Banggood PSU Enhancements

[soligen]

I bought a Banggood 0 - 30V PSU kit so that I could at least have something better than an old ATX psu to use.  Looking at the components, the 3A rating is rather generous, so I am looking to beef it up a bit to at least get the 3A safely, and maybe get 5A - 6A. Maybe this is folly and I should just accept that 2A is a more reasonable spec, but where is the fun in that   

Here is the item: http://www.banggood.com/0-30V-2mA-3A-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-p-958308.html

Schematics and parts list are here: http://img.banggood.com/file/products/201505080459530-30Vinstall.pdf

So here are my thoughts for changes. I'm looking for things I may have missed, traps for new players, or any reason this whole idea is shear folly.  I have my eye on a 24VAC 80VA transformer, but if I can beef up the current capabilities, I may go larger.

1) The rectifier diodes have a max rating of 3A (1N5408) so upgrade these to 6A or 8A diodes (recommendations? I'm looking at this http://www.allelectronics.com/make-a-store/item/p600m/6-amp-1000-volt-rectifier/1.html)
2) Q4 (2SD1047) move off board to a good heat sink.  It has a max of 12A, so should be OK at 6 if I can keep it cool. This is an area of concern about beefing this thing up.  Can I dissipate the heat?  I'm thinking of using a CPU cooler with a fan
3) The shunt resistor R7 (0R47 5W) is near its max rating at 3A (~4.5 watts). I can replace this with an aluminum case resistor, and drop the value to get more current limit adjustment range.  0R25 I think should get me in the ball park of 6A adjustment range. Also move off-board.
4)Beef up the traces for the main current path by piggy backing some wire along the traces
5) Add a fuse

Would I need to increase C1 and/or C7? (main input and output caps, respectively). If C1 needs to be doubled (second cap) I think I would need to move them off-board for the space.

I am thinking 16 guage wire for off-board connections in the main current path.

Any thoughts, concerns, tips are welcome.  Even if I stay with a 3A max, I'm thinking a diode and shunt resistor upgrade would be a good thing.

Just for full disclosure, I am also planning to add an Arduino with LCD readouts for set amps/actual amps/volt/watts, and of fan for the heat sink, so I am conservatively allocating about 500mA for internal load. I have already done this on my DC Load project, so I think I know how to do this part already.

Thanks

[Audioguru]

You bought a Chinese copy of a Greek kit that has almost every item overloaded. It was a project at www.electronics-lab.com when I saw it about 12 years ago and many people complained that it was unreliable. Its transformer is overloaded at 24VAC/3A because its maximum current is supposed to be 4.24A instead. The 24V transformer produces maybe 26V or more when it has a low current load then its peak voltage is 36.8V and the rectifiers drop the positive supply to +35.4V. The opamps also use its negative supply of -5.1V so their total supply is 35.4V + 5.1V= 40.5V but the absolute maximum supply for the TL081 opamps is only 36V. The 24V transformer voltage is too low anyway for the project to produce 30VDC at 3A.

I fixed it and I used a 28VAC 4.3A 120VA transformer, TLE2141 opamps rated at 44V and two output transistors each with an emitter resistor. Then TLE2141 opamps do not need many volts for a negative supply so I used -1.3V. Of course the value of C1 is much too low at only 3300uF, I used 12000uF. Unfortunately many of my modified parts will not fit on the Chinese pcb.
Here are my parts list and schematic: 

[electr_peter]

Hello, soligen
I saw this PSU kit in real life, so I can share my insights.

PSU kit works properly only with AC input from transformer. DC input is not enough because negative rail (derived from sine wave) is needed for proper operation.
I am not sure why you would want to go over 3A with this PSU - it barely achieves 3A output in factory condition. I would say it is 2A rated supply because many components overheat at ~2-3A output. Main transistor has to dissipate ~90W worst case - very serious heatsink with fan is required. Kit does not provide any heatsink by default - try not to burn it in testing. Also, two big resistors near main transistor heats up too much at 3A - additional cooling would help as well.

Your ideas about modding PSU are in the right direction. I would focus on increased cooling capacity and PSU case first, then think about better components.

[kolonelkadat]

I know its not the perfect answer for you, BUT if it were me, I'm not sure I would bother. Only because once you start buying transformers, massive great heatsinks, enclosures, components, the price starts to go way way up.

I dont know what country youre in, but for the hundred or so usd that it would cost me to make that one youve linked as good as it should be, id much rather start with something like this bad boy here
http://www.ebay.com/itm/Lambda-Electronics-LES-F-04-OV-0-60V-Digital-1-Output-Adjustable-DC-Power-Supply-/152017186599?hash=item2364ee2f27:g:EpcAAOSwAuNW6Mvb

real top quality 0-60volts up to like 10-15 amps. CC/CV OVP/OCP
https://www.us.tdk-lambda.com/ftp/catalog/le_fall85.pdf

since its a rack mount unit, theres still plenty of tinkering to be done to make it perfect for a bench setup, so you can still have some fun and learn something along the way.

[Kleinstein]

The more useful modifications would be increasing R18 by about a factor of 1.5 and choose a lower voltage transformer, e.g. 18 V and 3.5 A rating: The first change would bring the maximum current down to about 2 A and thus to a value more suitable for the resistor, filter cap and the output transistor. The lower voltage give the TL081 a chance t survive the voltage. Otherwise a an upgrade of the OP is needed and the current should be even lower.

It does not make much sense to use this circuit to much higher power. With emitter resistors to parallel two or more transistors about an extra 0.5-1 V are lost.  With higher power a different design with less heat generation is usually more suitable, e.g. using two or more transformer taps and a different kind of circuit that could give up to something like 2-3 V more output from the same transformer.

When you by a cheap Chinese supply you may end up with a similar circuit in a reasonable case and with a transformer that is slightly to small for the rated current. So it may also be a good idea to reduce the maximum current there. But it can still be cheaper than buying a case, the heatsink and the transformer.

[soligen]

Thank you all for commenting.  Between looking at the original schematic and the one from Audioguru, I am understanding this circuit much better now.  There are just a few things that I still don't quite get from the schematic, if someone could help me understand, I would appreciate it.

Referring to the original Banggood schematic:

What is the purpose of R1?  Looks like it is just a dummy load on the raw DC voltage.  Is this needed for stability, or is it just to discharge C1?

Is R2 just to limit the current to the negative rail? Wouldn't R3 already be limiting the current?

What is the purpose of D10?

What is the purpose of Q1?

Thanks

[Kleinstein]

R1 is just to discharge the main filter cap C1.
R2 is to limit the current used to charge C2. This reduces current spikes. The voltage is rather high anyway.
Q1 is there to turn off the output as long as the negative auxiliary supply is not there, as a kind of delay during turn on.

D10 is there to protect the output transistors from a to negative base-Emitter voltage, that other wise might happen with a large cap as a load and the voltage turned down fast. It can also help to bring down the output it happens to be to high.

[soligen]

Thanks for the help. I think I have a plan now.

I really do appreciate Audioguru's design, and If I were going from scratch, it is what I would do.  I definitely learned from it, but it's too big a deviation from the kit for me right now.

My goal now is to not try to increase it's amperage, but to bring the parts into reasonable spec/reliability.

As Kleinstein suggests, I am going to adjust R18 to limit this to 2A.  I want to try to keep the higher voltages (even if not at full amperage), so I would prefer not to go down to an 18V transformer (but I will if I need to).  Here is my alternate plan:

One of the things not on the schematic is that they added a 7824 regulator to drive a fan (odd since most fans are 12V). Q3, R19, U3, and U1 are all connected to the rectified DC, but none of these really need this much voltage.  I plan to add an output cap to the 7824 and drive all these parts from 24V.  I think this should also be a much more stable voltage, which should be a good thing. R22 will need to be adjusted accordingly.

I think this only leaves U2 to deal with. From Audioguru's numbers it isn't out of spec, but is very borderline.  I am going to experiment and see how close I am, and if an extra diode drop will give it a little more breathing room. If not, then I can either use a lower voltage transformer, or get a single TLE2141 like Audioguru used.

And one more question: Can two of these (isolated) be connected in series for positive and negative rails?

[Kleinstein]

There are quite a few OPs with a 44 V supply rating available for U2.  Already having only a minimal negative supply helps.  Another option is to use something like a 34-35 V zener diode and a resistor to limit the positive supply of U2. The zener diode mainly limits the supply for high mains voltage. Though a +34 V supply will hardly be enough to get 30 v out. So without regulation for the supply for U2, the TL081 specified at 36 V max will hardly get 30 V out while staying in specs.

One could use 2 separate diodes and a separate filter cap for the supply of U2 - so the voltage drop due to ripple will be lower there and thus a higher output (e.g. 2-3 V more) is possible with a given transformer.

One can have two isolated supplies in series to get a +- supply. In this case the diode at the output (e.g. D11) should be strong enough for the full current so more like a 1N540x or even P600.   

[soligen]

I see I had made a mistake in my thinking.  I was thinking U2 was connected to ground, not the negative rail. but looking again, i see I am wrong, so U2 is way out of spec, not borderline.  So, its either use an 18V transformer, or get the 44v op amp.

The 30V is not that important to me, but I would really like at least 24 at the full 2A. Will a 100VA 18V transformer achieve this?  If not, what will?

[Audioguru]

Referring to the original Banggood schematic:

What is the purpose of R1?  Looks like it is just a dummy load on the raw DC voltage.  Is this needed for stability, or is it just to discharge C1?

Its current is so low that it does not do much. It reduces the unloaded DC voltage a little.

Is R2 just to limit the current to the negative rail? Wouldn't R3 already be limiting the current?

R2 limits the high current in C2 and C3. R3 limits the low current In D7.

What is the purpose of D10?

D10 prevents emitter-base reverse voltage breakdown of the driver and output transistors if the project is turned off but a capacitor supplies a positive voltage to the output.

What is the purpose of Q1?

Q1 shorts the output of U2 to ground when the main power is turned off and the negative raw voltage drops before the positive raw voltage. It was used with the original TL081 opamps that had a problem called Opamp Phase Inversion that caused its output to suddenly go as high as it can when its negative supply dropped (then the input voltage becomes too close to its negative supply voltage). With the new opamps it helps prevent a voltage spike when the power is turned off.

[Kleinstein]

With an 18 v transformer you will not reach 24 V under all conditions, not even under light load.
To get 24 V out the raw DC voltage would need to be at least about 27 V (depending on the OP). With some reserve for ripple this would be an 30V unloaded. So a 22 V transformer might just work if line voltage is not to low. To be sure it's more like a 24 transformer is needed, and thus a little to much for a 36 V OP, even with only a small 2 V negative supply.

You can get a way with a little less if U2 gets a separate filter cap and diodes, as this could save on the ripple and get just away with a 22 V transformer.

2 A DC needs a transformer rated for 3.2-3.6 A AC, so the 18 V ,100 VA would be OK from the current.

[sosokruashvili]

Hi everyone, I am glad I found the topic about this psu in here. I purchased this kit and what I see is that everything works well when psu is in constant voltage mode but in current limit mode I have output ripple for above 250 mv peak to peak and it is proportional to load. I checked every part and is OK. I also tried to power current limit opamp with clean current from my bench but same problem. If anyone has the same problem please give me some advises  

See the iamge:

[Kleinstein]

Current limiting work with very low voltages and it thus a little more sensitive to disturbance / hum. The signal looks like some residual ripple. The capacitor C5 could couple some of the raw supply ripple over to the measured current.

The use of C5 could be for decoupling the supply of U1, the OP for the reference circuit. If needed one could have a resistor in the positive supply of U1 and C5 only after this to reduce ripple current.

Is there a fuse between the transformer and the rectifier ? This would reduce current peaks and thus ripple component due the filter caps ESR. This could be important with a low impedance (e.g. ring core) transformer.

Which circuit is used ? The "original" one gives way to much supply to U3 and thus U3 has every right to behave funny or release some of its magic smoke. So the modification with an extra zener (D13) diode in series is a good idea. However U2 still can have a problem !.

[sosokruashvili]

Fixed   Thank you Kleinstei  , for your suggestion. Yes, the main source of this ripple is current ripple generated on PCB ground trace. I just made wire connection between R17 ground side pin to current sense resistor pin(low side) and ripplce is almost gone. It makes sense, as this PCB has very thin traces for rated kind of loads.
This is the mod image:

[soligen]

Your image didn't come through.

I had ripple when using a constant current load to test, but I don't know if the cause is my DIY constant current load or the PSU (I assumed the load).  More capacitance between PSU and load fixed it.  Thanks for posting your solution.  I will add the mod to mine as well.

[soligen]

Just to round out this thread for future people who find it, here is the list of all the mods I made.  Target max is 2A at 15V to keep things in spec.

Used a 18VAC 100VA transformer
Replaced C1 with 2 4700uF 50V capacitors (Nichicon) that I had salvaged
R22 changed to 3K (2 1K5 in series) due to reduced rectified voltage
R18 changed to 94k (2 47k in series) This gives me just over 2A
P1 upgraded to 10 turn pot
D11 upgraded to 1N5402
Added .1uF bypass cap on all op amps
Fed power to U2 via a shotky 1n5819 diode and Added 330uF 50V bypass cap.  This is to reduce ripple
Added 10k resistor between P1 and U1 - this reduces the voltage range to just over 15V
Added a 20 ga wire between R7 and R17 to beef up the trace (per sosokruashvili above)
Moved Q4 off board - mounted on old CPU heat sink with fan.  Fan also blows accross the board

I also added a second board with an arduino to monitor and display volts, amps, current limit, and watts.  This board contains:

The new C1 capacitors
Inexpensive ebay buck/boost converter to get about 9V for the fan and arduino VIN
Arduino Nano
ADS1115 16 bit ADC
16 x 2 LCD with I2C backpack

The 4 channels of the ADC are connected to V-out, Both sides of R7 (current shunt) and the center tap of P2 (for the current limit setting).  Display calibration is done in software.

This project has been on the shelf for a while, but I hope to finish it up in the next couple weeks.

[soligen]

I am hoping one of you experienced guys are still following this thread.

I have a question about what kind of ripple/noise is generally considered acceptable.  Using my scope at 100MHz bandwidth I get some high frequency spikes.  They are not uniformly distributed and when they happen it looks like 1 - 3 waves of ringing, then it goes back to normal noise.  Each "ring" can be 100 - 200 mV peak to peak with a frequency from 10 to 80 Mhz.  Is it reasonable to be seeing these?

I also have had some stability issues, but I think I have that solved.I would get high frequency ripple that would come and go in a repeatable pattern.  Things got worse with more load.  It made the scope trace look a solid line that got thicker and thinner.  When I zoomed in on the thick part it was comprised a of a 1Mhz sine-ish looking wave.  I didn't nee the issue until I tested with a resistor because I had a 470u cap on my electronic load, which masked this issue, so I figured I need more output capacitance.  Not wanting to go too high and foil the current limiting, I reduced the extra capacitance to a 10u ceramic.  Things looked good and I think this is an acceptable level of extra capacitance.  I had put the cap on the binding posts to test, but when I soldered it onto the circuit the instability returned (although a little bit improved).  The only thing I could come up with was it having a lower ESR being soldered in, so I added a 0R62 resistor in series with the cap (lowest I had on hand).  This did the trick.  I also added a .1u output cap, not in series with the resistor.

Another change I made, although it didn't fix anything, is a 10u cap across the outside pins of P2.  Since the voltage reference from U1 will fluctuate in relations to U3's ground as current draw varies, my thought was to slow down that change.  I left the change in place because smoothing fluctuations across P2 seems like a good idea.  I will note that at one time I had some current limit ripple similar to what  sosokruashvili showed.  I dont know for sure if this has anything to do with it, but I don't see any of that kind of ripple now,

I considered an LC low pass filter to help with the 10 - 80 Mhz spikes, but my initial try was inconclusive, so I thought I would ask here about this to see if it is something I should even worry about.

Thanks for all the help I recieved on tnis thread. I know this thread spanned a long time.  I appreciate the learning it has provided,

[Audioguru]

I wonder why Banggood did not test the original faulty circuit they copied and also did not test the output of their kit.

[Audioguru]

It looks like Banggood is finished selling this faulty power supply kit (its price is reduced) and are replacing it with a similar kit but with its output voltage and current reduced to 28V/2A so it does not damage the opamps and its transistors do not overheat. Amazon and ebay are also selling the same kit. The new kit might have an LCD meter on it.

[soligen]

Well, I found Dave's video an evaluating power supplies, and, of course, I was doing it wrong.  I dont have the right probes to really do it right, but I was able to figure out that most of the noise spiking is common mode noise.

The best I could do to get readings is set bandwidth to 20 Mhz, earth reference the PSU, and use 2 10x probes with ground lead removed with a-b math function.  I got about 9 mV peak to peak not loaded and 12 loaded to 1.5 amps. Everything seems very stable, not even a hint any more of the oscillations like what sosokruashvili was getting.

I am going to declare this good enough for this PSU.  Considering I am running a fan, powered by a little SMPS inside the case, the design could probably do better if I used a linear supply for the arduino and convection cooling.  Attached is a schematic with my mods marked up.  I am compensating in software for the current limit reference voltage varying with load.

If this isn't going to be sold anymore, not sure others will find this useful, but I sure learned a lot.

Thanks to all those who contributed to this thread.

[soligen]

I ended up having to add a separate voltage reference for my current limit display to behave properly.  I removed R18 and added in it's place a TL431 voltage reference at 2.5V  I used a 3.7k + 3.3k resistor in series to feed to current from the 10ish voltage reference, then used 3 4.7k resistors in series to drop the voltage to approx 1V going into P2.

All is working good now.  The arduino code is at https://github.com/soligen2010/PowerSupplyMonitor

[Acanti]

Hello!
Why not doing it the hard way by adding a separate power supply for the op amps? a 2x10volts filtered and regulated indecently by a different transfo windings? that can eliminate problems to the opamps and simplify the power winding choice, dont you?

[Kleinstein]

This old Bangood circuit is using an output stage in emitter follower configuration. Thus is needs at least one OP to have the full voltage swing at the output. So for a 30 V output things get a little tricky.

Using a separate, floating +-10 V or similar supply for the OPs makes a completely different circuit. This is the classical version with the output stage in common emitter configuration. The circuit has different properties though, usually a high output capacitance. So it would be building a different regulator from scratch. It has some benefits when digital control is wanted, as the supply for the OPs can be combined with the control part.

[Zero999]

Some interesting op-amps have been suggested to replace the under-rated TL081: the MC34071 & TLE2141. There's nothing wrong with them but there are other options. It just needs to be able to handle a total supply voltage of 44V.

How about trying the OP07 which will have a much lower offset voltage, so much so the trimming circuit can probably be eliminated, but will be slower so might need a larger capacitance on the output to ensure good transient response.

Another possible option is the NE5534 which will be faster than the OP07 but will have a much greater offset and will need a compensation capacitor and a different offset trimming circuit.

The OP27 will give good transient response and offset but it's more expensive than aforementioned. It could be used for U2, with cheaper op-amps for U1 & U3.

[shobo]

apparently a new version of the psu has appeared.i'll reply with the parts

[Cliff Matthews]

More info on new rev. http://www.icstation.com/product_document/Download/12479_installation_instructions.pdf

[soligen]

I didn't trace through this in detail, but at a glance it looks like the parts in the schematic have just been re-arranged in the drawing, and they drew in the 7824 regulator which was part of the kit I bought, just not on the old schematic Maybe some small changes - I didn't look at it in detail. The board does look like it has been re-laid out, but at a glance I don't see that the big flaws were corrected.

Are they still rating it 30V @3A?  If so still way to high.  My mods de-rated the design to 15V 2A

[Audioguru]

The "new" kit has the same defective circuit as the old Greek kit and later Chinese copies of it.
It will not produce regulated 30V at 3A and many of its parts are overloaded.

It still uses a 24VAC/3A transformer that will smoke and burn.
It still uses TL081 opamps that will also smoke and burn.
It is still missing voltage and current calibration trimpots.

[shobo]

yay onther one:
https://www.banggood.com/0-28V-0_01-2A-Adjustable-DC-Regulated-Power-Supply-DIY-Kit-Short-Circuit-Current-Limiting-Protection-p-1060253.html?rmmds=search&cur_warehouse=CN

This one is lower rated but looks simpler in its design with no op-amps.Perhaps all it's done by the atmega?

[Cliff Matthews]

So will you trust in current limiting from an MCU?
- Coded by people who's first line in an assembly manual says: "The first step is the Resistance Welding".
- And on first boot up the MCU displays: PSU by: https://shop35395693.taobao.com/
- https://item.taobao.com/item.htm?spm=a1z10.1-c.w4004-4669527207.21.12cf9858jeH4Gs&id=532555910832

Time for a smoke.. and I don't even smoke!. (Well, maybe a magic smoke... )

FWIW, Dave seems to be highlighting RDtech power modules recently and they seem to support him responsibly..
The newest DPH5005 Buck-Boost unit is output rated for 0-50v / 0-5a and max 250w for $34 (with the stock heat sink and 6-50v DC input, you could maybe trust it up to 100 watts perhaps..) just throw it into a case. WMMV

[shobo]

well after much thought i'll buy a pre-made one.i was eyeing an old romanian source like this:

https://marebazar.files.wordpress.com/2008/05/sursa.jpg

I would have prefered to get one of those kits since there available close by (the same city) and not wait for post and stuff but since i dont have a scope to trouble shoot it i guess buying something already made would be wise.

[MrAl]

Hello,

The original point of the thread was can a power supply be upgraded to allow for a higher output current.

The answer is yes but there is some serious changes that have to be made.

First, the output power transistors all have to be doubled if the current is doubled.
Second the input transformer current rating has to be doubled, the input diode bridge current rating hs to be doubled.
Third, the filtering capacitance has to be doubled.
Fourth, any current limit detection circuit has to be modified to allow the higher output current.
Fifth, the heatsinks have to be doubled or enlarged.
Last, it has to be tested at full load.

If it's an old design it should be upgraded with a reference diode rather than a zener which the old supplies used to use.  The reference diode is much more stable over temperature.

As a side note, you can build 5 amp power supplies with the higher current version of an LM317 and they are pretty simple with much less parts.  That will get you there pretty quick if you just want t linear.  IF you want a switcher it takes a little more work.

[Kleinstein]

The original circuit already needs modifications to make it work reliable at all. Likely higher voltage OPs - maybe limiting of the supply of there OPs that don't need to be so high. It's also a good idea to have something to prevent a voltage spike on turn on. As the current limit is rather slow, it might be a good idea to add a fixed fast current limit.

Increasing the power is only what comes after that.

[shobo]

well if i get one,i'll use it with a 18 v transformer to get a min of 24v

[tpowell1830]

From looking at the picture from Bangood, the 2 adjustment pots are too close together to be able to put very big knobs on them.

[Cliff Matthews]

TIP142's (TO-247) sell at just $1.50, it would be interesting to see how stable this would be, and where it needs help.
https://www.digikey.com/product-detail/en/stmicroelectronics/TIP142/497-2541-5-ND/603566

[shobo]

From looking at the picture from Bangood, the 2 adjustment pots are too close together to be able to put very big knobs on them.

They provide 2 cables with jst connectors to move the pots off board.

[Cliff Matthews]

They provide 2 cables with jst connectors to move the pots off board.

Yes, but Soligen's premise was to take this cheap $5 kit and improve it, so budget tech's will have a resource that's worthy of making this design into a respectable linear supply. I don't see any thread achieving that goal for the benefit of those not ready (or able) the shell-out big $'s. We just see post's about mods (here and on the net) while simultaneously getting shot down and being told to cop-out and buy somebody's pre-made xyz-3003 box. Poppy-cock.. That's NOT the true spirit of electronics.

[Kleinstein]

There is one main point to change with the old Banggood kit, and that is rather simple: use a lower voltage transformer. To stay inside the specified max. 36 V supply for an TL081 this would be about 18 V max, if the transformer is not that small so that the no load voltage will not be that much higher than nominal.  Thus would limit the maximum output voltage to about 18-20 V with light load and maybe 13-15 V under heavy load. At the reduced voltage the 3 A current rating might not be that bad, but a lower value (increase R18) might be better.

A bodged in diode from the negative output side to the base of Q1 might be a good idea too (this gives an additional fast current limit at about 4 A). Another point are decoupling caps at the OPs, especially the one for the output.

It is a little odd they have a 7824 regulator an do not use it for U1 and U3, this way they could have gone away with the TL081 for these two OP.

There are a few additional changes that would help, but than it gets difficult to keep the board.

The other option would be to change the OPs to something like MC34071 (instead of TL081), with a higher voltage rating. As this one is single supply one could also change D7 to a lower voltage one (e.g. 2 V) and Adjust R14 accordingly.

[soligen]

The 24V regulator was just added on to run 24V fan for the heat sink (not included).  Seems odd to me since 12V fans seem much more available. I used a SMPS module to run the fan at about 9V and did not use the 24V regulator.

For those who may not have read the whole thread, the original goal of increasing the specs on the kit was abandoned, and instead the specs were reduced to about 15.5 V max and 2.25A. (as well as some other mods too).  An 18V transformer was used, and I also needed to add significantly more input capacitance.

I had to increase output capacitance to get rid of some oscillations, so it also seems the design is also not necessarily stable without some tweaking.

I did finally get it working well, but it was a journey.  At least I learned stuff. 

[shobo]

The 24V regulator was just added on to run 24V fan for the heat sink (not included).  Seems odd to me since 12V fans seem much more available. I used a SMPS module to run the fan at about 9V and did not use the 24V regulator.

For those who may not have read the whole thread, the original goal of increasing the specs on the kit was abandoned, and instead the specs were reduced to about 15.5 V max and 2.25A. (as well as some other mods too).  An 18V transformer was used, and I also needed to add significantly more input capacitance.

I had to increase output capacitance to get rid of some oscillations, so it also seems the design is also not necessarily stable without some tweaking.

I did finally get it working well, but it was a journey.  At least I learned stuff.

Do an instructable of the mods

[shobo]

TIP142's (TO-247) sell at just $1.50, it would be interesting to see how stable this would be, and where it needs help.
https://www.digikey.com/product-detail/en/stmicroelectronics/TIP142/497-2541-5-ND/603566

Can you give me link to the tip142 supply?

[Cliff Matthews]

http://www.ve2ums.ca/chasse/Serge/Atelier/Projets/Membres/VE2EMM/alimentation_ang.htm
Scroll to the bottom for complete archive download (several versions inside). For further comments on the LM723 supply, I'd recommend opening another thread.
 
It appears Soligen has not given-up on the thread (and.. maybe I should delete my distracting posts?).

[soligen]

It appears Soligen has not given-up on the thread (and.. maybe I should delete my distracting posts?).

For me, this project is completed, so I don't care that you have added a different design to the conversation.  I am happy to have you leave your posts so anyone finding this thread has a reference to an alternative.

Although I got mine working well, I would not recommend this kit to anyone, so having an alternative listed is a good thing.

[shobo]

Found a modified version of the kit made by a guy in his garage but the differences are the resistors are half a watt and uses LM741 op-amps and at the same price as the kit.

https://img01-olxro.akamaized.net/img-olxro/175166427_4_1000x700_kit-modul-sursa-de-laborator-sursa-de-curentelectronic-arduino-electronice-si-electrocasnice_rev001.jpg

[jaycee]

You could do a lot worse than base your design around the ELV 22532 circuit. This is pretty much what the Mastech supplies are.

Some people dont like the idea of having to have a separate transformer for the bias supplies, but why not ? It only needs to be small. Mine uses a 9-0-9VAC transformer from an old digital clock/bedside radio... probably 2VA at the most, pretty small. It's enough to drive the opamps, a relay for changing between transformer taps, and run the display meters

[Cliff Matthews]

That's using LM741 a/b (44v) but not c version (same 36v problem as TL081). You pic comes from here I think..
https://www.olx.ro/oferta/kit-modul-sursa-de-laborator-sursa-de-curent-electronic-arduino-ID9v3vx.html
The highest voltage he shows is 28.5 and that's not even loaded..

I searched for schematic "sursa reglabila 0-30" it's the same. Maybe try this simple one with another 2n3055??
http://www.circuiteelectronice.ro/power-circuits/sursa-de-alimentare-reglabila-0-30v-cu-lm723.html

[shobo]

That's using LM741 a/b (44v) but not c version (same 36v problem as TL081). You pic comes from here I think..
https://www.olx.ro/oferta/kit-modul-sursa-de-laborator-sursa-de-curent-electronic-arduino-ID9v3vx.html
The highest voltage he shows is 28.5 and that's not even loaded..

I searched for schematic "sursa reglabila 0-30" it's the same. Maybe try this simple one with another 2n3055??
http://www.circuiteelectronice.ro/power-circuits/sursa-de-alimentare-reglabila-0-30v-cu-lm723.html

As an option i have this too. The guy sells only the pcb but not a kit.i might end up saying "fuck it" and buy one.

Trimis de pe al meu VFD 1100 folosind Tapatalk

[wrongtwins]

Hi , I have build the power supply but i dont have any voltage at the output ? Do you have any trouble shooting guide to help me out. I'm a hobbyiste i a dont know were to start to find the probleme. The 24V for the fan is working. At D1047 i have +28V, 0V, -28V.

[Kleinstein]

The first step in troubleshooting is usually a visible inspection. So check if all solder joints are done and diodes the right way around.

A nest step would than be to measure voltages following the supply. A first point could be something like just after the rectifier, than maybe the negative supply and voltage at the OPs.

Be aware that the cheap PSU kit gives too much supply voltage to the OPs if the intended 24 v transformer is used. So a 16-20 V transformer would be the more suitable choice and the resulting maximum output voltage lower. Depending on the voltage used, chances are the OP dis not like it. So a broken OP is a likely fault with a 24 V transformer.  So as a bonus you get an example for fault finding.

[Cliff Matthews]

The voltages you gave are referenced to 0V (-) from the rectifier? I've no idea where the -24v would come from if you used the same reference.. Tell us about your transformer setup and perhaps why you didn't start the project using 18 Vac as described earlier in the thread.

[wrongtwins]

Hi Kleinstein,

Thanks for the info.  I have already check all the solder joints and diodes.  i will mesure de voltage and trace it back were it's stop.  Can i replace the OP ?

Cliff,

I have comme alone this thread only todays.  I have follow the intruction from instructables/DIY-bench-power-supply. So i have used  110v 24VA 2Amp for the power supply.   i will print the circuits and begin meseauring along the circuits to see where can be the probleme.  And i will poste some picture of my setup.

Or do you sigest to us another  board/option.  I preffer to keept what i have already.

Thanks

p.s. English is not my primary language

[Kleinstein]

The kit is more suitable for a 18 V or similar transformer - the 24 V from the instructions is too high.  24 VA and 2 A would mean a 12 V AC transformer. This should be OK and not deliver too much voltage. It could be more on the low side so that the negative supply might not have enough current. This would be relatively easy to fix by using a smaller value for R2.

IF needed one can replace the OP with a different type to work with a little higher voltage. But first one should check a few more points, like the supply to the OPs. This should be something around -5 V and a positive voltage around 20-30 V depending on the transformer. More would be a risk to damage the supplied OPs.

Seeing -28 V at the SD1047 would be kind of odd as there should be no -28 V on the circuit - it could indicate a problem with the negative supply, like a wrong diode where the 5.x V Zener is supposed to be. This in turn would have a good chance to fry the OPs.

[wrongtwins]

Kleinstein,

i have 30V at PIN7 on each IC.  I cannot fin the -5V any were ?  And my R2/R3 are hot ?  I notice D12 is not working to. 

Thanks for your help.

[Cliff Matthews]

  ..So i have used  110v 24VA 2Amp for the power supply

We assume you are connecting the PCB directly to an AC to AC transformer. Are you?
Do not connect DC to the PCB or it will not function as intended.
What DC voltage do you measure between pin 4 and 7 of all 3 op amps?
One of them (U1) should be less, and all should be under 35v DC for stable operation.

[wrongtwins]

Yes i use a Avalanche Transfo 12vac-0-12vac 2Amp.

I have -29.4v, -29.7v and -30.2v.

[Cliff Matthews]

It looks like the charge pump circuit is not working. Using pin 4 on U1 as a positive reference, you should see 5v less to pin 4 of the other two IC's (this is established from the junction of R3 and D7). What did you mean by Avalanche? A photo helps...

[wrongtwins]

It's the company that made the transfo.

For pin4 i have 0v with the other IC.  I have mesure D7 and got 0v ref to ground.  I have remove it from the circuit to test it and i have 0.700 my diode tester.

What is the charge pump circuit ?

Thanks

[Zero999]

It's the company that made the transfo.

For pin4 i have 0v with the other IC.  I have mesure D7 and got 0v ref to ground.  I have remove it from the circuit to test it and i have 0.700 my diode tester.

What is the charge pump circuit ?

Thanks

https://www.google.co.uk/search?q=charge+pump+circuit&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjC6YW_uvTZAhUDI8AKHWgOBY0Q_AUICigB&biw=1920&bih=953

[Cliff Matthews]

I think you could visualize how C3 charges to become the -5.6v rail in this way.. C2 charges on the negative half cycles, and during the positive half cycles, C2 and the transformer add together (like batteries) to reduce charge on C3 even more (or.. with reference to the 0v rail). Not a lot of current available, but it's enough to establish a negative rail for the op-amps. (I've seen others forward bias 2 or 3 diodes instead of the zener so the op-amps don't release the magic smoke.. but I think a TL431 would make it more temperature stable).
*restored GIF animation*

[Kleinstein]

That R2 is getting a little hot is kind of normal. One could reduce that by making C2 and R2 smaller, but this would be more like the last polish, the original circuit should be working in this respect too. D7 is required to limit the negative voltage. The TL081 needs something around -3 V to work around ground at the input. There can be even a slightly negative input voltage. As the TL081 can react with phase reversal the OPs should get at least around -4 V to work well. Even this might require to more extra diodes to limit the possible negative voltage at the OPs input. So just 2 or 3 forward diodes don't sound right if the original TL081 is used, it may be OK with other OPs (like MC34071).

A 700 mV forward voltage is OK for a zener diode - so it is not burnt. The more important voltage would be the reverse voltage - especially to make sure it is not mixed up with a normal diode, that might look very similar.

Measuring 0 V across D7 indicates a short to ground somewhere else. This could also be a damages part.

[Calambres]

I've got two of these kits for a symmetric supply project. Both work OK but for some reason I cannot set the zero voltage with the trimmer in none of them.
Both were thoroughly checked and both fail miserably when trying to set 0V.

Any clues on this failure?

[Cliff Matthews]

@Calambres, maybe this is similar to @wrongtwins problem (talvez). Read post #57. What voltage do you see?

[wrongtwins]

I have donne somme reading on the board :

Like i said U1, U2, U3 have 30v from pin7 to 4.  i have 30V at R1, 0V at R2 (heat on the ressistor), D5 3.2V, D6 2.7V, R3 5.8V.  At Q3 i have 30 v emitter(1), 30v base(2) and 0V at Collector and  0V at R22/D12.  I have 30V at pin 3 U3 and i have a lot of heat on U3 ?  I think my probleme is arround there ? 

[Cliff Matthews]

U3 should not get hot.. replace. An age-old problem: 5.6v is too much for the negative rail and 24vac pushes things to the limit.
Most who take this cct seriously, will opt for higher Vcc in op-amps (MC340xx or etc..) or settle with 18vac on the transfo.
IMO, Banggood and other sellers should encourage buyers to assemble and test in two stages.

[wrongtwins]

Ok,

I will replace all of them with  MC34071/tle2141 ?

[Cliff Matthews]

Yes, I'd socket them.. still something wrong in charge pump, since U2 and U3 are getting the negative rail from somewhere. Lift one end of D7 and test to see if it's shorted in both directions.. R2 heat might be explained too.

[Calambres]

@Calambres, maybe this is similar to @wrongtwins problem (talvez). Read post #57. What voltage do you see?

With positive reference in pin 4 of U1, I see 5.162V in both pin 4 of U2 and U3.

No matter how much I fiddle with the trimmer, the zero adjustment won't vary at all...

What DC voltage do you measure between pin 4 and 7 of all 3 op amps?
One of them (U1) should be less, and all should be under 35v DC for stable operation.

U1: 31.22V
U2: 35.18V
U3: 35.18V

All this measurements with:

- Input:   25.7V AC. The xfrmr is 220V to 23V, 4.2A toroidal. As I have a mains out of 240V, the output is a bit higher.
- Output: 27.8V DC (maximum) @ 1.462A.

[Cliff Matthews]

The measured 5.162v difference between op amps showed the zener is regulating, but after you measure op-amp voltages with a 3.96v disparity..    (was the later measurement under load?). If the transformer has 25.7VAC during load tests, it could be 27-28VAC with no-load.. so rectified, that 36v could fry TL081's (considering where they came from..) without even considering the other two op-amps at 36v + 5.16v across them.

If you want to continue testing with this transformer, a quick way to drop 5 or 6 volts would be with a Variac, or as a noisy last resort, a light dimmer on the primary side - be careful of mains..

*Oops, an updated post (I saw your earlier version). At ~1.5amps out you'll have 1.5v drop on R7, so the +rail is 32.7v! (V(U1) + Vr7). So you have 5.16v more (37.8v) on U2 and U3 under load, and likely 39v with no load.. Any smoke happening?

[Calambres]

...Any smoke happening?...

No... it works OK. The only complain I have with this kit is that the zero volt trimmer is not working at all...and as I said in a previus post, I've got two of these kits and the trimmer issue is happening in both of them!

I've got somewhere a couple of 4A 18V AC toroidal transformers. I'm going to use them just in case and/or swap the Op Amps for some uA741 or LM741 (not the "C" version).

[Zero999]

...Any smoke happening?...

No... it works OK. The only complain I have with this kit is that the zero volt trimmer is not working at all...and as I said in a previus post, I've got two of these kits and the trimmer issue is happening in both of them!

I've got somewhere a couple of 4A 18V AC toroidal transformers. I'm going to use them just in case and/or swap the Op Amps for some uA741 or LM741 (not the "C" version).

Perhaps the trimmer potentiometers are bad. Have you tried replacing them with a decent brand?

[Calambres]

No, but I'll give it a go!

[Calambres]

Perhaps the trimmer potentiometers are bad. Have you tried replacing them with a decent brand?

No, but I'll give it a go!

Nope!... both trimmers are good 

On the other hand, I've seen other's reports with the same issue: the trimmers do nothing to set the zero volts.

[IanMacdonald]

Just my ten cents worth but using a single TO248 for this kind of power level is indeed stretching the thermal capability of the package. No matter how large the heatsink, internal temperature difference between chip and case will be the issue. Two would be more sensible.

Also, in any lab PSU running the opamps from the main supply rail and directly controlling an emitter follower output stage, is neither robust nor efficient. Apart from limiting the max output to ~30V, if any spike or reverse voltage gets onto that rail from a load, pop go their clogs.  Better to drive a C-E transistor from the opamp output, that in turn driving a PNP or P-channel output stage. That isolates the opamps from any nastiness on the PSU output, and has the bonus of a near-zero minimum volt drop instead of wasting a good volt or two of output. 

[Cliff Matthews]

Putting in numbers for visual effect..
Here's how 25v might look across this lonesome 2SD1047 (silently dying at just 2 amps)

This is why an 18vac transformer makes better sense
*Numbers plucked from Step 8 at http://www.instructables.com/id/Superb-Lab-Power-Supply/.

[Calambres]

...the trimmer do nothing to set the zero volts...

Issue solved! I changed U2 for a real Ti TL071CN and a real Ti UA741CP I had in my parts bin and problem solved! Now the trimmers work like a charm in both circuits.

It is weird how those chinese Ti TL081CP, obviously fake, do not seem to have a valid circuit for OFFSET NULL 1 and 2 (pins 1 and 5) but the rest of the circuit seem to be at least compatible.

I'll try with a set of true-non-fake LM741 (not "C")

[Cliff Matthews]

It is weird how those chinese Ti TL081CP, obviously fake, do not seem to have a valid circuit for OFFSET NULL..

Hi Calambres, good to be aware of. For those buying cheap kits, can you post a close-up photo of these fakes? Thanks!

[Calambres]

It has been quite difficult to take a shot of such a faintly printed chip, but I've managed to get a couple of pics.

The one with a single chip isf the fake Ti TL081CP that were bundled with the kit and the other is a comparison with a real legit Ti TL082CP.

[Cliff Matthews]

I bought 2 of these kits and all 6 op-amps had the pin-1 dot, and compared with those bought locally the letters are not sharp.

[Calambres]

If you look closely at some photos of these kits from diffrent sources you'll see different versions of the PCB and slight differences in the components.
I was unlucky with the OpAmps I got, but that is not a big inonvenience as I originally put IC sockets and replacing the chips is quite easy and cheap. Besides, the TL081 was never a good option for this circuit so I'm heading to my local components store to get some LM741.

[Kleinstein]

Especially if using a relative high transformer voltage ( e.g. >18 V) it is a good idea to use sockets for the OPs, as the supply voltage might be outside the specified range of the TL081.

[Calambres]

The D1047 power transistor in one of my boards went south! 

No short circuit, no mishaps, no overpower, no zap!, no magic smoke... just normal operation 10V @ 1.5A 

I think I got a "very chinese" sample. First was my issue with the Op Amps and now this!
Now to find a replacement for this tranny. That reference is not available locally.
I'm going to try with a good old 2N3055.

[Kleinstein]

The 2N3055 is a little slower, but the circuit should still work with it. The TIP3055 would be a more similar case - but maybe to weak.  A TIP35 would be a little more robust and similar case and might thus work better with the same heat sink.

The heat for the transistor is highers at low output voltage. If the heat sink is not very large 1.5 A and 10 V output could already be the limit for an SD1047.

[Cliff Matthews]

That Chinesium transistor failed under where it should have.. 36Vrail-10Vout=26Vdrop@1.5a=39W (about 80C over ambient).

[patacon]

Hi guys,
Thanks all of you for the fruitful discussion about these PSUs. I stumbled upon this Forum today while searching for info about these PSUs
(un) Fortunatey I had purchased one of these kits a few days ago. As I am total beginner I thought it would be a nice first project.
After assembling it I tested it and it worked ( supplied from 0 to 30 DCvolts output) for 5 minuts. Then output plunged to 0.4 volts and never came back, even if I tried several times. As I dont have  anyone to ask for advice nor much experince I started troubleshooting by trial and error:  checked every component several times, tested resistances, capacitances, all diodes and transistors for shorts, etc etc but still dont get what  the reason of the failure is. I also replaced all transistors but no luck.
As a final take,  I bought a second set and built it all over again. This one never yielded more than 0.4 volts at PSU output
My best wild ass guess ( see schem with voltage readings attached) is that the output at U2 is so low that  Q2 and q4 are never triggered , but again I am a total newbee and this could be all wrong. In anycase I couldn't tell why U2 AND u1 might be failing...

I have read above in  this thread  the valuable advice given to Soligen and Wrongtwins but I am not capable of solving my issues with that info

In anycase , all I want is to learn what the problem is and why this happened. I dont care too much about the PSU performance.I just want to learn about electronics.  So if you guys could give me a hand to spot the issues I'd more than appreciate it.
Thanks in advance.

Remarks:

• Input is provided by a 220 VAC to 24  VAC- 10W transformer- Actual output is 26 VAC.
  all other Voltage readings in red fonts at the schem are measured  by inserting negative  multimeter lead into location #4 (negative PSU output) and positive lead into the spots marked with a small red dot.

R2 and R15 get very hot, almost burning but I havent gotten any "smoke" from ICs.
As a last trial I had inserted  a 100 ohm resistor in series to one of the outputs of the secondary of the AC-AC transformer which reduced input voltage for PSU to 16 VAC but this didnt work ( probably some component is already damaged...)
[/list]

[Audioguru]

As discussed a few pages ago, the TL081 opamps will fail soon because the total supply is too high for them.
Your 26VAC has a peak of 36.8V and the rectifiers reduce the positive opamp supply to +35.4V. The negative supply should be -5,6V so the total opamp supply is 35.4V + 5,6V= 41.0V but the absolute maximum allowed supply for the TL081 is only 36V. That is why I suggested using 44V opamps in this project many years ago.

Your output voltage is too low because Q1 is always turned on reducing it. Q1 is supposed to have -5.6V on R14 to keep it turned off but your -5.6V is much too low at -0.7V or maybe it is very wrong at +0.7V. Maybe because D5, D6, D7, C2 or C3 is connected backwards, so check them.

[Calambres]

That Chinesium transistor failed under where it should have.. 36Vrail-10Vout=26Vdrop@1.5a=39W (about 80C over ambient).

The transistor was not hot when it failed. Besides, it had a very generous heat sink attached. I guess it was defective.
As I previously said, I've got two of these circuits and the other circuit is working fine with different loads and voltages.

By the way... when it failed I was feeding the circuit with 18V AC 

R2 and R15 get very hot, almost burning but I havent gotten any "smoke" from ICs.

In my failing board, R15 was VERY hot when the D1047 transistor failed!

[patacon]

Your output voltage is too low because Q1 is always turned on reducing it. Q1 is supposed to have -5.6V on R14 to keep it turned off but your -5.6V is much too low at -0.7V or maybe it is very wrong at +0.7V. Maybe because D5, D6, D7, C2 or C3 is connected backwards, so check them.

Thanksk for the feedback Audioguru. I will replace opamps with 44 V. I have ordered MC3 4071 and sockets.

In the meanwhile, it seems that there is another issue with negative rail as you  also pointed out. I cant have proper negative voltages there. I have measured it all over again just now. All diodes and capacitors are well connected, polarities are ok. The only negative values I get are :  -0,001 at D3,D4 anodes and   -5,5 V at negative pin of C3 but not at R3 or elsewhere, meaning no opamps get negative voltage at pin 4 ( they actually get +0,8V), same goes for R14 obviously. It seems that voltage pumping arrangement is not working right.

Any thoughts guys ?

BTW, I have also finished this DIY little function generator. It works all right!
https://photos.app.goo.gl/ya7ZzxYdlKS7N2Zf2

Thanks

[Audioguru]

The MC34071 is not made in a DIP package anymore. The tiny surface mount ones might overheat. The TLE2141 is also 44V and available in a DIP package.
The MC34071 and TLE2141 do not need and do not want the -5.6V negative voltage needed by the TL081, their negative voltage is only -1.3v because zener diode D7 is replaced with two 1N4148 ordinary diodes in series. You should use the latest schematic and parts list.

Your -5.5V at C3 is almost correct but since your negative voltage is positive then maybe your R3 is 220meg ohms (too small and is burned out?) instead of 220 ohms 1/2W.
Measure voltages with the black lead of your meter on the negative wire of C1 because the output 0V goes positive when the load causes current and a voltage drop in R7.

[cspwcspw]

I built the kit with the MCU control, superficially it seems working just fine.   But when I put a scope on the power I had doubts.  I found no specs about levels of ripple current or stability, so a few questions...

a) What do we guess "sensible" specs would be for ripple at quite modest voltages and load (6v, 500ma) say?

b) Has anyone looked at their supplies (either the original one without the MCU, or the computer-controlled one) with a scope?  Can you confirm your observed levels of ripple / noise?

c) My picture is below, 50mv per division, 5ms per division in the time axis.  Clearly, with no changes in load or voltage, and nowhere near current limit kicking in, I have something turning on then off.   Even with zero load, this artifact is present.    Can anyone else with an MCU-based supply let me know if they see similar artifacts? 

Thanks
Peter

[Kleinstein]

IN voltage regulation more the ripple should be relatively low. It depends a low on the quality of the layout. So I would expect ripple similar to cheap small supplies. So a few mV of ripple at most.

However the layout is bad when it comes to ripple in the current regulation case. This is due to a poor layout on how to connect to the shunt. So current ripply is rather high, but could be improved a lot with some small modification on the layout.

The shown curve does not look really good.

[soligen]

Peter,

The scope trace I get is nowhere near that bad, but did a major upgrade on the main filter capacitors and added another output capacitor to clean up some oscillations.

Can you link the exact kit you are using?

[Argiros]

Hallo to the group. I build one too works fine excep the led does not lights up. The Volt and Curent potentiometer are working and the volt and ampere are adjusting but the led does not light. I test it with load. The catch I found is that the resistor in series with the amp potentiometer above the TL081 the R18 was 46K I test it also with 75K and the same results.

** finally it was a stupid mistake: I put the red led in wrong direction. Everything works fine. As for the R18 value from 56K or 46K or 75K is only for limiting the curent of the board.

[Fabius_89]

Hi there.

I bought this psu kit from aliexpress. And started reading forum after forum

I made some of the modifications noted by Paul´s DIY Electronic Blog. (http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html?m=1)

And wanted to share some of my insight.


First, modifications.

I replaced R3 (220 ohm) with an LM337. First configured to -3.3V output, then -1.8V.
This caused the maximum voltage output to fall from around 32V (with -3.3 Vref) to 29.1V (with -1.8 Vref). I´m thinking in replace it with the 220 resistor again and putting 2 1N4148 in series in place of D7 as Audioguru´s approach to leave -1.3V (needs testing)

I'm using a 24V transformer with 4A DC rated output.

I also put a 200K trimpot in series with R18 and reduced the output current (which originally was 3.81A to 2.5A) Mainly to reduce power dissipation in Q4 and the bridge rectifiers, which were pulling 4.95A from the transformer and this also reduced the input raw voltage from 35V to 26V. Also past 3A high ripple was present at the output at 100 hz rate (I believe the C1 3300 uF capacity was overloaded and the output was "sucking" the unfiltered voltage. I have plans to add a 2200uF (the one I have around) in parallel to C1).

Aside the Lm337, and the 200K trimpot I also added a 470uF cap in parallel with C7 which was 1uF, at the output (this caused that the CC mode is present in the milliamps spectrum so further testing is needed to see at which current setting the CC is disabled) the rest of the PSU is original. I'm also planning to add a cutoff switch to ground the non-inverting input of U1 in order to kill the output voltage of Q2. and some more stuff.

So it seems until now that reducing the negative voltage to -1.8V using the LM337 AND reducing the max output current to around 2.5A leaves this kit pretty much usable.

I measure the ripple with a DSO138 oscilloscope with 50mv and 5us divisions and the noise was 18mV peak to peak.

Hope this information benefits beginners (like me) who just bought this kit and are overwhelmed by its flaws and how to fix them. And also I'm not saying that all that was discussed (like changing the op-amps to 44V ones is not good)

It's just that in some countries is more difficult to come by with supplies (I live in Uruguay for example and my best chance to get good op amps are buying from Ebay. I cannot even get a D1047 or a SC5200 here)

Still my psu is under test and construction my next step is to get a K-Type thermocouple and start measuring the heat sink temperatura of the D1047.

Last but not least, I putted a 5A fast blow fuse and a NTC thermistor rated for 5A between the output of the transformer and the input of the PCB. And I can still hear a "PLOC" in my Pc speakers when I turn on and off the PSU. Even with the 3300uF cap (still not upgraded to 5500uF)

[Kleinstein]

Hi there.

I bought this psu kit from aliexpress. And started reading forum after forum

I made some of the modifications noted by Paul´s DIY Electronic Blog. (http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html?m=1)

And wanted to share some of my insight.


First, modifications.

I replaced R3 (220 ohm) with an LM337. First configured to -3.3V output, then -1.8V.
This caused the maximum voltage output to fall from around 32V (with -3.3 Vref) to 29.1V (with -1.8 Vref). I´m thinking in replace it with the 220 resistor again and putting 2 1N4148 in series in place of D7 as Audioguru´s approach to leave -1.3V (needs testing)

I'm using a 24V transformer with 4A DC rated output.
....

Reducing the negative supply does not make much sense with the original TL081 OPs. They may need the negative supply to stay inside the common mode range, that may start at some 3 V from the negative supply. So some reduction to -3.3 V may be OK, but not below. Outside the comon mode range the TL081 can show phase reversal and thus possibly nasty spikes.

I see no reason why the maximum voltage should drop with less negative supply - maybe a variation in mains voltage happened.
With a 24 V transformer the TL081 are already used to beyond there rated maximum voltage. One may be lucky for some time, but there is a risk to this. The reduced negative supply help a little.

Reducing the current is a good idea. Even 2.5 A is a hefty load for the SD1047.

Increasing C7 to 470 µF may be too much of a reasonable idea. A relatively small capacitance at the output is one of the few good point's of this supply. A little more (e.g. 10 µF) may be good though.

[Fabius_89]

Thanks for the fast response.

The max voltage of 29.1V is with no load attached (is being measured by the amp/voltmeter) Maybe it would be a good idea to check it with a DMM to be sure. Maybe the difference is there, sorry my bad I just realized now.

The idea of lowering the negative voltage was to bring the infamous TL081 closet to 36V. In my current setup they are at 37.3V with no load and around 35V with load. I checked for transients at the output and saw a difference with the added 470uF capacitor (I can see it´s discharge curve in the oscilloscope once I cut mains power, this did not happened before adding it) also with your input I will consider removing it again.

As for the rest I´m still tinkering with the supply but if anyone is interested I can continue to post my findings.

For now I will stick to the TL081 as per said in my country is not easy to find electronic spares (the electronics store here charged me around 7 dollars each LM337, so you get the picture).

Still I assembled the op amps with socket so my idea in the future is to remove the +VCC pin of the three of them from the socket and put 10V zener diodes in series or feeding them with a Lm338 configured to 33V and leaving the negative rail at -3.3V as pointed out. The irony is that a LM337 cost me the same as the whole PSU kit (with no heatsink nor transformer) haha so, it sucks but still learning is fun

[Fabius_89]

Reducing the negative supply does not make much sense with the original TL081 OPs. They may need the negative supply to stay inside the common mode range, that may start at some 3 V from the negative supply. So some reduction to -3.3 V may be OK, but not below. Outside the comon mode range the TL081 can show phase reversal and thus possibly nasty spikes.

Thanks for the valuable info, this behavior can occur at anytime during operation, or in a current/voltage range? Sorry for the ignorance I´m trying to grasp all the concepts and learn some more

[Kleinstein]

The U3 Op does not need the full positive supply - so for this OP the 10 V zener in series to the positive supply could be OK.

There are simpler ways to reduce the negative supply (e.g. just change D7 to a 3.3V or 3.9 V zener). A small BJT wired as a VBE multiplier could also work.

With to little negative supply, to low a common mode voltage could be reached at output voltage zero or close, e.g. during a short or turned down to zero.

To be on the safe side for the supply to the TL081 one could use a series resistance and a zener to limit the voltage to some 30-33 V.  If one wants one could compensate for the voltage lost by using 2 extra diodes and an extra filter cap for the supply of U1. This way U1 would not see the ripply and might even give a little higher output under high load.

[Fabius_89]

Thanks for your valuable insight and time Kleinstein. Much appreciated.

I will continue to tinker with the supply and if wanted will post here the workarounds. If this post was closed then please just let me know, so I do not restart a closed discussion.

Best regards to all!

[Fabius_89]

There are simpler ways to reduce the negative supply (e.g. just change D7 to a 3.3V or 3.9 V zener). A small BJT wired as a VBE multiplier could also work.

I went with the Lm337 approach by Paul´s blog because the original zener is a bit noisy and as he found in constant current mode there is a 1.7V peak to peak ripple at the output which in some way the negative charge pump is responsible. Being driven by mains frequency. So eliminating D7 and changing R3 to and LM337 reduced some mains hum in CC mode. Still I must say that I did not experienced that much ripple testing my supply with the mentioned modifications (LM337 as R3, 470uF cap at output and 2.5A max output limit) as Paul did, and he limited his PSU to 1.5A (for more information go to http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html?m=1)in his, maybe I got a better version of the PCB or who knows?

[Kleinstein]

Ripple in the CC mode is to a large part due to the poor board layout, not properly sensing at the shunt. This could be fixed with a bodge. Cut at the blue line and connect the green.

For less negative side ripply one could increase C3 a little.

[PinheadBE]

I intend to use the modified kit with a 2x12 VAC transformer, and with the Arduino controller described on Instructables, modifying the code to switch a relay to change the input voltage to either 12 VAC or 24 VAC depending on the output voltage needed, so that the power dissipation in the output transistors could be limited for low output voltages.

Do you think it might work as such, or do I need to foresee other changes to the already modified schematics ?

Any opinion appreciated.

Thanks

[Kleinstein]

Reducing the AC voltage to 12 V may result in only relatively low current for the charge pump. So one may have to increase the capacitors there or still run the charge pump from 24 V in some way. At high current and thus high ripply 12 V AC may not be enough to power the OP used to provide the 10 V reference - in some suggestions this part of the circuit has changed anyway. So it could be a problem with some versions only.

There are quite a few modified / improved versions of the this supply around. As there may be a significant drop in the supply, when switching the AC voltage, one may have to have sufficient buffer capacitance. It may help to have the separate filter cap for the OPs, as in some versions.

[montyx]

Hello,
I've just bought this PSU from Aliexpress, and since then I've found this topic about. You have mentioned there are different and updated versions of the PCB itself. Can you help how can I find them please?
Thanks in advance

[Kleinstein]

The main weakness of the circuit it that some of the OPs (TL081) that come with the kit get too much voltage - so either change to other OPs that can stand the higher voltage or reduce the transformer voltage (e.g. 18 V AC). With changes to the circuit (some could be done as a bodge) one could reduce the voltage to the OPs a little, but this type of circuit is limited to output voltages smaller than the output range of one of the OPs. So 30 V from the TL081 is very hard.

The PCB is a main part of the kit. So there is little use in buying the kit and get a new board. The main weakness of the PCB layout is the ground connection that leads to ripply in CC mode. This can be fixed (see the picture a few posts before).
AFAIK the kits still come with the old layout.

[montyx]

Honestly this kit is a good sandbox to play with. The price indicates that, so I haven't got so big expectation
But also I'd like to purchase a low noise psu for a affordable price, and I prefer kits, because I can learn during the assemble. This was the most important reason why I've purchased this kit.
So if you have better ideas about more reliable kits, or any linear adjustable PSUs which are affordable, I could really appreciate any small help to a beginner guy.

[Urshurak776]

I’m
Building one but using a UA741CN chip instead of the TL081’s.  Transformer is 24V/2A.  Not sure how well this will work but giving it a whirl 🤣

[m3vuv]

Hi all,im currently building one of these,im checking voltages as i go, here goes,dc input from rectifier is 28v,u1 pin 7-4 is 42v,pin 4 against rectifies ground is -15v,u2+pin 4+rec -ve =-5.1v pin 7+4=33v, u3 pin 4+rec -ve = -5.1v,pin 7+4 =33v,why do i get  42v on pin 7+4 on u1?,any ideas,i presume this is wrong?,this is without the ic's fitted,just the sockets,cheers in advance.

[Zero999]

Hi all,im currently building one of these,im checking voltages as i go, here goes,dc input from rectifier is 28v,u1 pin 7-4 is 42v,pin 4 against rectifies ground is -15v,u2+pin 4+rec -ve =-5.1v pin 7+4=33v, u3 pin 4+rec -ve = -5.1v,pin 7+4 =33v,why do i get  42v on pin 7+4 on u1?,any ideas,i presume this is wrong?,this is without the ic's fitted,just the sockets,cheers in advance.

That sounds right to me. The rectified voltage is equal to the RMS AC voltage, multiplied by √2, giving around 40V.

Did you read the first page of this thread? The TL081 included in the kit has too lower voltage rating of jus 36V and should be replaced with an op-amp rated to at least 44V. A few alternatives have been suggested.
https://www.eevblog.com/forum/beginners/bangood-psu-enhancements/

[Kleinstein]

Hi all,im currently building one of these,im checking voltages as i go, here goes,dc input from rectifier is 28v,u1 pin 7-4 is 42v,pin 4 against rectifies ground is -15v,u2+pin 4+rec -ve =-5.1v pin 7+4=33v, u3 pin 4+rec -ve = -5.1v,pin 7+4 =33v,why do i get  42v on pin 7+4 on u1?,any ideas,i presume this is wrong?,this is without the ic's fitted,just the sockets,cheers in advance.

The description is a bis confusing. It is not clear where the voltages are measured, but it sounds like there is something wrong. One point to check may be the shunt. The circuit has a negative supply of some -5 V, so the -5.1 V are OK. If needed one may be able to reduce this a little (e.g. -4 V), but this still does not help with the main weakness of the circuit: it gives too much voltage to some OPs.

Are the 28 V from the rectifier measured without a filter cap ?  The capacitor gives the high DC voltage close to the peak voltage.

[m3vuv]

yes thats after the big input cap,basicaly i get 33v  to a coupe of opamps and 40 odd to the other,thats between pin 4 and 7 on all opamps.

[floobydust]

Then I would think your shunt resistor R7 0.47R 5W is open. With no IC's in place, there might be stray charge on C7 but ultimately U1 should have lower rails than U2, U3.

[m3vuv]

forgot to mention the shunt resistor wasnt fitted as thats off board,was just checking supply voltages to the opamps.