shitty chinese psu kit

[magic]

The relays are supposed to turn off when output voltage is low to reduce heat on TIP42. So this seems to be working as intended.

Probably CC or CV acts up and regulates the PSU down to zero. So it's gonna be U1 today.

For CV adjustment, the output should simply follow CV pot voltage delivered to U1 pin 5 (output should be ~80mV less because it's fed to pin 6 through divider R9/R10). If this works under no load than the problem is in CC.

On CC side, R6,R14 bias the low side of the pot to ~18mV less than the output - check if this works right. R3,VR1 limit how far the high side of the CC pot can go - this may need adjustment. CC kicks in when current sense voltage coming from those 0.22 resistors through R11 exceeds the threshold set by the CC pot. Because 5A·0.11Ω is 0.55V, you should adjust VR1 so that maximum CC pot setting gives 0.55V above the output voltage going to U1 pin 3.

Once all load is removed, CC should disengage and allow the output to rise up again.

[m3vuv]

is that u1 a or b?

[xavier60]

is that u1 a or b?

U1A is the CC opamp
U1B is the CV opamp.
You have not responded to my earler questions.

[m3vuv]

im using a 5w 12v bulb to load it,as soon as i remove the load voltage on the output recovers,havent measured u1a or b yet,its late,will look tomorow.

[magic]

So it's working as intended, but the CC setpoint is too low. Likely a matter of VR1 tweak or some resistors are wrong.

U1 is one chip and pin numbers on the schematic correspond to those in LM358 datasheet .

[m3vuv]

one thing i do remember is when i was messing with it was the tab of the 7912 shorted to the tab of a 7812 as there back to back,that popped but i renewed it,the 7812 seems ok tho.

[m3vuv]

Well not tested yet as have to put it back together,i checked resistors around the lm7912 that shorted,i found R5 33k to be open,looks like this is one of the feeds too pin 2 of u1A,fingers crossed.

[m3vuv]

i find these new metal film resistors just go open at any sign of abuse,not like the carbon resistors that could glow red hot and still sort of work!.

[m3vuv]

Well i dont know if i mentioned it but i found r5 open,replaced that also added more  pass transistors and current sharing resistors x2 so now 6x pass transistors,and it worked,so started testing into a resistive heater element as a load,had ,i also was running the heatsink fan from the output of the lm7812 vreg,it was going well for as i wound up the  output up from zero on max amps ,it got to 10 amps think it was reading 25v,then the fan slowed down and output vanished!,i found r5 open again so replaced it with a 2 watt item,now all transistors ie pass transistors and the tip41 driver test good but still no output,fan is normal tho and voltmeter works,any ideas?,seems i am chasing my tail with this! lol TIA.

[m3vuv]

forgot to say i renewed all of the opamps too.

[xavier60]

It is impossible to electrically damage  R5 33k with the available voltage.
Is the +12V control rail ok?
If so, measure the CC and CV opamp outputs WRT the + output terminal, unloaded then loaded.
Also say what the output voltage is for each test condition.

[magic]

Check if C4 isn't shorted and if the resistor you are replacing isn't R6 or some other one.

[m3vuv]

Well,the 12v power rail from the 7812 is ok,c4 is not shorted and it was r5 i renewed to get it working after the first blow up,to test the output ot the cc/cv opamp,against the + output,do i need my red+ve meter lead in the +ve output jack or black-ve?,all diodes test ok as well in zeners ,but havent tested zener voltage,just normal voltage drop in foward direction,they drop .7v others 0.6..

[xavier60]

Have the black meter probe on the + output terminal which is the same as the 0V for the control rails.

[m3vuv]

Well,dont know if this is good or bad news! lol,i was removing the 7912 with a hotair smd heat gun and when i pulled it,one leg stayed in the pcb,i had been bending it a while ago when fitting the added pass transistors,i assume this will cause the zero output?,i have a lot of resistors removed from the board at the moment that i removed to test as in cuircit they gave strange readings,i was just gonna lift one leg then thought no,i am going to change them for carbon types as they seem more resiliant to abuse!( is that a good idea?),i have checked all resistors associated with the cc/cv opamps and they are all good,also originaly had q7 blowup when i first connected the transfo taps wrong on the first powerup after building it!,i replaced that with a tip122,now thatd testing as a double diode!,may test the 8050 driver too,also is there a simple test for the tl431 out od circuit,i have spres so may just replace it.TIA.

[xavier60]

Well,dont know if this is good or bad news! lol,i was removing the 7912 with a hotair smd heat gun and when i pulled it,one leg stayed in the pcb,i had been bending it a while ago when fitting the added pass transistors,i assume this will cause the zero output?,i have a lot of resistors removed from the board at the moment that i removed to test as in cuircit they gave strange readings,i was just gonna lift one leg then thought no,i am going to change them for carbon types as they seem more resiliant to abuse!( is that a good idea?),i have checked all resistors associated with the cc/cv opamps and they are all good,also originaly had q7 blowup when i first connected the transfo taps wrong on the first powerup after building it!,i replaced that with a tip122,now thatd testing as a double diode!,may test the 8050 driver too,also is there a simple test for the tl431 out od circuit,i have spres so may just replace it.TIA.

The broken pin on the 7912 could have been causing a problem, only a voltage measurement would have made it more certain and might explain the intermittent nature of the fault.
Replacing the resistors isn't necessary at all.
I'm having difficulty following circuit references. Which schematic are you using?
Like a lot of things, the proper function of the TL431 is best done with voltage measurements. Just check that it's shunt regulating 2.5V first. If it is, then move on. If it isn't, then investigate further.

[m3vuv]

Hi the schematic and info i am using is on the link here :https://gswagner.com/35%20V%205%20A%20Supply/35%20V%205%20A%20Power%20Supply%20Schematic%201%20of%203.pdf
too late on the voltage ref tl 432?,ive replaced it already,will post when ist all back together.

[xavier60]

Hi the schematic and info i am using is on the link here :https://gswagner.com/35%20V%205%20A%20Supply/35%20V%205%20A%20Power%20Supply%20Schematic%201%20of%203.pdf
too late on the voltage ref tl 432?,ive replaced it already,will post when ist all back together.

I couldn't get that link to work, I assume it's this one, "ledtester" posted earlier.
https://gswagner.com/35%20V%205%20A%20Supply/
"Vpwr Control" and "Apwr Control" are interchanged between sheets 1 and 2.
R5 circuit position is also wrong. It should connect to the RH end of R14. R5 is not important to PSU operation.
R6 and C4 as shown, do nothing useful.

[magic]

So halfway in the thread you started using a different schematic with different component numbers without telling anyone anything?

Isn't that guy's schematic wrong? You said on the first page that your board agrees 100% with the schematic and BOM from the auction.

[m3vuv]

magic,my board matches the schematic i got from the seller(sort of - the lm7912+7812 vregs,same with the bom,i am now using the one on wagners site as that seems more complete,hope that clears it up.

[m3vuv]

magic,my board matches the schematic i got from the seller(sort of - the lm7912+7812 vregs,same with the bom,i am now using the one on wagners site as that seems more complete,hope that clears it up.

[magic]

Wagner shows a pair of LM7812/LM7912 being used to power U1 and the AliExpress schematic shows 680/1W resistors and 15V/6.2V zeners instead. The AliExpress board photo somebody found also has space for those resistors and zeners, but not for regulators (except those 7812/7805 on page 3 of Wagner).

And there are those weird differences in resistors that xavier60 pointed out.

Obviously, both schematics can't be true at the same time

Given that you talk about 7912 regulators and I can't see any of that on the AE schematic, I suppose that your board version is the same / similar to Wagner's?
Are the CC/CV pots also connected to the board with three wires instead of two?
Are those resistors R5, R6 and cap C4 exactly as drawn by Wagner?

[m3vuv]

my pots connect with 3 wires not 2,r5+6 also c4 are as per wagners scematic om my board.

[m3vuv]

are there higher power pass transistors with the same tab mountings i could use in this that could be driven to 30+amps,without killing the opamps or driver ?,several of them in parralell of course.

[magic]

Okay, so what's the current state of this PSU?

There are several regulators (I use Wagner's numbers now).
U6 7812, U7 7805 that power some opamps, relays, fan etc. AFAIK there is absolutely no problems with this part of the circuit, no magic smoke, no nothing, correct?
U3 7812 which powers the U1 CC/CV opamp. This regulator is still OK too?
U4 7912 which powers U1 - this blew up once or twice already?

And the failure is always the same: blown 7912, blown U1 and blown R5 between these two ICs? I'm not sure what could be causing it, but at least it makes some sense because these components are connected together.

What sort of power supply are you using for U3,U4? It's supposed to be some low voltage transformer, separate from the main transformer.
What's the voltage on Vcont+ and Vcont- and on the inputs of U3,U4? (Black probe on Vcont each time).


There are two things I would change right off the bat:
1. Most negative regulators require output capacitors for correct operation. The big LM79 regulators in TO220 are supposed to work with at least 25µF aluminium electrolytic per National datasheet. So I would bodge some 33µF or more into the circuit.

2. LM358 really doesn't like its inputs pulled below the negative supply. I would add a diode from Vcont to U1 pin 5. I posted a pic with this mod on the first page. It's nice that they corrected pot wiring on this new board revision, but protection diode is still missing.

In general, I can't see any sane reason for U1 to blow up here, except for these two problems above: misbehaving supply regulator or input voltage out of range.

Output base current is provided by R4 from U3 (a few mA) and U1 only removes some of that current before it reaches Q1 in order to throttle down the output. It really has no good reason to blow up.