Ron Hooper wrote:
>
> Diode protection:
> How the general purpose diodes are suppose to work accross relay coils
> is that are intentionally put on in reverse to arrest the voltage
> generated caused by back EMF, which is the opposite polarity from the
> voltage applied. This back EMF can slow the pick time of the relay when
> energizing. Back EMF is at its highest when current is high and drops
> to nothing when the relay is fully picked. Now when you open the relay
> the diode does nothing to arrest the collapse of the magnaetic field.
Hi Ron,
I wasn't following the idea behing an external conventional diode, I was
curious how it was being used.
Back EMF is highest at the moment the current flow is broken, when the
magnetic field just begins to collapse. Of course shunt bypass
capacitors slow the peak down a little.
Remember the inductance of the coil tries to keep relay current flowing,
so when the radio relay opens the voltage soars.
At the output jack of the amplifier, the polarity is the same as the
open circuit voltage. So the spike is positive and very high. That's why
any diode external to the PA must be a zener, or if a conventional
EXTERNAL-to-the-PA diode is used it must connect from the keying line to
a voltage source in a direction that dumps the relay transcient INTO the
voltage source.
If you look at internal back pulse cancelling diodes, they are connected
across the coil with the cathode towards the positive voltage source.
The addition of a diode always slows the relay's release time down.
You can avoid all this by just using a simple one or two transistor
buffer.
73 Tom
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