shitty chinese psu kit

[m3vuv]

Hi all,i have one of these:https://www.aliexpress.com/item/32986139693.html?spm=a2g0o.productlist.0.0.627b1d98IrSbAW&algo_pvid=9607a120-ddef-4f47-bfbb-cc527331571d&algo_exp_id=9607a120-ddef-4f47-bfbb-cc527331571d-6&pdp_ext_f=%7B%22sku_id%22%3A%2266875672074%22%7D&pdp_pi=-1%3B12.8%3B-1%3B-1%40salePrice%3BGBP%3Bsearch-mainSearch
Firstly it came without the 15v and 6.2v zeners as marked on the schematic that arived with it,ive just built it but have no place/holes/silkscreen markings for the 6.2v or 15v zener diodes,has anyone else built the same if so whats the score,do i have a reject board,i havent put power to it yet ,i would rather know what is what with it before doing that,i have mailed the seller but no responce as of yet,maybe due to time zones as im in uk,TIA m3vuv.

[Algoma]

Simplified URL: Links can exclude everything past that first: ?

https://www.aliexpress.com/item/32986139693.html

Oh, those kits often copy and paste other listings and then ship the cheapest knockoff version they can get from a local market.  Can you post a few pictures of the board you did receive?

[magic]

Why did you start another thread on this?

Anyway, if there is no zeners in the kit and no place for zeners on the board, it goes without saying that you must have received wrong schematic. At best, they use open circuit output voltage of the transformer without any regulation, at worst it's a completely different circuit and you haven't noticed during assembly

[m3vuv]

not a lot of point sending pics of the board as its now fully populated with components,it looks %100 identical too the one in the listing tho,even the bom checks out against the schematic,but lists d10+d9 that it doesent appear to have.

[magic]

If lack of zeners is the only difference, the CC/CV circuit supply voltage will be unregulated and higher (how much? no idea, disconnect the CC/CV transformer and measure its output). And you may have trouble with adjusting CC mode regulation, perhaps. Maybe they gave you different resistors in this area to compensate for different supply voltage.

Furthermore, the problem with pot connections still stands. It should be done like below (the diode can be anything, like 4148 or 4007, it's reverse biased under all sane conditions).

[MK14]

This kit seems the same:
https://www.aliexpress.com/i/32946735505.html

But it seems to have clearer pictures, including of the PCB.

Although I can't see the markings for D9/D10, I can probably see where they go, in the bottom left hand corner, between the pair of 680 Ohm 1 watt resistors.

If you have the same PCB layout as the picture in my link (click view details and buy, then please scroll down, to see big picture of PCB).

[magic]

This kit seems the same:
https://www.aliexpress.com/i/32946735505.html

Finally a legible schematic

I see that negative rail voltage has little effect on CC setpoint, just 15~20mV to compensate for LM358 offset voltage. It's still better to have it regulated, though. I don't think there would be problems using 5.1V instead of 6.2V.

The positive supply looks like it could be reduced too, if not to 5V then maybe to 10V at least, but some resistor would need to be recalculated.

If you feel like it, those stupid shunt regulators could be replaced with a 78L/79L pair. (With some care).

[Electro Fan]

Maybe it's just the particular supplier's parts, or maybe trying to squeeze out a bit more performance (35V, 5A) than can be expected for the price.

These kits (30V, 3A) from various sources sometimes including Amazon perform pretty much as advertised:
https://www.ebay.com/p/1474757007
Some versions come with a heat sink and small fan (see photo below).

Some learnings from building the kit:
https://www.eevblog.com/forum/beginners/question-on-power-supply-diodes/

Also, might try a slightly less derogatory subject title too.  Jus' sayin'

[ozcar]

. . .
If you feel like it, those stupid shunt regulators could be replaced with a 78L/79L pair. (With some care).

Take a look at this other listing.

https://www.aliexpress.com/item/32946735505.html

From a quick look the schematic looks similar. However, this other listing has a pretty good PCB picture, and it is easy to see where the two zener diodes and the associated 1W resistors are placed.

In contrast, the PCB photos in the listing given in this thread appear to show fixed-voltage regulators, 7912 and maybe 7812 in the equivalent place on the PCB. So, may not be necessary to make any changes after all.

I’d say the schematic and PCB in this thread are only distantly related to each other, so maybe follow the advice given, and stand well clear of it when turning it on!

Ah MK14 beat me to finding that...

[magic]

Those big regulators are for other stuff. We are talking the CC/CV regulator (top of the schematic) running from a small floating PSU.

This design seems reasonably foolproof, because current from the output can't pass through the CC/CV circuits to ground, except through that dangerously connect CV pot.

I would rewire the CV pot, start the CC/CV circuit without power on the main transformer and verify if everything works, then power the rest up. Also, I would mod CC/CV to run on ±5V because I have a suitable crap transformer with no better use for it.

Try to stay away from the motherboard when powering on for the first time! Prevent electric gun incidents caused by various welding errors! ! !

If the potentiometer is disconnected during use, there will be 30V voltage and 5A current output! ! !

Exactly what I said. And moreover, zeroing / shorting out the CV pot with high voltage present on the output could blow up the CV opamp. And the circuit can be fixed easily, they are idiots

[xavier60]

The control transformer doesn't need a center taped secondary if it's being half wave rectified as shown. A single 12VAC secondary could be wired to work just as well.

[xavier60]

The CV Pot could be replaced with a fixed divider and the Pot used to adjust the reference in place of R9 and R10.
The stability will need to be checked and possible compensation tweaks.

[m3vuv]

I think its possible they use the linear regs as mentioned,im just finishing the case then will power it up and try it,will be tomorow as 20 to midnight here,if i wanted to hook up a cv indicator where would i hook it up too?,cheers pAUL M3VUV

[magic]

The control transformer doesn't need a center taped secondary if it's being half wave rectified as shown. A single 12VAC secondary could be wired to work just as well.

Only if the capacitors are sufficient. A dual secondary recharges them twice as often. But you are right, it's another missed opportunity for improvement.
LOL, you are right. Those guys really were on some hard drugs.

The CV Pot could be replaced with a fixed divider and the Pot used to adjust the reference in place of R9 and R10.
The stability will need to be checked and possible compensation tweaks.

IMO not worth it, just fix the damn pot. It's nice as it is, the opamp runs consistently at unity gain regardless of output voltage thanks to C7. It's like a discrete build of LT3042 with a slightly noisier buffer

if i wanted to hook up a cv indicator where would i hook it up too?

Same way CC is done

[m3vuv]

ok,the cc led gets its from ua output so saying the same as cc dont help!,so what pin should i use for a feed for a cv led indicator,or are you taking the piss? lol.

[xavier60]

ok,the cc led gets its from ua output so saying the same as cc dont help!,so what pin should i use for a feed for a cv led indicator,or are you taking the piss? lol.

D17 could be replaced with an LED also but LEDs can and do go open circuit.
A safer option is an LED with series dropper resistor from the +15V control rail to the output of the CV opamp, pin 7.

[xavier60]

If D17 is replaced with an LED, a Zener diode or even a resistor placed across it would make the LED failure safe.

 Extra: The LED might dim noticeably with increased PSU loading. 

[xavier60]

Actually the position of VR2 is rather clever. It makes an open wiper contact fail safe, the PSU's output voltage will go low.
But its position would also normally prevent the PSU's voltage from being able to be adjusted to 0V. The R9/R10 divider fixes this.

[magic]

VR2 is
It's J2 where you plug the CV pot. OP said so in the previous thread.

[xavier60]

VR2 is
It's J2 where you plug the CV pot. OP said so in the previous thread.

So it's not fail safe at all.

[magic]

Obviously.
And it gets better. Consider such cases:
1. the output cap is charged, CV pot is rapidly turned to 0Ω
2. CV pot is set to 0Ω, 12V is backfed from outside

[m3vuv]

Thought i would just mention it,this thing has what looks identical to a zener diode,but aparently its a thermistor,ive measured it and it is in fact a thermistor,has anyone else seen one like that?,all the ones ive seen are totaly different to this either a bead type or what looks almost like a disc ceramic cap.

[Terry Bites]

You cannot gild a turd!

[MK14]

Thought i would just mention it,this thing has what looks identical to a zener diode,but aparently its a thermistor,ive measured it and it is in fact a thermistor,has anyone else seen one like that?,all the ones ive seen are totaly different to this either a bead type or what looks almost like a disc ceramic cap.

Are you sure it's not just a faulty zener diode ?

[magic]

DO35 thermistors are a thing.

Sometimes even a good zener could be confused for something else if tested at insufficient current. Low voltage ones need a few mA.