Automotive relay question

I have a 12V SPST cube relay in my junque box. The diagram molded into the

which looks suspiciously like a resistor.

(using a current-limited power supply) and the same current draw is measured.

What is the purpose of this resistor?

Thanks, Dave

Reply to
DaveC
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I have a 12V SPST cube relay in my junque box. The diagram molded into the

which looks suspiciously like a resistor.

(using a current-limited power supply) and the same current draw is measured.

What is the purpose of this resistor?

Thanks, Dave

Reply to
DaveC

And what voltage is across your coil as you measured this 'same' current?

Reply to
RobertMacy

Don't know about that specific case, BUT envision current flowing through the inductor, then you turn it off and the current has to go somewhere, it goes through a resistor. Since you knew the current through the coil, you know the voltage that can 'pop' across the resistor [equal to the drive current times the resistance, but reverse polarity]

This technique is sometimes used to 'dump' the current out of an inductor faster. A small catch diode can take a LONG time, but the higher voltage of the current going through the resistor dumps the energy pretty fast. The area under the curves is pretty constant. High voltage is fast, low voltage takes a long time.

Reply to
RobertMacy

Congratulations. You found the perfect way of killing a protection diode. After this the diode will bother you no more. Dont touch my car please.

Reply to
Sjouke Burry

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.

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Reply to
Maynard A. Philbrook Jr.

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

Jamie

Reply to
Maynard A. Philbrook Jr.

Not really, could be a bidirectional TVS of sorts.

Jamie

Reply to
Maynard A. Philbrook Jr.

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

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Reply to
Kevin McMurtrie

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

3 of the Tyco application note
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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

Reply to
mroberds

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

Reply to
Maynard A. Philbrook Jr.

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

Reply to
Tom Biasi

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.

Reply to
DaveC

Pretty much.

Reply to
Tom Biasi

:) Jamie

Reply to
Maynard A. Philbrook Jr.

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.

Reply to
Ian Field

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

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

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