flyback/snubber diode in relays

To all the pro installer,

Do you always use snubber diodes in relays?

https://en.wikipedia.org/wiki/Freewheeling_diode

always, always without exception?
No.
It depends on what is "triggering" the relay.

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Let's say one end connects to 12V+ and the other connects to a neg output signal from alarm system.

Probably yes unless you know if what is "triggering" the relay tolerates the relay.


FWIW - I never use snubbed relays - I add external diodes etc to plain relays.

During the 7 or so years I installed alarms/remote starts for a living I don't think I ever installed a diode across the coil of the relay. There was an occasional unit I purchased that had the diode preinstalled, but that was rare as well.

Example. . .
Alarm (+) trigger to power locks.
Diode? most likely no. unless 07 or newer car.

Alarm (-) trigger for say a GM reverse polarity power locks. HELL YES.

Its situational. It basically avoids that voltage spike when the relay lets go.


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I, too, never added a diode to a relay when installing alarms. Any respectable alarm will have the diode built in to the (-) output.

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Kevin Pierson

Just about all alarms that have a 200Ma output are expecting you to use that wire to power a relay.  Most of them have diodes across that wire to the power path. 

The backlash from a relay is sometimes severe.  A transistor or logic circuit that is only capable of 200Ma wouild not survive for long without a diode across it.   They know that they can not count on everybody installing an alarm to install a diode.  They put the diodes in the alarm brain.

 EDIT: I did not see Kevin's post, so just like Kevin said.

Kevin is right, but the same goes for any respectable anything - they should be self protecting (withing reason).
(At first I was going to disagree, then I re-read and realised Kevin wrote "Any respectable alarm will have the diode built in..." - not "Any respectable relay will have the diode built in...". The latter I do dot like!)

Keep in mind, the protection is for voltage spikes (not current surges etc).
As icearrow wrote, they are essentially when the coil de-energises, hence a HUGE negative spike that reverse-biases & fries electronics, and that's what the reverse-biased diode "quenches". Varistors would be required for positive spikes, but they are rare.

From what I understand is that the collapse of the magnetic field would cause induced voltage greater than 12V+ so current will flow toward the 12V+. Would this cause any damage to other components in the car that connects to 12V+?

BTW, I have the viper 5901.

Since I buy the damn things in 200 batches (4x 1N5404 and 5 x 1N4004 on one E46 BMW) yes. I solder them across the relay base on EVERY alarm output.
I'm taking no chances.
Seen the bit on the 5901 where it says use diodes? You have to with some Euro locking systems.
I've even had Euro GM and others where if you add a BT phone kit or aftermarket head unit and trigger it from the ACC, you can't lock the doors, residual voltage to the ignition switch tells the processor the ignition is on, it refuses to lock the doors, so diode required.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

sequoia2006 wrote:

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From what I understand is that the collapse of the magnetic field would cause induced voltage greater than 12V+ so current will flow toward the 12V+

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It's the same as an ignition coil.
The coil tries to keep the current flowing, or fight the change (as a capacitor does for voltage). That reaction is a voltage. (Direction - every direction.)

The spike at the coil is present at any point connected to it (noting that the worst spikes probably occurs because of the break - hence one end is floating....)
But like a lighting front it collapses due to path impedance (resistance, capacitance, inductance) etc.
If not, some component will absorb the spike - eg, a battery, a transistor, a CPU.
And with luck, that component will survive the spike.

Hence the above answers.
Good auto accessories expect the harsh environment and thus handle (say) 8-16V supplies with 200-400V spikes, and inputs/outputs incorporate noise filters. Low quality devices do not.
Hence we might place quenchers across relays to prevent damage. I don't, but I don't have CPUs and OEM electronics.