I have electrical (general power) understanding but little electronic experience.

I am trying to assist my son in installing supplementary lighting onto his motor cycle. The bike is a Yamaha 2018 with LED head lights. I identified a wire that became +ve on high beam and connected a wire. From this wire, the power went through the driving light relay (coil) and to earth to initiate the reply. The relay didn't engage but we ended up with the dash high beam light coming on dim and when going to high beam, went bright.

So I'm thinking of adding a diode into the circuit feeding the new relay coil (someone mentioned teh issue may be back feed). The 30A relay coil is under 100 Ohm (Inductive I assume) and the bike is 12V. I understand the purpose of the diode but would like help with selection.

Is a 400V 1A diode acceptable?

Thank you in anticipation of your help.

 

The relay coil will draw 120mA so a 1N4004 will handle both the current and the back emf.
Regards,
Keith

 

The relay coil will draw 120mA so a 1N4004 will handle both the current and the back emf.
Regards,
Keith

Thanks for the info.

When looking at Diodes, do I just ensure the voltage of the diode exceeds my requirements and the current carrying capacity are suitable? So, in this case 400V vs 12V and current is less than 1A.

Or is a matter of ensuring the power (W) of the diode are managed (P=VI). The Diode is 400V x 1A = 400W. The relay is 12V x 0.1A (the relay coil ish) is around 2.5W. Or is my thinking just wrong .....

 

The back emf caused when the power to the relay is switched off can be very much higher than the supply voltage. How much higher will depend on the resistance and inductance of the relay coil and the resistance and capacitance of the wiring between the coil and the switch. If you are concerned that this may exceed the voltage rating of the diode, you can connect a snubber across the relay coil. This will suppress the back emf and will also prolong the life of the switch contacts. The diode forward current rating just has to be high enough to carry the energizing current of the relay.
Regards,
Keith

 

A back emf diode is most always a must when driving a relay with electronics, but I doubt it will solve the relay not working and the dash light problem.

If the diode doesn't work, you might think about using an interface between the head light output and the driving light relay.