Cheap PSU: Piece of junk or semi-decent?

[tom66]

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

The two caps are in series, creating a 400V capacitor (with half the capacitance of the two capacitors.)

The peak voltage from 230VAC mains is about 325V.

[TELVM]

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

Thanks Tom!

[TELVM]

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

Well I don't think these resistors will cook my fresh caps anymore .

[momaka]

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

Well I don't think these resistors will cook my fresh caps anymore .

https://www.badcaps.net/forum/attach...5&d=1353379557

https://www.badcaps.net/forum/attach...6&d=1353379557

https://www.badcaps.net/forum/attach...7&d=1353379557

Wait! You might want to put those resistors back the way they were! Those are not the same as the minimum load resistors on the secondary - the resistors in the 3 pictures above are for current sensing and putting them on long leads is not good! Besides, those don't run that hot anyways.

[goodpsusearch]

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

Wait! You might want to put those resistors back the way they were! Those are not the same as the minimum load resistors on the secondary - the resistors in the 3 pictures above are for current sensing and putting them on long leads is not good! Besides, those don't run that hot anyways.

+1

This looks scary

[TELVM]

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

Aye aye. Lesson of the day: Don't fiddle with current sensing resistors .

The gray resistor is 2.6Ω, the pink one is 47.1Ω. As they are similar in form and size to the other four I thought it would be interesting to put them higher in the breeze .

Which ones are the bleeder resistors, those that discharge the two big caps after power off?

[Behemot]

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

There is most often none. Important is only the one which is on input filtration to discharge X and Y caps which can give you minor shock from the energy they store.

Usually nobody cares about input caps as the energy they store cannot go back through rectifying diodes. For that reason if you will ever test supply on live, have some 10k+ resistor and discarge the caps manually after disconnectign mains. I help myself by measuring their left voltage too. Otherwise you can get shocked or destroy some parts of the PSU if you will accidentally short some parts with the caps for example over the metal casing. I have some experience in this area

[momaka]

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

Aye aye. Lesson of the day: Don't fiddle with current sensing resistors .

Well one of them (the 47 Ohm) is actually for the snubber network for the half bridge, so it's not that critical to have it with short leads. However, it's still better not to fiddle with it.

Which ones are the bleeder resistors, those that discharge the two big caps after power off?

Should be R1 and R2, found between the primary caps and the primary heatsink. They should be a high value - usually at least 100 KOhms and above. On my LC-B300ATX, they are 330 KOhms (orange, orange, yellow, gold).

There is most often none.

Not true.
Whenever there are 2 caps in series like that, you will always find 2 bleeder resistors - if you don't, the manufacturers are, as tom66 would say, doing it wrong.
The bleeder resistors have another purpose that's actually more important than just discharging the caps - at very low loads (such as when the main power supply is off and only standby section is working), they balance the voltage between the primary caps.
They become even more important if the PSU is to be used in countries with 230 VAC mains. That way, if a slight mismatch in capacitance and ESR exists between the primary caps, the resistors will balance the voltage.

Important is only the one which is on input filtration to discharge X and Y caps which can give you minor shock from the energy they store.

That "bleeder" resistor across the mains also has a more important purpose than just discharging the X/Y caps - and that is to ensure that if power is disconnected from the PSU abruptly, the energy stored in the input common mode choke and/or PFC (if any) won't peak too high to destroy the X/Y caps and MOV (if any).

Otherwise you can get shocked

Been there, done that, didn't get any T-shirts though. Both times it wasn't an PC PSU, however - it was TV/monitor PSUs with a single large 400V.

[Behemot]

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

Guess I have been looking into very wrong PSUs mostly, and not very thoroughly too

[TELVM]

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

Again thanks for the help gentlemen , you are managing to slowly extract me from the darkness of ignorance. Here, another token of gratitude:

... Should be R1 and R2, found between the primary caps and the primary heatsink. They should be a high value - usually at least 100 KOhms and above. On my LC-B300ATX, they are 330 KOhms (orange, orange, yellow, gold) ...

Here they are:

I've tried to measure its resistance with the multimeter, but while in circuit with the big caps the readings are all over the board.

There are two empty parking slots near them marked 'Z1' and 'Z2', could these be for MOV varistors? If so, which MOV specs (volts, joules, ns) would be adequate for this PSU?

[Behemot]

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

Seems so. If they are on the potential between the two caps, than it would be something like 115 Vrms/150 V DC. I use 9mm diameter, always tie them into some heatshrink, you get significantly less residue if it goes

[TELVM]

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

... always tie them into some heatshrink, you get significantly less residue if it goes

That's foresight .


If I read the colour codes correctly R1 is 330K (orange/orange/yellow/gold), R2 is 470K (yellow/violet/yellow/gold).

[momaka]

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

If I read the colour codes correctly R1 is 330K (orange/orange/yellow/gold), R2 is 470K (yellow/violet/yellow/gold).

That's not quite right - if those were the bleeder resistors, they should've had identical color codes. So I guess I was wrong, - R1 and R2 are NOT the bleeder resistors.

Perhaps R3 and R4(?) are (the two resistors with color codes Green, Blue, Yellow, Gold). If they are in parallel with the primary caps, then those are it for sure. Check me this time .

[Pentium4]

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

I also learned from what TELVM did, I didn't know that. But I admire his creativity Although those Panny's will still last probably longer than you'd ever use them for

[TELVM]

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

That's not quite right - if those were the bleeder resistors, they should've had identical color codes. So I guess I was wrong, - R1 and R2 are NOT the bleeder resistors.

Perhaps R3 and R4(?) are (the two resistors with color codes Green, Blue, Yellow, Gold). If they are in parallel with the primary caps, then those are it for sure. Check me this time .

Resistors are so marked:

And so connected:

R1 is in parallel with second bigcap, R2 is in parallel with first bigcap.

R4 has one leg on negative leg of first bigcap (the other leg of R4 displays 0.002 while beeping with bigcap 1 negative leg).

R3 has just one leg connecting with bigcap 1 (+) and bigcap 2 (-), the other leg or R3 does not connect to bigcaps.

R5 does not connect at all with bigcaps.



Z1 is in parallel with one big cap, Z2 is in parallel with the other bigcap.

[Behemot]

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

Z1 is in parallel with one big cap, Z2 is in parallel with the other bigcap.

That's it. There is half the rectified mains over them, so you would be fine with similar voltage of the varistors.

Maybe for example VE09M00131K would be safer despite my previous recommendation. This one is for 170 DC and it starts conductig at 184 V minimum (compared to 162 V of the previous ones). This is enough to protect those caps yet not to conduct too often and destroys itself sooner.

[TELVM]

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

Yay, I busted my first cap .

Plugged my creation to the wall, and just after five or so seconds, when I was beginning to believe, heard a venting noise .

Unplugged INSTANTLY , smelled no napalm in the morning, opened it cautiously, looked for casualties, and there it was, poor number 15, the Rubycon 470uF 16V for -12V, had bought the farm.

And the cause of the crash was of course pilot error . A couple pages ago, when we talked about caps 15 & 19 (positive on ground, negative on blue -12V / white -5V respectively), for some reason only my weird neuronal processes know, I left with the fixed idea that both 15 and 19 should be soldered reversed (cap positive on PCB negative, cap negative on PCB positive ). And so I did, and so happened what had to happen ...


Now the good news.

The other Ruby 470uF 16V, number 19 for the -5V rail, seems to have survived the ordeal (no vent, no bulging, and healthy as far as my simple DMM can say), although it was subjected to the same torture for ~5 seconds.

And there seems to be no other casualties, which is amazing (during the countdown I was peering through the slot in my bunker expecting 20-megaton explosions, or worst ).

So there could be still hope.


A couple questions if you don't mind .

· Can I trust the (apparently) surviving -5V number 19 Rubycon?

· As I got no Rubycon 470uF 16V spare, and will not order for just a single cap or a pair, the replacement has to come from the strategic reserve. Which one of these options would you use?:

Left: Koshin originally in this PSU // Right: Choyo from a mobo (it's a bit shorter)

· If we can trust the (apparently) surviving Ruby, should I left it where it is, or should I move it to position 15, and put the crappy in 19? (my feeling is leave it where it is ... )

· Would putting a crappy cap in -12V (or -5V) be a great tragedy?

[tom66]

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

I can't see much problems with -12V usingf a crappy cap.

It's generally poorly regulated, and rarely under any more than a few hundred mA load, if that...

You can likely increase the capacitance without significant concern.

[mariushm]

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

Both -5v and -12v are hardly ever used on modern systems.

-5v is already optional in newer ATX standards, other than isa slots I don't remember off the top of my heads what used it... -12v is still used sometimes on serial or com ports, I think.

So I wouldn't bother replacing that -5v capacitor.

As for choosing between those, I think either one would work, like I said it's unlikely your system will even use -12v so it's no cause of concern.

If you would force me to pick one, I'd probably go for Koshin, just because I've seen them more often in hardware.

[Behemot]

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

You are 5 years back, -5 V is not present from ATX V2.01. Not optional, it is not in the specification. The -12 V is there but rarely used, mostly by older audio cards.