One other possibility - Is the feed to the relay decoupled? If there
is significant inductance in this line & it's not decoupled properly
then there will be an additional transient from this inductance. While
I can't quite see how it would damage a diode in parallel with the
relay, it could damage the switching device.

Also - is there anything special about the relay. Relay coil
inductance does change as it operates & there could be "something" to
do with the relay that produces a waveform different from what we're
talking about. For example a polarised relay will produce different
waveform from a non-polarised one.

And it is DC energised isn't it?

RP


On 08/02/06, Herbert Graf <mailinglist2@farcite.net> wrote:
> On Mon, 2006-02-06 at 23:04 +0100, Wouter van Ooijen wrote:
> > > I don't think that the problem is speed or current - the
> > > problem is the voltage. A 1N4148 is rated with 100 Volts and
> > > this could be less if you switch off a relay.
> >
> > How can you even approach 100V when switching a low-voltage relais? When
> > the relais is energised there is just the power supply voltage (5, 12,
> > 24V), when the transistor switches of the diode will conduct so the
> > voltage is just 0.6V.
>
> I'm no expert, but remember we are dealing with non ideal components
> here.
>
> The diode isn't an ideal device. When reversed biased the charge
> separation is basically a capacitor (very useful in FM transmitters).
> When the relay is turned off it'll take a while for this "capacitor" to
> discharge and the diode to become forward biased and start conducting.
>
> The relay, being an inductor, will "want" to keep the same amount of
> current flowing and will do anything it can to make that happen. It'll
> ramp it's voltage up so that the current continues to flow. It's during
> this time of going from reverse biased to forward biased that the
> voltage "spikes"; and you can EASILY see voltages WAY above the power
> supply voltage.
>
> Considering how widespread and successful the practise of putting a
> reverse diode across a relay coil is, calling it "bad practise" is a
> false generalization. I will agree that IN SOME CASES it is prudent to
> go for a "better" device, but if the problem were that bad we would
> notice.
>
> TTYL
>
> -----------------------------
> Herbert's PIC Stuff:
> http://repatch.dyndns.org:8383/pic_stuff/
>
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