[chbot] Snubber circuits

[Peter Harris]

 Volker Kuhlmann wrote:

> > It turned out that there was a spiral cut in the metal film at one end of
> > the resistor presumably to trim the value and when a voltage spike hit
> the
> > snubber it would spark across the film at the end of the spiral burning
> > some film away.
>
> Very useful posting, thanks Peter! What was the voltage rating of the
> resistors? And the spike voltage?
>

Sorry this happened in the mid 80's when data sheets were actual sheets of
paper, I was a lowly technician and never saw the resistor specifications.
We never tried to measure the voltage spike, we just left the test jig
running with the new resistors until the boss was convinced the problem was
solved.


> Your experience would confirm 2 things if my suspicion is corect: The
> snubber resistor must be rated for the spike voltage (the capacitor
> probably too), and carbon film resistors are more tolerant to voltage
> spikes exceeding their design rating. Or, resistant to spikes exceeding
> their design wattage??
>

Under normal circumstances the capacitor will not see the full spike
voltage, the resistor certainly will and must dissipate the energy in the
spike.
Something I learned with surface mount resistors is that a resistor's
ability to dissipate heat is reduced when the energy is delivered in pulses
that exceed its nominal rating. E.g. A 1W resistor array can take short
pulses of several watts but the average power being dissipated under these
conditions must be less than 1W. There are tables in the data sheets to
calculate the required derating.
I have no idea if similar derating has to be applied to through hole parts.

  P
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