Automotive relay question

Hi Dave,

It's probably a snubbing capacitor, used to absorb the energy produced from back-EMF voltages when the relay is de-energized. It also cuts down on harmonic RF energy, which causes interference on AM radios.

David

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more than likely a snubber network, they normally have a resistor and cap in a single component.. Many times, they simply show it as a R across the coil because it's in series with a cap.

Also, it could be a bidirectional TVS diode, those will clamp either polarity when ever the voltage exceeds the coil rating.

Jamie

I haven't ever seen a snubber in a relay. It usually has the iron designed to short-circuit fast field changes.

It suppresses the inductive spike when the relay turns off. See page

I knew you could get automotive relays with nothing, a diode, or a resistor across the coil, but I'd never heard before that any of these shorten the life of the relay. I wonder what the mechanism is... does it damage the coil winding insulation, or does it try to move the armature a little, or what?

Matt Roberds

We buy 24VDC relays with snubber networks in side on the coil, it removes the need to put one on the terminals. It comes in handy when you're doing a few rows of relays with PLCs and micro controllers involved. They don't like the little pulse noise in the lines.

Those with diodes in them are ok for driver component protection but they still can generate a noise pulse, just not a damaging one. When you have bundles of wires tightly packed together, in race ways and wire harnesses, like in cars, it can cross talk very well.

Jamie

Why can a diode across the coil shorten relay life?

It's usually a suppression resistor. Bosch used them a lot.

So the resistor dissipates the coil energy as the magnetic field collapses? You still get back-emf; more than with a diode but less than without anything?

This one is a Bosch.

Thanks.

diode can shorten life

(sorry if this is a dupe)

Pretty much.

:) Jamie

Why might a diode shorten the life of a relay?

(3rd attempt to post this)

There are such things as bidirectional transient suppressor diodes.

Although I've never seen it on a car relay - a resistor isn't impossible, it could act as a "Q-spoiler" to damp inductive ringing.

Or a bit more likely - a VDR.

Think of it as an RC snubber without the C. The capacitance will be whatever the coil and wiring supply.

Cheers

Phil Hobbs

Makes sense. I wasn't thinking about the contacts opening more slowly and possibly arcing.

Matt Roberds

Don't you mean 3rd attempt to get notification of posting?