odd failure of zener in supper supply

[cybermaus]

Hi Guys

I have something that I find rather odd. Trying to fix a Dell power supply for a monitor with internal power. Dust had build a bridge between some pins of the high frequency transformer and blew out the FET and controller chip.

I know for sure the FET and chip were busted: FET component tested as diode, and chip had a short to ground on a pin that his new replacement did not. The diodes around that area tested fine, except one zener. Based on the service manual spec I am pretty sure it is supposed to be a 4.7V zener: "ZD901 0DZ470009BC GDZ4.7B TP GRANDE DO34 0.5W 4.7" Its actually labeled 4B7, so that should be correct.

But it tests as a 3.3V zener. Forward 0.7V, reverse 3.3V !

How common is it for a zener to fail to a lower value? I would expect either fully open or close. But to fail down to a working 3.3V?


Of course, I'd simply put a new 4.7 in (or even 4.3 or 5.1) if I had one lying about. All I have is another 3.3 and a 9.1. I guess I'll have to find one. (the nearest proper electroshop is 45 minutes hour away by car)

But I am interested, anyone thinks it is normal to fail like this, or would this be a 'normal' factory spec variation or even component lifetime drift.

[krivx]

It's more likely the wrong part was installed at the factory. Or the service manual is incorrect or out of date.

[mij59]

Hi,

Zener diodes tend to short out, or become leaky, at what current did you test the zener diode ?

[sleemanj]

I'd guess that the actual value was not critical so they just used 3v3, especially if there are 3v3 anywhere else on board.

[wraper]

I've seen a few times zener to fail acting like lower voltage zener. What is written on the case? Where on schematic is it used?

[cybermaus]

As mentioned, the actual zener print is "4B7", so the idea they used a lower value on purpose does not hold. 4B7 seems a very plausible print for a 4V7 zener.

Tested first with a component tester (unknown test current) and then to confirm with a 22K and then a 10K resistor on about 8V power supply. All 3 gave about 3.3V. Also used tried two different multimeters, just to make sure. Weird.

I now have all active components (FET, chip, diodes) in that part of the circuit replaced. Trafo has continuity. But 340VDC goes in (I checked the DC voltage after the full wave rectifier), nothing comes out. Guess I have to start measuring if I get any waveforms on the chip. Not sure I like hooking my scope to the hot side of the board as I have no diff-probe. (and yes, I know about the ground loop trap)


Anyway, about the zener. Still unsure how it is possible that a 4.7V zener fails to become a seemingly good working 3.3V zener. But assuming the manufacturer accidentally mislabeled a batch of zeners seems equally unlikely.

[wraper]

Tested first with a component tester (unknown test current) and then to confirm with a 22K and then a 10K resistor on about 8V power supply. All 3 gave about 3.3V. Also used tried two different multimeters, just to make sure. Weird.

Too big resistance, you need something like 300-600 ohms or even less to be sure that it is not just leaky.

First measure if there is enough voltage on IC power. Small electrolytic capacitor on IC power also often becomes bad with time (before mosfet and all other parts blew).

[rob77]

test the zeners with at least 10mA current... it might be that with current <1mA the voltage will be much much lower than the spec (many zeners got a quite soft "knee")

[Rufus]

As mentioned, the actual zener print is "4B7", so the idea they used a lower value on purpose does not hold. 4B7 seems a very plausible print for a 4V7 zener.

Low voltage zeners have soft knees, soft enough to often make them worthless at low currents. It didn't take long to find a datasheet for a 4.7v zener in DO-34 package with 4B7 marking. The zener voltage test current is 20mA. You need to test somewhere around that current to get a useful answer.