How often does a zener have a failure mode where it acts like a good (not leaky) much lower voltage zener? I usually see leaky or shorted ones rather than substantial drift...
Fault was 185v line in 'scope low at 170v and slight ripple, failure was
24v zener which was acting like a good 3.3v zener. 'Scope was still working, just had slightly reduced intensity and thought it better to check voltages before twiddling the sub-intensity pot ;)Lee
That is common for a zener.. excessive heat usually does that.
From what I understand the zener is fabbed a little differently so not to destroy itself when doing the break down. Normal diodes tend to not like being used as zeners for any length of time. I guess it could be possible you may had a zener that had some impurities in the process that could of allowed it to break down like non-zener semi's or, it just got wacked some how. If this reference is supplying some out side device as a source of energy or ref on the scope? It's possible it got whacked.
Also, if only supplies internal references, check to make sure the connecting circuit is still using it. If not, the zener could be clamping too much and operating hot..
Normally the evidence is showed with a over heated board that it's attached to.
Jamie
Thanks.
I'm not sure I understand the reasons it's used in this application either, it's a 5W part and it seems to be used to create a 185v supply from a 245v tap (+ bridge obviously) on the mains transformer. This
185v supply is then regulated by a comparator/series pass transistor to 160v. The weird thing is that even with the zener failed, the 160v supply was still just about right, (it was at 163v). Replacing the zener has put the 185v rail back to the proper value and the 160v rail is now solid as well, minus the ripple.The zener was mounted off-board with fibreglass sleeves on its leads so they expected it to get hot ;) Hasn't burnt the board though. Replacement runs hot as well, but not uncomfortably so.
Lee
Ah, penny drops. I had to model the circuit in Spice before I understood it ;) It's used to create a stable 160v for the positive end of the resistor divider at the input to the comparator, the negative end of the chain is connected to a separately regulated -12v (from a different transformer tapped supply) and the comparator (other input is ground) and series pass transistor then regulates the 245v supply from the bridge to 185v. Which in turn is dropped by the zener to 160v
If the zener is shorted then the 160v supply stays at 160v, but with lots of ripple. The 185v supply isn't used for anything else other than as a marked test point and to supply the 160v rail via the zener. That's the thing that confused me originally ;) Why have a series pass transistor but then drop that supply through a zener?
I guess there is a good reason to do it this way, but it confuzzled me ;)
Lee
sounds like a poor design.. maybe it should have a heat sink hanging off it.
Jamie
Possibly two things..
Jamie
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