Cheap PSU: Piece of junk or semi-decent?

[TELVM]

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

While you're at it, replace those two small electros. They, along with some resistors and diodes, bias the two switchers ...

Beg your pardon: 'Electros' ? Do you mean those two small 10uF 50V caps?

[Th3_uN1Qu3]

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

"Electros" = short for "Electrolytics". So yes, those two is what he was talking about.

[TELVM]

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

Oh, roger , two Panny FC are underway.

[TELVM]

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

(Most amazingly) my switcher swap looks like a success.

>Expand . . . >Expand . . . >Expand

No explosions and the PSU seems to work OK giving coherent voltages.


Next I'm going to swap the +12V rectifier diode, let's see if lowering the Vrrm brings fireworks or not ...

.

[kc8adu]

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

i wouldnt do that.if anything use a higher rated part.lower vf however is good!less drop=less heat.

(Most amazingly)

Next I'm going to swap the +12V rectifier diode, let's see if lowering the Vrrm brings fireworks or not ...

.

[TELVM]

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

i wouldnt do that.if anything use a higher rated part ...

I'm confused now ... :

... i doubt you need anywhere near 100v though.
i like twice the output voltage.
vf is lower on lower voltage parts so less heat ...

https://www.badcaps.net/forum/showthread.php?t=3759

[kc8adu]

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

didnt remember you mentioning a 100v part.
hell a 50v would give plenty of margin.

[Behemot]

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

Problem are the spikes, according to the unique (sorry, not using those strange-stuff numbers ) quite high reverse-voltage spikes can occur and the rectifier has to handle them.

[TELVM]

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

Swapped the MBR20150CTP +12V rectifier for a STPS40M60CTN.

>Expand . . . >Expand . . . >Expand

No fireballs but no joy, the PSU doesn't start .

When switched on the fan doesn't move even a bit. With the multimeter on the +5V rail I see just ~a couple volts that very quickly drop to zero.

+5vsb is alive showing ~5V.


Testing with the MMF:

+12V rectifier (the new one)
Leg 1 Leg 2 (center) = 0.0 continuous beep
Leg 3 Leg 2 (center) = 0.0 continuous beep
Leg 1 Leg 3 = 0.0 continuous beep

+5V rectifier
Leg 1 Leg 2 (center) = 0.48 short beep
Leg 3 Leg 2 (center) = 0.48 short beep
Leg 1 Leg 3 = 0.0 continuous beep

+3.3V rectifier
Leg 1 Leg 2 (center) = 0.15 continuous beep
Leg 3 Leg 2 (center) = 0.15 continuous beep
Leg 1 Leg 3 = 0.0 continuous beep


+12V YELLOW WIRES GROUND WIRES = 0.0 continuous beep
+12V YELLOW WIRES Ω GROUND WIRES = 1Ω

+5V RED WIRES GROUND WIRES = 0.48 short beep
+5V RED WIRES Ω GROUND WIRES = 44Ω

+3.3V ORANGE WIRES GROUND WIRES = 0.15 continuous beep
+3.3V ORANGE WIRES Ω GROUND WIRES = 15.4Ω


What do you think?

.

[Behemot]

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

Either there are minimum load resistor of quite low values or it's shorted.

[domas]

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

12V one seems to be shorted
leg 1 to 3 of rectifier and vice versa are normally shorted over pcb, that is normal.

2 to 1/3 and vice versa should not be shorted.

[Th3_uN1Qu3]

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

Sounds like your replacement 12v rectifier was shorted to begin with, or there's something wrong with the soldering. Remove it and measure again. BTW, legs 1 and 3 are not "shorted over pcb" in a half bridge PSU, they are connected to opposite ends of the transformer winding. But since the winding is made with thick wire, it measures (almost) shorted. An inductance meter would tell you a different story.

[domas]

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

yes, after you desolder 12v rectifier measure both pcb without rectifier and the rectifier itself separately. There might be a short somewhere else around rect.

[TELVM]

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

OK, removed the newcomer STPS40M60CTN and out of the board reads:

Legs 1 or 3 to Leg 2 (center): .150 , Ω 58
Leg 2 (center) to Legs 1 or 3: 1.

^ Does that sound like shorted ?

For comparison, the original MBR20150CTP out of the board reads:

Legs 1 or 3 to Leg 2 (center): .240 , Ω 88
Leg 2 (center) to Legs 1 or 3: 1.


The holes in the PCB (sans +12V rectifier) now read:

Holes 1 or 3 to Hole 2 (center): .390 , Ω 1.
Hole 2 (center) to Holes 1 or 3: .390
Hole 1 to Hole 3, or vice versa: .001 continuous beep , Ω 01.0

Hole 2 (center) to YELLOW WIRES: 0.01 continuous beep , Ω 01.0
Holes 1 or 3 to YELLOW WIRES: 0.390 , Ω 1.0
YELLOW WIRES to GROUND: .390 , Ω 1.

[tmiha71]

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

http://en.wikipedia.org/wiki/Avalanche_breakdown

Probably too high reverse voltage (for that part)...

[TELVM]

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

Soldered back the original MBR20150CTP +12V rectifier and the PSU works OK . Once on board the MBR20150CTP reads thus:

Legs 1 or 3 to Leg 2 (center): .213 , Ω 80.6
Leg 2 (center) to Legs 1 or 3: .390
Leg 2 to YELLOW WIRES: .001 continuous beep , Ω 01.0
Leg 2 to GROUND: .390 , Ω 1.


This means the gremlin dwells in the new rectifier somehow.


I wonder why the new STPS40M60CTN reads like it is shorted while on board ... and I'm thinking ... could it be that, unlike the MBR20150CTP, it needs to be isolated with a mica from the (grounded) secondary heatsink?

[kc8adu]

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

yes it does!
now smack yourself on the forehead and try it again with insulator!
i was thinking solder short.your original had an isolated tab.

[TELVM]

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

... now smack yourself on the forehead and try it again with insulator! ...

Aye aye!



OK at last, once insulated from the secondary heatsink, the new rectifier works as advertised . ~12.3V & 5.07V with a couple fans plugged in, and no explosions thus far .

.

[Behemot]

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

You were supposed to smack yoruself, not somebody else!

[tmiha71]

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

Now, if you really didn't put insulator where it was needed, i'll skip all your posts in future :-))