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I need to use a relay in my car to power a piece of equipment when the ignition switch is off.

Relays / relay sockets have diodes to suppress flyback voltage spikes and prevent damage to switch contacts / transistors.

A relay generally switches to ground whereas, in a car, the ignition switch is in the positive lead with the negative lead connected directly to chassis.

Is flyback diode protection still required with the excess voltage going to ground?

Without diode protection could the voltage spike still damage components on the positive side of the relay?

I suppose the flyback voltage spike would occur on the negative side of the relay.

The relay socket I have ordered already has the diode wired in.

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3 Answers 3

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In brief, yes, you still need the diode

You have a couple of things wrong in what you're thinking:

in all schematics, relay uses switch on ground

This is false. It's certainly more common to switch to ground with an NPN relay, for example, but you can find plenty of circuits with top-side switching.

My thoughts are that overvoltage build up is in the nagative side of the relay.

This is also false. The thing that builds up is the magnetism in the relay. When the powering current is removed, we now have magnetism which dissipates next to the coil. A changing magnetic field will produce a current in a coil -- it's how all generators and transformers work -- and so you get electricity out from the relay, in the reverse direction. The voltage back out can be higher (a lot higher) than the input voltage, because it's not really anything to do with the input voltage. (In an experiment with a very ordinary 12V relay I was seeing 50-100V voltages back.)

The diode provides a low-resistance circuit in one direction only -- the opposite direction -- so this reverse voltage dissipates (as heat) through the diode. Sometimes other components serve the same function.

  • To find out more about the cause, read about Back EMF.
  • To find out more about the solution, read about Snubber Circuits
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  • \$\begingroup\$ Does diode completely eliminate back voltage for further circuit? \$\endgroup\$
    – dummygear
    Jul 6, 2023 at 13:14
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Here's the schematic.

enter image description here

With the ignition switched off and relay 'K1' de-energised, its 'NC' contact enables the load to be switched on through the SPST switch 'S1'.

When 'K1' is energised the load is disabled.

When 'K1' is de-energised, the flyback current is dissipated in the closed circuit comprising 'K1' coil and its flyback diode.

The load would also require a flyback diode should it be inductive.

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Without flyback diode

schematic

simulate this circuit – Schematic created using CircuitLab

STEP 0

STEP 1

With damping resistor

schematic

simulate this circuit

STEP 2

With flyback diode

schematic

simulate this circuit

STEP 3

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