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high voltage and resistors

What happens when HV rating of a resistor is exceeded? There is no power dissipation - I am talking about ~500V across 20MOhm. There is no breakdown - I see no flying sparks ;o)

Here is the story: I need to get 500V out of voltage multiplier ladder used for 300V or so. The only "underrated" parts are couple 10MOhm resistors used in feedback divider, they are specified to have 150V maximum working voltage, 300V overload (Yageo

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The only mean of adjusting output voltage is to change "bottom" resistor in the feedback divider. The output does not want to go above

460V or so. Is there any chance that he voltage coefficient of resistance jumps up when the voltage across the part exceeds some value? Thanks!
Reply to
Michael
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** Groper Alert .

** The life span is shorter - maybe dramatically shorter.
** The damage is done very gradually.

Resistance value increases and then it finally goes open.

** Depends on the phase of the moon ...

....... Phil

Reply to
Phil Allison

I think that Phil's answer is only correct for carbon composition resistors and for carbon and metal film resistors, all of which are vulnerable to the ozone produced in corona discharges along the length of the resistor. The ozone will react with carbon to produce a partially oxidised surface layer in the first instance, and probably carbon monoxide in the longer term, while metal films will be oxidised to less conductive metal oxides.

Metal oxide resistors, commonly used for high resistance parts, don't oxidise and are not vulnerable to the is particular mechanism.

In general, non-thermal maximum voltage rating for resistors are determined by arc-over and creepage distances.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

** The OP asked simply about "resistors" - what happens when the maker's voltage rating is exceeded.

The particular 0805, SMD ones in the URL he posted are " thick film ", ie "cermet" or "metal glaze" types with a voltage rating of only 150 volts.

Do try to make your posts relevant some day.

........ Phil

Reply to
Phil Allison

First of all: thanks for the replies

As to using bunch of resistors in series - this is for rev2, now I have to make do with what I have without hacking the board too much. What I do not know is if resistance voltage coefficient can jump at some point (it needs to be negative to explain what I am observing). ... I do not wan to experience premature hair loss due to excessive tugging... ... P.S.: the relevant area of the PCB is clean and dry...

Phil Allis>

Reply to
Michael
** Groper Wanker Alert !

** This is what YOU claim to have actually observed:

" The output does not want to go above 460V or so."

Work on that for a bit.

....... Phil

Reply to
Phil Allison

Since cermet and thick film resistors are all made by silk screening a metal-oxide/glass slurry onto the alumina surface, then firing it to fuse the glass, your degradation mechanism is irrelevant in those two cases, and pretty slow for metal glaze resistors, where the metal particles are similar protected by fused glass.

How does your foot taste? I used to claim that you knew your electronics - and you did catch me out a day or two ago - even if if you social interactions were on the sociopathic side of barbaric, but now you are putting the electronics into question.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

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