltage limiting. I haven't simulated the transient response, which is probably horrible. U1 is shorted by Q3 and current limits, when in voltage mode, which some may not like, but that can be avoided if necessary. The Darlington pair could be driven by a current mirror based source, with each op-amp diode AND'd pulling it down.I don't see the problem with the negative rail. I'd probably use the OP07 run off regulated +34V and -3V, derived from a 30V transformer.

author: iMo
author_rank: Super Contributor
timestamp: March 04, 2019, 01:23:05 pm
content: Update: The stability analysis..

author: iMo
author_rank: Super Contributor
timestamp: March 04, 2019, 02:24:52 pm
content: And some minor resistor's values changes and notes..Update: Added V/A(out) as a function of R_Load.

author: Audioguru
author_rank: Super Contributor
timestamp: March 04, 2019, 03:25:06 pm
content: I first saw the original Greek kit 15 years ago at Eletronicslab.com where its problems were discussed in the forum there. Many parts were overloaded and failed.Paul's version will not produce 30V at 3A because his 24VAC transformer voltage is as low as the original Greek kit:1) 24VAC has a peak of +34VDC which is reduced to +32V at 3A by the bridge rectifier.2) Ripple from the small filter capacitors drops 2V to +30V.3) The voltage amplifier TLE2141 opamp maximum output drops 1.6V to +28.4V.4) R15 that is not needed drops 2V to +26.4V.5) The driver transistor drops 1V to +25.4V.6) The output transistors drop 0.5V plus 0.3V for their emitter resistors to +24.6V.7)At 3A the 0.47 ohms current sense resistor drops 1.4V to +23.2V.If the opamps and transistors have maximum spec's then the maximum regulated output might be +26V.The negative supply is not needed for the voltage amplifying opamp since its output can go down to 0.5V above its negative supply that can be the circuit's 0V.

author: iMo
author_rank: Super Contributor
timestamp: March 04, 2019, 03:31:08 pm
content: And finally the stability with a capacitive load.15deg phase margin - it needs to play with compensation..

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2019, 03:55:27 pm
content: One could reasonably simple modify the kit circuit to work essentially without a negative supply. The main point they use the negative supply is for the current control to pull down the set voltage alle the way to zero. So if one shifts the zero point for the set voltage to some +2 V there is no more need for the negative supply. So the feedback divider would go towards this auxiliary 2 V instead of GND and the set voltage too.There is a slight downside that the output would not drop all the way to zero without a minimum load - depending on the minimum load circuit there could be some 50 mV or so as a minimum. To really go to zero the minimum load would need the negative supply. So the need is back, but the regulator could still work without it, just not below some 100 mV with no load.The main limitation of the circuit is that the output OP needs to make the full voltage swing and a little more voltage lost. The good point is the the voltage compensation can be simple.

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2019, 04:29:39 pm
content: Quote from: imo on March 04, 2019, 03:31:08 pmThe first iteration had a gain of 10, but I changed it to 12V, because 2.5V is a common voltage reference: there was a reason for that seemingly odd number.You'll need a higher input voltage than 30V to get 30V out, probably at least 34V.Try simulating the transient response. Change the load current from 0A to 1A and 1A to 3A and back to zero again. If you use a current sink, then don't forget to click "This is an active load" in the parasitic properties section, otherwise it will try to force the output to sink current, when the output transistors are off.Another option is to use an if function with the load resistor.It works like the Excel.if(condition, value_if_true, value_if_false)Suppose you set the value of RL to:R=if(time>10m&time<20m,1,10)It would set the value of the load resistance to 10Ohm and change it to 1R between 10ms and 20ms.See example attached. Note the current and voltage spikes. I imagine the OP07 will be worse, then the ideal op-amp model I used with GBW=10Meg Slew=10Meg. You're right, adding a parallel capacitor would reduce the voltage spikes but make the current spikes worse. A series inductor would do the reverse: better current regulation at the expense of voltage. Care needs to be taken to avoid oscillation.I've noticed the current regulation is poor for lots of bench supplies, which are optimised for voltage. I've toyed with the idea of building my own, with a switch to select between better voltage or current regulation.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2019, 05:23:36 pm
content: Current regulation is often not that good for different reasons:One is having a relatively large capacitor at the output. This limits voltage overshoot, but can add significant transient extra current.Another point is that a good shunt is expensive - so they tend to use a relatively low value shunt to limit the power dissipation and self heating. Still heating can be a problem in cheap supplies. Without range switching it get's tricky to measure small currents with a shunt made for 3 A.Also current regulation is often considered less important - more like a way to protect the supply and maybe circuit.Many commercial supplies use the floating regulators, that are in principle quite good in current regulation, except for the output capacitor.So there is not much option to switch the supply to a mode better for current regulation, as the output capacitor is required to get stability.In principle one could use a faster, tighter tuned controlled loop for the voltage control, so that less (and with well defined ESR) capacitance at the output is sufficient. So it would be more like improving voltage control to alow a smaller capacitor and this way faster current regulation. Adding range switching would be another step. I don't see much need to really switch the regulator to a separate mode - maybe add an optional addition capacitor to the output if needed to support the voltage regulation.The other point is a delay before the current liming sets in - this sometimes is desirable, but sometimes not. So this could be a switchable point.The circuit as shown, with an emitter follower at the output tends to produce current spikes from the regulator, as it takes some time for the CC regulator to kick in and the OP has to slew down - which essentially limits the rate on how fast the voltage can drop. It can work with less cap at the output, but there is a kind of simulated capacitance.

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2019, 08:39:32 pm
content: Quote from: Kleinstein on March 04, 2019, 05:23:36 pmYes a capacitor on the output doesn't help with current mode, but I think a glitch when switching back and fourth constant current and voltage is inevitable. When in CV mode, the CC amplifier will be trying to force it to limit the current and vice versa. Whether there's a voltage or current spike, depends on the design. Even if the rapid change on the output of either error amplifier can be greatly reduced, there's still the issue of the output having to rapidly change from a very low, to high impedance and back again.Yes, whether allowing current surges or not depends on the application. Recently I've being using a bench supply to power some LEDs and have had issues with huge current spikes at power up. Fortunately it didn't damage the LEDs, but it isn't good. It isn't the output capacitor because it should be discharged when the power is applied. I will try connecting a short circuit, in parallel with the LEDs, during power up.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 04, 2019, 10:27:14 pm
content: The power up phase is a different issue from the normal regulation. In better supplies there is some extra circuit to keep the output off before the supply to the control is fully there and when the supply breaks down (turn off - which can be the more difficult case to detect). In the simple form this could be a way to disable the output if the raw voltage is below a certain level, needed to operate the OPs.The simple circuits have usually quite some glitch when switching between CC and CV mode, because the non active regulator will often run all the way to it's limits. In normal control theory this is call windup and in better regulators handled separately. This is not standard for lab supply circuits, but it is possible to add this to the circuit. With some extra circuitry the change over can be quite smooth.The circuit from the start of this thread uses a slightly unusual form, as the CC mode reduces the set voltage and thus works with both regulators in series. So there would be no (or only a small) spike when going from CC to CV mode as the voltage regulation is always active.

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2019, 10:59:34 pm
content: Quote from: Kleinstein on March 04, 2019, 10:27:14 pmI'm interested to see how the seamless switching between modes works.The original draft of my Howland current source based design did a similar thing to the circuit at the start of the thread, except it was the CV amplifier reducing the CC amplifier's set current. I only changed it to challenge the myth that a negative power supply is required, otherwise I probably wouldn't have posted it, since it was just a quick sketch, rather than a complete design.

author: not1xor1
author_rank: Frequent Contributor
timestamp: March 09, 2019, 06:31:33 am
content: Quote from: Zero999 on March 04, 2019, 12:32:32 pmI ran a few tests on variations of this circuit with LTspice.The circuit might work without negative supply with a LT1013. It has a better phase margin with low loads if you use 100k/470k resistors.The main drawback is the resistor tolerance. Even just 1% of umbalance would cause about 15% of current variation vs. load value (or output voltage if you prefer). 0.1% tolerance resistors would provide about 1.7% current variation which might be acceptable in a cheap DIY PSU provided that aging and temperature drift do not make it much worse.BTW the output current can be calculated like:I = (Vref * Rhot) / (Rsense *Rgnd)whereRhot is the resistor connected to the emitter/outputRhot are the 2 resistors connected betwen the BJT emitter/output and the opamp inputs, Rgnd are the resistors which connect the opamp inputs to ground... and Rsense... guess it.

author: soldar
author_rank: Super Contributor
timestamp: March 09, 2019, 09:34:20 am
content: Quote from: Zero999 on March 04, 2019, 04:29:39 pmI think this is natural for starter, basic power supplies because they are used mainly in voltage mode and the current limiting is just a rough setting for protection and is not as critical as the voltage setting.

author: Tek Tech
author_rank: Contributor
timestamp: April 24, 2019, 12:41:34 am
content: Quote from: Kalvin on March 01, 2019, 02:55:51 pmThat's a brilliant suggestion! Do you have any idea how to make it relatively simply?

author: Zero999
author_rank: Super Contributor
timestamp: April 24, 2019, 07:41:39 am
content: Quote from: soldar on March 09, 2019, 09:34:20 amPerhaps in the past but nowadays lab PSUs are often used to power LEDs, in which case the constant current setting is more critical.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 24, 2019, 10:36:29 am
content: A good current regulation is not often required and it also adds quite some costs for the shunt. To get good resolution at low currents the shunt can not be very small and with more drop the self heating of the shunt can limit the stability of the current regulation.Another point is that quite often the capacitance at the output is quite large and this makes the dynamics of the current regulation not that good.

author: Zero999
author_rank: Super Contributor
timestamp: April 25, 2019, 10:39:38 am
content: Quote from: Kleinstein on April 24, 2019, 10:36:29 amI would quite happily pay good money for a lab power supply with both decent voltage and current regulation. I don't demand anything fancy, just reasonable current regulation (5%) from 30mA to 3A, without high current pulses, more than double the setting.If it's well-designed, an output capacitor shouldn't be necessary. Any circuit which needs a power supply, with a low impedance at high frequencies, should have a decent decoupling: the transient voltage response, of a lab PSU, isn't as critical, as many believe it to be.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 25, 2019, 03:15:39 pm
content: The output cap is there for 4 reasons:1) some capacitance could be needed for the stability of the CV loop (usually not very much though), especially if made to also tolerate a larger capacity.2) To avoid to much voltage drop on pulsed load the capacitor can reduce the drop, though making the time longer. Kind of having the cap to cover up a slow regulator and making it even slower3) To reduce possible voltage overshoot on transients (turn on, CC to CV transition)4) for the stability of the CC mode with inductive loads (this usually does not need much).Especially point 3 may need quite some capacitance.A very fast acting current limit can also be quite confusing as it may be too fast for some applications.

author: Zero999
author_rank: Super Contributor
timestamp: April 25, 2019, 07:57:36 pm
content: Oh I know why it's there. I just think it can be greatly reduced, if not eliminated.Local decoupling (which you should have anyway) will overcome the issues with pulsing and overshoot for sensitive loads. Seriously, who connects a microcontroller up with no decoupling? Even with the capacitor, inside the PSU, there can be spikes due to lead inductance.I think a switch to select between optimal current and voltage modes. One day I might get round to putting together a proper design.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 26, 2019, 06:08:17 am
content: The usual circuits don't have much anti-windup for the regulators. This gives some extra current peak when going from CV to CC mode - this may be not that bad and even could be a good thing in some cases. However for the CC to CV transition this gives an extra charge pulse and thus possible voltage overshoot - a larger cap at the output is the easy way to limit this overshoot. Here one can not really rely on the cap at the circuit itself. In addition it does not look good if there is a 5 V overshoot without an extra cap at the output.Just for stability a capacitor in the 1 µF range could be sufficient for many circuits.I agree that the transients in CV mode are not that important, but a fast regulation is also the obvious way to reduce the capacitance at the output.

author: wandows
author_rank: Contributor
timestamp: July 28, 2022, 05:56:56 pm
content: Hello everybody! I would like to count on the help of friends!!!In the offset adjustment, my power supply has the output voltage >= 32.9V, and does not reduce to 30V.Any tips on what can be done to solve this problem?

author: antoniodv
author_rank: Newbie
timestamp: September 07, 2023, 08:32:14 pm
content: Hi everyone.Thank you all for the information you have shared.Inspired by the best electrical diagrams of linear power supplies made in the past and available online, I digitized them by adding a display microcontroller and DAC to have everything in a single PCB.As you can see in attached pdf , for the final stage I used the K7200 scheme.The firmware for ATtiny3216 is also ready.I'm waiting to complete the final PCB, all tests went well. I just doubt the voltage peak at shutdown I would like to remove it. Advises?

author: MrAl
author_rank: Super Contributor
timestamp: September 07, 2023, 11:59:38 pm
content: Hello there,My question is mostly for the original circuit.Why use a circuit so complicated? What are the specs and do the specs warrant such a complex circuit design?If it was 30 amps I could see it, but 3 amps, that's nothing these days.As to the newer circuit, still a bit complex. What are the specs.Maybe a related question:What specs are you looking for.

author: xavier60
author_rank: Super Contributor
timestamp: September 08, 2023, 12:05:05 am
content: Maybe the -3.3V is discharging too quickly. Im curious about the CV loop's stability?The design that I like, doesn't need a negative control rail,https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg3582664/#msg3582664

author: MrAl
author_rank: Super Contributor
timestamp: September 08, 2023, 08:13:47 am
content: Quote from: xavier60 on September 08, 2023, 12:05:05 amHi,I've used the LM358 and LM324 variety of op amps for these things, but in one power supply I wanted faster response so I went with an IC that has a similar pinout but has a much faster slew rate.

author: xavier60
author_rank: Super Contributor
timestamp: September 08, 2023, 08:49:22 am
content: I used the TLC072 in my build of the deign I linked. It gave faster transition from CV to CC during abrupt overloads.