oh no!, that 0-2mA; 0-30V power supply kit again! - Page 1
Forum section: electronics / beginners

author: digik
author_rank: Regular Contributor
timestamp: January 10, 2025, 04:39:51 pm
content: Hi guys, sorry to annoy you again with this kit but after reading hundreds of posts in various forums, i found this post:https://www.eevblog.com/forum/beginners/0-30v-0-3a-psu-audiogurus-version/25/in reply # 42 the user "jaycee" says that the opamps can be powered with an extrenal floating regulated transformer.I have a 12V transformer but how sould I connect my 12V to the opamps? should I simply stick my 12V across pins 7 and 4 of the TL081s?But then, how can my PSU give me 30V if the opamps supply is limited to 12V? the user jaycee said that he powered the opamps with an old clock radio transformer of 9V?Sorry for the stupid question, but I'm not familiar with opamps.Thanks in advance

author: special_K
author_rank: Regular Contributor
timestamp: January 10, 2025, 11:05:19 pm
content: Transformers create alternating current. You can't just feed it directly into the power rails of an op-amp.You need a bridge rectifier and a smoothing capacitor, and also ideally a fixed voltage regulator.

author: digik
author_rank: Regular Contributor
timestamp: January 11, 2025, 06:57:38 am
content: of course, the transformer is rectified and stabilized before feeding into the opamps. But the question remains: is it possible?

author: ledtester
author_rank: Super Contributor
timestamp: January 11, 2025, 06:14:30 pm
content: Quote from: digik on January 11, 2025, 06:57:38 amIt's possible but it's a very different design and I doubt the existing PCB could be reused.Quote from: digik on January 10, 2025, 04:39:51 pmThe idea is to reference the ground of the op-amp supply to the output of the power supply.You can see this happening in this schematic:https://www.eevblog.com/forum/projects/linear-power-supply-based-on-hpagilent-e3610a/(look at the e3610.JPG attachment)Note how "+S" (the power supply output) is also the ground of the Reference and Bias Supply. This makes the operating range of the op-amp equal to the supply output voltage +/- 12V.Some more discussion on the original design:https://www.eevblog.com/forum/beginners/bangood-psu-enhancements/https://www.eevblog.com/forum/beginners/0-30v-0-3a-psu-audiogurus-version/https://www.eevblog.com/forum/beginners/diy-0-30v-0-3a-again/https://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.htmlhttps://www.paulvdiyblogs.net/2017/07/my-new-power-supply.html

author: Jwillis
author_rank: Super Contributor
timestamp: January 11, 2025, 08:57:58 pm
content: Quote from: ledtester on January 11, 2025, 06:14:30 pmThat's not entirely correct. The TL081 is capable of the voltage having a maximum operating voltage of 36V. But the problem is when the circuit is turned on. The inrush current drives the voltage to high. That's what kills TL081. The solution is very simple. Use a different opamp like the humble LM741 which has a Maximum operating voltage of 44V. Or The LT1637 with a maximum operating voltage of 44V , which is the one I use. You can swap then out directly and the power supply will work without the hazard of the op amps burning out.It's a misconception that you need a high slew rate for DC. Even for the current control it's completely unnecessary.Minimum Slew rate = 2*Pi*Frequency*VoltageFor DC,being Zero Hertz, The minimum slew rate will always be zero.In fact having an op amp with very high slew rates can increase the possibility of instability and oscillation.So just replace the op amps with LM741 an it will work just fine. Theirs no need to redesign the board at all. Lm741 op amps are super cheap.One problem that will occur is high ripple. That's because the filter capacitor is inadequate for 3 amps. You'll probably get at least 1V of ripple under full load.Adding extra filter capacitors only increases the inrush current.putting the problems with the op amps burning out right back where you started.The solution for that is to remove the rectifier and capacitor from the board and rebuilding that part, separate from the board, with a rectifier and LC filter. The LC filter won't require more capacitance and you will be left with next to no ripple at your output. For a 3 amp load you'll need a 10mH choke(inductor) minimum.

author: digik
author_rank: Regular Contributor
timestamp: January 12, 2025, 07:30:29 am
content: i give up: too much troubles for a stupid linear supply. shame that i have bought the kit before searching infos. I thought that linear supply was mature and simple enough so every kit would work just fine. i have found another much simplier schematic but i will open another thread soon for that. thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 12, 2025, 12:04:24 pm
content: The kit has some issues, but is fixable. The main point is to change the TL081 to something like 741 that can tolerate the supply voltage.With the original TL081 OP-amps the AC voltage should be no more than some 16-18 V and thus a maximum output of maybe 16 V.The circuit follows an old article that used 741 OP-amps.As usual the current rating is a bit optimistic for the power transistor, if a higher input voltage is used. So ideally limit the current to more like 2 A or so.Another lesser issue is in the layout, that leads to quite some hum in constant current mode.

author: digik
author_rank: Regular Contributor
timestamp: January 12, 2025, 08:41:04 pm
content: Quote from: Kleinstein on January 12, 2025, 12:04:24 pmKleinstein!, you were one of the greatest contributor for this project, along with audioguru of course.I've read almost all of your posts, but at least two of your suggested modifications/improvements are not clear to me:1) someone said that if this supply is used as a battery charger you get backfeed from the output transistors E-B junction to the op-amp and it damages it. Adding a 1N4004 protection diode across C-E of output transistor would fix that. But if i use the audioguru version with two transistors to share the current should i add this diode to both of them? also this diodes are connected with the cathode to the collector, right?2) you suggested to add a diode at the output pin of U2 and this diode are connected with anode at the output pin, right?Are this two modifications going to further drop the output voltage?Also, why are you saying that you cannot reach 3A? I thing that the audioguru version with 2 transistor can do!If I use a 25V 120VA transformer and TLE2141 Ops, what could be the maximum voltage?

author: digik
author_rank: Regular Contributor
timestamp: January 16, 2025, 07:13:33 am
content: Quote from: Jwillis on January 11, 2025, 08:57:58 pmdid you mean just throwing a 10uH inductor in series with the existing 3300uF capacitor solves the problem?

author: silly sausage
author_rank: Regular Contributor
timestamp: January 16, 2025, 03:33:52 pm
content: someone needs to correlate all the mods to this and present a sorted working solution, there are hundreds of posts about this so it needs putting together into one final version, as is its a mess!

author: digik
author_rank: Regular Contributor
timestamp: January 16, 2025, 03:39:08 pm
content: Quote from: silly sausage on January 16, 2025, 03:33:52 pmwhy? the conclusion is always that the project is crap, it never reaches 30V, it never reaches 3A it hums, it oscillates, and so on

author: Grandchuck
author_rank: Super Contributor
timestamp: January 16, 2025, 03:40:24 pm
content: 10 millihenry not microhenry

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2025, 04:08:38 pm
content: For 3 A of output current the diodes at the rectifier, the filter capacitor and the single output transistor from the original are too small. The whole circuit is more made for 1.5 maybe 2 A max. For a higher current I would really consider a ciruit with transformer tap switching to also reduce the heat.Keeping in mind the limits the circuit is not that bad, just not the promissed ratings.For protection against voltage at the output, when the power is off a single diode would be enough, even with multiple transistors. The transistors are in parallel with only the relatively small emitter resistors to make a small difference. Of cause the diode is with the kathode at the collector - otherwise it could conduct under normal operation.For the maximum voltage, one looses about 1.4 V for the darlington output stage, some 1 V with an TLE2141, some 1 V at the shunt and some 2 V at the rectifier. With the current peaks the transformer resistance will likely also some 1-2 V. This are some 7 V lost from the theoretical maximum. The emitter resistor with 2 transistors in parallel would add another 0.2 V. This would leave some 25 V if the mains voltage is 10% below nominal (or reduced peak from rectifier loads).The big point is however the ripple: With 3 A, 3000 µF and 10 ms to bridge this would be 10 V. With 2 A and 4000 µF one could reduce it to 5 V.So more like 15 V with 3 A and maybe 20 V at 2 A with a slightly larger fitler capacitor. Using a higher transformer voltage is not really an option as the max voltage also for the 741 is limited.It would need a rather large inductor (mH range) to make a difference at the rectifier. This can help with the power factor an also reduce the ripple a little, but the voltage still drops under load. This tends to be even more than the ripple without the inductors. 10 µH would be more a thing for EMI, not for the power factor.

author: digik
author_rank: Regular Contributor
timestamp: January 16, 2025, 04:59:27 pm
content: thank you kleinstein, so a 10mH in series with 4700uF cap (or more) solves the ripple.unfortunately vdrop is too high: 7 Volts lost are way too much

author: digik
author_rank: Regular Contributor
timestamp: January 16, 2025, 05:36:30 pm
content: also there is one thing: if i put the inductor in series before the caps (right after the transformer) what happen when power switches off? the magnetic field should collapse and create a big spike damaging the caps and the rest of the circuit?also can you show me how to calculate the inductor?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2025, 06:21:43 pm
content: The inductor at the rectifier is not a solution here. It can help a little with ripple, but the voltage still drops under load. The energy in the inductor is not that large. Usually it should be just enough to bring the capacitor voltage back to about the no load case. If turned off at the transformer the transformer is also relatively high impedance for the inductive kick back. So much of the energy can go to the transformer.Todays it is probably easiest to use a simulation like LTspice to simulate and understand the effect of an inductor at the rectifier.In part the voltage drop is a problem if the circuit, but parts (e.g. the shunt, diodes and drop at the transformer itself) are also hard to avoid. A simple rectifier will have some loss and variations. There are ways to reduce the voltage loss from the OP and power transistors with a separate filter capacitor.A larger filter capacitor can help with the ripple, but one usually still has to accept some residual ripple, like 10% of the voltage. A very large capacitor comes with even more pronounced peaks in the current.

author: Jwillis
author_rank: Super Contributor
timestamp: January 18, 2025, 04:20:05 am
content: Quote from: digik on January 16, 2025, 05:36:30 pmNo it won't hurt anything. The best way is to have a PI filter. After the rectifier theirs a Capacitor, Inductor (choke), Capacitor. The first capacitor is the "Bulk" The 10mH is the Critical Value (Minimum value).You can use this formulaLoad resistance/1200 for 60HzLoad resistance/1000 for 50hZLoad resistance is equal to peak output voltage divided by peak current R=E/ISo for example 30VDC and 3A we have 10 ohms, divide by 1200 = 8.3 milliHenries critical inductanceor 30V/3A=10ohms 10/1000=0.01H or 10 milliHenries critical inductanceI'm using a 2mH 30A choke with 2 50000uF capacitors on a 40V 20A project and get No ripple at 40V 20A load. Without the choke theirs over 2V of ripple. Chokes work.The need for a massive choke is a throw back from the vacuum tube days with very high voltages and low current. With low voltages and higher current the choke value needed, drops significantly.Now the reason you get sagging in your voltage has nothing to do with the choke. Its all about the transformer. The RMS voltage of the transformer Must be equal or greater than the peak output voltage of the power supply. So for example, If your required peak output voltage is 30V you should use a a transformer rated at 30V or greater. If your using a 24V transformer, the Peak voltage after rectification will be around 33V. But most of the 9V above the 24V RMS is basically unusable under high load.This is why I suggested a different op amp that works at a higher voltage. Unfortunately the LT1637 is fairly expensive.There are other less expensive ones that will work at 44V, but require an adapter PCB to plug into the provided sockets.I suggested the LM741, but in hind sight I realized that the maximum input voltage is only ±15 V which won't work. You need an Op amp with an input voltage that is much closer to the supply voltage.The power supply design in your first post doesn't match the original design from before the 1980's . The original design had per-regulation for the opamps. Pre regulation insures that the supply voltage never exceeds the maximum rating of the opamps. The original design works very well. In fact that basic architecture is used in many reliable linear supplies on the market. Unfortunately these cheap kits have given the original design a bad reputation.You can still use the kit, but it will work better using different opamps, regulating the opamp supply, using an adequate transformer with the addition of a PI filter.You can still use the TL081. but you will need to regulate the supply voltage to the opamps to 30V maximum. I also suggest reducing the negative supply Zener Diode to 3.3V to 4V. This will keep the op amps within their operating voltage.

author: BillyO
author_rank: Super Contributor
timestamp: January 18, 2025, 05:09:03 am
content: Quote from: digik on January 16, 2025, 03:39:08 pmIf you are okay with 20V/2.5A/50W you can follow this video series about this power supply:https://www.youtube.com/playlist?list=PLAoY7fARPMX_rywPLGCvLJU9BJl97MIJgThe results are quite good.

author: digik
author_rank: Regular Contributor
timestamp: January 18, 2025, 02:40:15 pm
content: Quote from: Jwillis on January 18, 2025, 04:20:05 amThank you very much, very valuable infos here.So, the first cap is the bulk (let's say 10000uF), then comes the choke (10mH) and what about the second cap?

author: Jwillis
author_rank: Super Contributor
timestamp: January 19, 2025, 05:56:05 am
content: Don't get to high value on the caps. Around 4700uf then the inductor then another 4700uF. That's more than plenty for 3amps. The more capacitance there is the higher the Inrush current. To Much can put a lot of instantaneous load on the transformer. This can damage the transformer and other circuits attached to it.It looks like the -3db cutoff with 4700uF and 10mH is about 23Hz, so that should be OK.I use large capacitors because of the extreme high amperage for my own project. 40V 20A is 800W. That's a lot of power. My circuit includes soft start to slow the charge up of the caps. Yours is 3A so you should be OK with an exceptable inrush current and no soft start.

author: silly sausage
author_rank: Regular Contributor
timestamp: January 19, 2025, 02:38:40 pm
content: were there not a few big companys in the 1970's using opamp psu designs similar to this turd?, if so how did it get turned into this abortion that sells on aliexpress etc?.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 19, 2025, 05:04:51 pm
content: It is a bit odd that the kit is still this way. It would not need that many changes to make it at least reasonable working (e.g. 741 OP-amps, PCB layout to avoid the CC ripple, ggf. extra filter / limit for OP-amp supply).The other problem are the exaggerated ratings.

author: BillyO
author_rank: Super Contributor
timestamp: January 19, 2025, 06:28:29 pm
content: Quote from: Kleinstein on January 19, 2025, 05:04:51 pmIf you switch to 741s you will be running two of them at the absolute maximum rating continuously. Not a great formula for success. And you better make sure you don't have the lower spec "C" version. The original 741 and 741A are out of production so you'd need to search for some old NOS parts or opt for the 741J @ $30 a pop. Makes no sense to blow $90 on op-amps for this. Then suppose you are foolish enough to do that and fix some other niggling problems, you'll be faced with a worst case 130 Watts of dissipation on that single pass transistor. Not going to happen any time soon with anything less than chilled water liquid cooling.By reducing the "specs" to 20V @ 2.5A (20V transformer, not 24) and making a few component changes, not limited to but including tripling the filter capacitor size, adding a very large heatsink and providing fan cooling, you can make this into a decent power supply. All the details are in that video playlist I posted above.No need to speculate about how to fix this, someone did the work already and ended up with a serviceable bench power supply.

author: Jwillis
author_rank: Super Contributor
timestamp: January 20, 2025, 03:23:34 am
content: Quote from: Kleinstein on January 19, 2025, 05:04:51 pmI agree. It's probably because people still get suckered and don't know any different. So it sells.There is another kit that works better. 0 to 30V 5A. It works really well and has tap switching built in. Unfortunately people would have to get a custom built 0V-15V-23V-31V Current 5A with extra 15V 1A Line Transformer or use multiple transformers. It's a shame they don't sell a transformer with the kit.I got a transformer custom made and the unit works way better than the one being discussed here.

author: BillyO
author_rank: Super Contributor
timestamp: January 20, 2025, 03:28:26 am
content: Quote from: Jwillis on January 20, 2025, 03:23:34 amWhere did you get this done and how much did it set you back?

author: Jwillis
author_rank: Super Contributor
timestamp: January 20, 2025, 04:14:55 am
content: Quote from: BillyO on January 20, 2025, 03:28:26 amThis is back in 2022, so prices may be different now.https://www.antekinc.comComplete order was for two transformers.AS-3435 - 300VA 35V Transformer Custom made 0-15-23-35V was $52.00 USDAlso got a AN-10440 - 1000VA 40V Transformer Custom made 0-10-20-30-40V for $119.00 USD

author: Electro Fan
author_rank: Super Contributor
timestamp: January 20, 2025, 05:22:07 am
content: A couple threads from a similar project:https://www.eevblog.com/forum/beginners/looking-for-power-supply-kit-to-be-assembled/msg3338264/#msg3338264https://www.eevblog.com/forum/beginners/question-on-power-supply-diodes/In the end the completed power supply seemed to operate reasonably well (at least for the price), and the project was fun and a good learning experience.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 20, 2025, 08:57:46 am
content: The nice point if the kit is that it gets away with a simple transformer. Though the choice of voltage is a bit tricky as one usually is rather close to (or above) the maximum voltage of the OP-amps. The more universal circuit is using 2 separate windings or transformers. Tap switching adds more voltages and the more commone center taped winding is not ideal, though it would probably be the best choice for a kit that does not come with the transformer.The other kit looks good and may resemble one of the simple lab supplies. Here simpler 2 tap swiching may have been the better choice.