Relays and Protection Diodes

Hi, how are you doing?

I'm making this post just to learn more. I want to use a relay in my motorcycle, and I'm not sure if I should use a diode in anti-parallel for spike protection. The solution is easy, in case of doubt I should use one. But... most automotive relays don't have a built in diode, only special ones. Why is that?

The idea is to use a diode in anti-parallel with the coil of the relay, to protect the electronic circuit from spikes (mainly transistors and chips), like this image:

In case you were wondering, my motorcycle is a Honda NSR 150 RR from 1998. The original wiring doesn't use a relay, and I want to add two of them for the headlights, so I don't have a drop in voltage.

In case that you want to know, Daniel Stern explained all of it in deatil in his website. I already measured my instalation and I have a 2v voltage drop in the wiring and switches of both low and high beams, so I need it. Daniel didn't mention anything about using diodes for protection.

This would be my circuit:


The thing that I want to protect the most is the CDI unit. That is, the computer of the bike. As you can see, it is in parallel, not in series with the coil. So I guess it won't affect it. But, I can't find any info about it.

A guy in another forum told me that the only part affected would be the switches. So, by using a diode, the life of the original switches is increased.

What do you think?

If you are serious about protecting the CDI from spikes when the relay is de-energized, don't use garden variety rectifiers (like 1N4000 series).
Use Schottky diodes (like 1N5819).

eguru] wrote:

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If you are serious about protecting the CDI from spikes when the relay is de-energized, don't use garden variety rectifiers (like 1N4000 series).
Use Schottky diodes (like 1N5819).

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Why is that? I saw a lot of people use the 1N4000 series for that purpose

BTW I found the answers:

https://www.autoshop101.com/forms/hweb2.pdf

This pdf explains the basics in relays. There are two comonly used ways to add protection to the circuit on the relay: the diode method and the resistor method. A lot of relays don't have any protection because the computer itself has the protection.

So, resistor is what for example, my mom's car has. It has a few cons, but anyway it's used in practice.

The use of 1N4000 diodes is fine for wired-or logic in order to prevent backfeeds.
The 1N4000 diodes can't respond quickly enough to suppress spikes that can damage your electronics.

Some info on relay coil suppression
https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3311_AppNote&DocType=CS&DocLang=EN

niksfish wrote:

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Hi, how are you doing?

I'm making this post just to learn more. I want to use a relay in my motorcycle, and I'm not sure if I should use a diode in anti-parallel for spike protection. The solution is easy, in case of doubt I should use one. But... most automotive relays don't have a built in diode, only special ones. Why is that?

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Bosch makes automotive relays with both diodes and resistors. This might help you make a choice, though very long after the fact....:oops:

https://www.boschautoparts.com/en/auto/relays/mini-relays

Sorry if I might be misunderstanding your goal, but that link is pretty good for choosing a possibly appropriate one.