Cheap PSU: Piece of junk or semi-decent?

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

Yes. Exactly

Thanks C_hegge . Piece of cake then .

[c_hegge]

Re: Cheap PSU: Piece of junk or semi-decent?

to add a PI coil for say caps no. 1 & 2, I should cut the direct trace from 1+ to 2+, then bridge 1+ and 2+ with the PI coil. Right?

Yes. Exactly

[Behemot]

Re: Cheap PSU: Piece of junk or semi-decent?

Well it's not typical, but still SOME output filter in there. See the big toroid coil? I bet it sits on +3,3 V.

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

Thanks!

Hmmm ... I'm wondering about adding PI coils to a PCB that lacks parking slots for them ... ...

If I understand it correctly,


to add a PI coil for say caps no. 1 & 2, I should cut the direct trace from 1+ to 2+, then bridge 1+ and 2+ with the PI coil. Right?

[momaka]

Re: Cheap PSU: Piece of junk or semi-decent?

^ what goodpsusearch said. Those are good values to use. The Panasonic FJ may actually work due to lack of PI coils and small output inductors. Also make sure to change #14, 15, and 16. Use same or higher voltage and same capacitance values. #13 looks like a filter cap for the auxiliary rail that powers the PWM controller. Not necessary to replace unless it plays part in the 5VSB regulation. On older Deer/L&C units, I know it doesn't, but not sure about this one. Probably doesn't so you can leave it alone.

I can see one from here on +3,3 V

You must have the ability to see ghosts too then because I don't see any PI coils either .

[goodpsusearch]

Re: Cheap PSU: Piece of junk or semi-decent?

1,2 (+5V) · 1000uF · 10V · 105C · ASIA
3 (+5VSB) · 470· 16V· 105C · ASIA
4 (-12V) · 470· 16V· 105C · ASIA
5,6 (+12V) · 1000uF · 16V· 105C · ChengX
7,8 (+3.3V) · 1000uF · 10V · 105C · ASIA

12 (+5VSB) · 1000uF · 16V· 105C · ChengX
13 (+5VSB) · 47uF · 50V · ChengX

1,2 (+5V) · 2200uF
3 (+5VSB) · 1000uF
4 (-12V) · 470· 16V · don't change the value of that cap
5,6 (+12V) · 2200uF · 16V
7,8 (+3.3V) · 2200uF

12 (+5VSB) · 1000uF · don't change the value of that cap

[Behemot]

Re: Cheap PSU: Piece of junk or semi-decent?

This crappy PSU doesn't have any PI coil in the output

I can see one from here on +3,3 V

If you remember that Eurocase of mine, it actually had two coils on +3,3 V. I think that's the reason why this rail stayed in spec almost under all circuimstances. Other rails were so-so, barely in spec even with 50 % higher total capacitance

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

I'm thinking about a moderate recap of the 'TOOQ' PSU from the beginning of this thread, just the output filtering and +5VSB critical caps:

1,2 (+5V) · 1000uF · 10V · 105C · ASIA
3 (+5VSB) · 470· 16V· 105C · ASIA
4 (-12V) · 470· 16V· 105C · ASIA
5,6 (+12V) · 1000uF · 16V· 105C · ChengX
7,8 (+3.3V) · 1000uF · 10V · 105C · ASIA

12 (+5VSB) · 1000uF · 16V· 105C · ChengX
13 (+5VSB) · 47uF · 50V · ChengX

This crappy PSU doesn't have any PI coil in the output , so I wonder if it could be a good idea replacing all the output caps with higher capacitance ones, say 1500~2200uF replacing the 1000s, and perhaps ~1000uF replacing the 470s. Or something like that.

I have four spare Panasonic FJ 1500uF 6.3V 105C caps scrounged from an old mobo, apparently in good health. What do you think about using them for 1-2 (+5V) and 7-8 (+3.3V)?

[Behemot]

Re: Cheap PSU: Piece of junk or semi-decent?

I am hungry and cant focus Which has what resistance?

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

Is the bigger one accross first, or second cap? If second, isnt there higher voltage (from first AND second cap), so they maybe placed it there to limit current to +- same value as from first cap? Just a wild guess

As in the pic, R2 is in parallel with left cap, R1 with right cap.

[Behemot]

Re: Cheap PSU: Piece of junk or semi-decent?

Is the bigger one accross first, or second cap? If second, isnt there higher voltage (from first AND second cap), so they maybe placed it there to limit current to +- same value as from first cap? Just a wild guess

[c_hegge]

Re: Cheap PSU: Piece of junk or semi-decent?

Maybe Deer ran out of 330K resistors half way through assembling it?

[momaka]

Re: Cheap PSU: Piece of junk or semi-decent?

Seems they knew what they were doing after all. While on educational patrol I found this thread, and grew uneasy on the prospect of one of my brand new big Pannys eating excess voltage due to the weird assimetry in the bleed resistors .

Funny, I remember reading that thread but I don't remember those 2 particular posts in it. I didn't expect those bleeder resistors to make that much of a difference. Seems like they do, though. Time to check some of my PSUs that have bulgy primary caps.

So I checked on the live with DMM, and found with relief that both big caps read 158V each .

That's good. I'm still puzzled by the mismatched resistors though.

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

... R1 is 330K (orange/orange/yellow/gold), R2 is 470K (yellow/violet/yellow/gold) ... Either they knew something that we don't ... or they simply screwed up and put a resistor with the wrong Ω .

Seems they knew what they were doing after all. While on educational patrol I found this thread, and grew uneasy on the prospect of one of my brand new big Pannys eating excess voltage due to the weird assimetry in the bleed resistors .

So I checked on the live with DMM, and found with relief that both big caps read 158V each .

The Thing has been working like a charm for two or three dozen hours flight time now.

[tom66]

Re: Cheap PSU: Piece of junk or semi-decent?

Yes essentially.

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

... If you use a single MOV in a half-bridge PSU, you will not get full protection. The two big caps in a half bridge work as a voltage divider, so if you only clamp the top to the bottom you could still get voltage imbalance ...

And what if instead of two 180V MOVs, one in parallel to each bigcap ...

... I lay just one 360V MOV, in parallel to both bigcaps?:

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

Thanks! So if I get it right the formula is:

Max continuous operating voltage of MOV = 230 * 1.1 * 1.414 = ~360V

For two MOVs in parallel with bigcaps (half-bridge topology), halve that value (~180V each MOV).


Dang, just scavenged six MOVs from an old broken UPS and none of them suits :

· 561K -> 560V
· 471K -> 470V
· 2x 320K -> 320V
· 271K -> 270V
· 101K -> 100V

[tom66]

Re: Cheap PSU: Piece of junk or semi-decent?

The cap just breaks down internally -- it isn't a chemical reaction (so doesn't affect the electrolyte), but simple arcing across the dielectric. The dielectric is really thin, and decides the breakdown voltage and working voltage of the capacitor.

If it gets too bad, it creates local hot-spots, which damage the capacitor by decreasing capacitance. But this usually takes a few hundred ms.

If it gets REALLY bad, it creates a short circuit, the cap vents violently from the heat (it's got a few ohms of resistance so the fuse doesn't pop immediately), and then the fuse goes pop.

[Behemot]

Re: Cheap PSU: Piece of junk or semi-decent?

Yeah but degrading electrolyte is what kills the caps in the first place…

[tom66]

Re: Cheap PSU: Piece of junk or semi-decent?

Well, I've only repaired a few 115/230V jobs, and the primary caps haven't blown on them. Always the fuse. Sienoki has done a lot more... only had one which had more substantial damage.

The caps basically behave like short circuits at those voltages due to dielectric breakdown. The heat from this would quickly damage it but it is so quick it doesn't usually do any harm.

Perhaps it reduces the life of the capacitors but this just moves the problem to the MOVs. So you end up changing more of them.