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What are the effects of a blown Zener diode?

 
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FalconFour



Joined: 22 Nov 2009
Posts: 35
Location: Fresno, CA

PostPosted: Mon Mar 29, 2010 8:02 am    Post subject: What are the effects of a blown Zener diode? Reply with quote

Okay, since nobody's viewing my main topic except a few ghosts Laughing I figure I should reach a broader audience with a more broad question.

I'm trying to track down the cause of a blown MOSFET in the high-voltage side of a switching power supply. I've found a few "casualties" of the blown chip, like a couple resistors. But I haven't yet been able to figure out what caused the failure to begin with - until I found a Zener diode connected directly to the chip that's completely shorted, 0 ohms in both directions.

If a Zener diode malfunctions, could that cause such a catastrophic effect on the regulator that it completely blows the chip? I'm working on a relatively modern power supply from 2002, but it doesn't appear to have anything but discrete components on the whole power supply - no logic ICs in sight, or anywhere in the schematic. I imagine it's pretty susceptible to a brain-damaged blowout from some sort of runaway current... or a bad signal from a Zener.

Any thoughts?
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The_AntiRAM
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PostPosted: Mon Mar 29, 2010 7:47 pm    Post subject: Reply with quote

Blown Zener like your case is the final effect not the cause.

Zener going to short circuit in that manner is a conseguence of very high
direct current.

In this circuit The zener diode is (was) probabily acting as a reference
voltage source, and anyway a schematic helps exact diagnosing.

Why a SMPS goes faulty ??

Complex to say. Generally spoken, Capacitors leaking in circuit is a
common culprit but as before mentioned a schematic helps exact diagnosing.

Finally, nobody cannot diagnose properly without "see" "THE PATIENT"!!!
('Wink')
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FalconFour



Joined: 22 Nov 2009
Posts: 35
Location: Fresno, CA

PostPosted: Mon Mar 29, 2010 11:18 pm    Post subject: Reply with quote

Heh, true, that would come in handy. Here's the schematic of the affected area. I can post a more full image if it would help...



Really just want to figure out why it blew to begin with, before I end up blowing out another set of replacements. 300v is a heck of a high voltage and certainly not something I want to be "playing with"... Shocked

edit: TP30 is the MOSFET that killed itself. DP30 was shorted. RP30 & RP31 were smoked; RP31 was totally smoked to a crisp and fell apart when trying to remove it. RP33 checks out OK. Nothing else around that area is shorted, I poked around quite a bit with my multimeter. I took each of the electrolytic capacitors out and they tested OK on a capacitance meter. So I'm relatively confident that the chip killed itself by a power surge or thermal runaway... I don't see anything else gone wrong in the system... =\
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The_AntiRAM
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PostPosted: Tue Mar 30, 2010 8:37 pm    Post subject: Reply with quote

- TP30 has NOT killed by itself !! There is a Cause producing this !!!!!
Find IT Before ALL !!!

Anyway when TP30 is shorting during working, those Supplyes have
tendency to destroy or alter other parts wich are in need to be changed.

- RP32 AND DP32 are Surently GONE change it all !!
- CP30 Change it.
- DP31 (Pulse aplitude limiter + prot ) should be changed.
- RP31 Change it.
- RP30 Change it.
- DP30 Change it.
- CP12 Change it.
- Check / verify RP10, RP11, RP12, RP13.

- TP33 , TP27, TP28 Change all.
- TP24 and CP23 Change it.
- RP28 Change it.
- Cp42 and tp45 and rp45 Change it.
- There is a part that I can't see, error ampl + photocoupler verify it even
on secondary part.
- Change the photocoupler + rp44
- rp23 + rp22 + tp20 + tp21 change all.

Capacitor shouldn't be checked but changed without questions !!!
Resistors could be checked but those near faulty parts
should all be changed without questions !!!

Check for burnt parts/dry joint.

Remember that you have to do with a MOSFET output power stage !!!

This is not expensive to repair regarding component side
but requires a little TIME.

& patience
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FalconFour



Joined: 22 Nov 2009
Posts: 35
Location: Fresno, CA

PostPosted: Tue Mar 30, 2010 10:39 pm    Post subject: Reply with quote

Thanks!! Very Happy

I already replaced the two resistors RP30 & RP31; RP32 is actually a big-fat 3w resistor & it tests OK, but I'll still replace it on your suggestion Smile

DP32 and DP31 I already have on order along with TP30, both 16v/1.3w Zener diodes. Aside from those, though, I haven't done any other replacements...

Didn't think about CP30. If the capacitors are all available, I'll buy those replacements as well. The transistors, though, that's where I run into problems. Two of those, TP27 and TP28, are surface-mounted transistors on the other side of the board, and aren't shorted... is there a way to test those? Or even, replace them?

I'll have a look at RP10-13 when I get back home tonight. What about that photocoupler - do they tend to go bad?

Thanks A MILLION for all your advice so far... I'm glad to have someone at least giving me some sort of direction here Smile
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The_AntiRAM
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PostPosted: Wed Mar 31, 2010 7:24 pm    Post subject: Reply with quote

FalconFour wrote:
Thanks!! Very Happy

I already replaced the two resistors RP30 & RP31; RP32 is actually a big-fat 3w resistor & it tests OK, but I'll still replace it on your suggestion Smile

DP32 and DP31 I already have on order along with TP30, both 16v/1.3w Zener diodes. Aside from those, though, I haven't done any other replacements...

Didn't think about CP30. If the capacitors are all available, I'll buy those replacements as well. The transistors, though, that's where I run into problems. Two of those, TP27 and TP28, are surface-mounted transistors on the other side of the board, and aren't shorted... is there a way to test those? Or even, replace them?

I'll have a look at RP10-13 when I get back home tonight. What about that photocoupler - do they tend to go bad?

Thanks A MILLION for all your advice so far... I'm glad to have someone at least giving me some sort of direction here Smile



TP27 and TP28 they aren't shorted ??

May be. But they may go to leak by B-E inverse or something.
Result: You give power and two things may happen:

1) - You broke in few milliseconds another MOSFET + all components.
2) - Nothing happens / No start at all.

CP30 may be leaky or going to be inefficient. Change it to avoid future
side effect going to faulty behaviours during operating of the PSU.

YES the optocoupler should be changed for two reasons:

1) - May be one of the culprit part that generated all the fault by chain
reaction (No regulation ----> no feedback ----> overvolt -----> ALL
KO !!!)

2) - May be broken or damaged or even "stressed" during the
fault and, by a look to schematics, ruined in insulation requirements.
Furthermore reasons are the same aforemetioned for TP27 and TP28,
and the other parts directly/indirectly related to main fault.




Check secondary parts and change the electrolythic (Damn) capacitors.

For SMD surface-mounted components they are replaced with the
equivalent normal PCB mounted type. (good skill needed)

For this check and give a look of datasheet database of the parts,
after this check pins connections and put them in place.

Remember that this type of Supplyes DON'T ADMIT ERRORS during
repairing or you're going to have hard bad times (Nightmare).

At last try to check if the manufacurer of the set or the supply has a
repair kit for PSU ! (just to see)

(the link for the electrical diagram seems off......)
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