Relays With Diode Protection

For anyone contemplating wiring additional lights and/or heated grips via a relay.

It doesn't seem quite as common now but some brands of automotive relays had a suppression diode internally connected across the coil Pins 85 and 86.

Here is a diagram showing an example of a diode across the relay coil.



I was about to make a bold statement that relays with these internal diodes need to be wired with +12volts to pin 85 and ground to pin 86.


Further research has revealed that relays from different manufacturers (Hella 3056 for example) have diodes wired the opposite way.

The consequence of wiring the other way around is probably to damage the relay or worst case possibly blowing the bike fuse supplying the circuit.

My suggestion is to check the documentation for the particular relay. There is often a diagram on the relay case to assist you.

Of course, if your relay does not have an internal diode this is not an issue

Good thread BD.

Why have the diode there in the first place ?

Glad you asked Grasshopper....



The relay windings form a coil, when voltage is applied to this coil a magnetic field is produced thus attracting an armiture which in turn causes the contacts to close, all pretty simple stuff.  When voltage is removed, the magnetic field collapses and the contacts are released, no surprises here either.

However, the collapsing magnetic field across the coil windings creates its own voltage - this is essentially how your generators work on the car/bike (i.e. a coil moving thru a magnetic field, just so happens with the relay the coil is static and the field is moving).

This is where the problems come in, the voltage that is produced by the collapsing magnetic field can be as great as 200v or higher, and in the reverse direction.  In some highly sensitive electronic controlled engine management systems this is enough to cause damage.  

My rule of thumb would be anything that has a CANBUS management system should have reverse protection diodes on all relay windings, especially where the battery is small (such as a bike) or the relay is a long way from the battery but close to the ECU/black boxes.  The battery acts as a sort of buffer/filter, but if the reverse voltage cannot be curtailed quickly enough the chance of damage is increased.

Most good electronic management systems (ECU's) should have their own protection diodes and other power filtering circuitry, but for the sake of a 20c part why chance it.

BTW, I have no diodes on any of my relays on my 07 Bandit   

Next lesson, how to tell if your relay has a diode and how to wire it correctly without letting the magic smoke out......tbc