[chbot] Snubber circuits

[Charles Manning]

On Wed, Nov 21, 2018 at 8:36 AM Volker Kuhlmann <list57 at top.geek.nz> wrote:

> On Wed 21 Nov 2018 00:14:56 NZDT +1300, Peter Harris 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 is 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?
>

Not just the resistors but the PCB tracks etc too.

I have done quite a lot of fiddling with pulse induction metal detectors.
These use a transistor (or preferably a MOSFET) to turn on a coil up to 2A
or so, then switch it off quickly and watch the decay time of the flyback
voltage (metal = slower).

It is common for flyback voltages on a 10V or so coil to reach over 400V
even with a parallel resistor soaking up some of the energy.


> 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??
>
> I remember it being very easy to blow up 5W ceramic resistors by
> discharging a photographic flash capacitor into it. It didn't even get
> warm... ;-)
>
> Volker
>
> --
> Volker Kuhlmann
> http://volker.top.geek.nz/      Please do not CC list postings to me.
>
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