Bosch

I recently replaced my AirCon relay and it raised a question when I was sourcing a replacement. It seems some 5 pin relays have a diode placed between 85 and 86. This is apparenly referred to as a tamping diode. When would you use this versus a standard relay?

Veek said:

I recently replaced my AirCon relay and it raised a question when I was sourcing a replacement. It seems some 5 pin relays have a diode placed between 85 and 86. This is apparenly referred to as a tamping diode. When would you use this versus a standard relay?

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The purpose of the diode is to suppress a high-voltage spike that the relay coil spits back when the current through the coil is turned off, i.e. when you "turn off" the relay. This is good for the contact life of whatever switch is upstream from the relay coil. If the relay is driven by an electronic device (i.e. a transistor) it will protect that transistor from being fried, although a good electronics design would provide that protection in the electronics themselves. It also would reduce "radio frequency interference" (RFI) being transmitted by that spike to nearby electronics. It has no effect when the relay is turned on.

Most likely these are used where the relay is drive by solid-state electronics or some small-geometry switch, like a reed switch, that has delicate contacts.

I can't think of a case where the diode causes any harm, so all other things being equal I would always take the version with the diode.

If there is a big cost difference I would go with whichever configuration was specified by the system designer in the first place.

In any event, if the original system design called for one with a diode, I would NOT replace it with a relay without one.

Thanks for the insight...