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

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