Should a diode be used with a relay from upfitter switch?

2020Raptor said:

Thanks very much for joining in.
I want to have two methods of turning on my light bar.

1. Upfitter switch
2. High beams (solid state relay, per a different thread)

My understanding is that use of the appropriate diode will prevent backfeeding the unpowered leg.
That is the goal, unless there are additional considerations of which I am not yet aware.

Which is what I discovered here while performing my due diligence.
Apparently, you cannot just use any diode labeled "12V".

Googling this topic will make me want to toss the light bar in the trash, ha ha.
Thus my desperate plea for assistance here on the forums.

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bryanb said:

Hey,

I think maybe two different diode use cases are being mixed in this thread. Is your question about:

1) Using a diode to protect against damage from the transient voltage created when power is turned off to a relay coil; OR

2) Using a diode to ensure current only flows in one direction

The thing you quoted doesn't have anything to do with case 1 and a diode doesn't have a "voltage rating" in the sense it is being used in that sentence. A diode has a forward and reverse bias voltage and the former is not a "rating" bur rather a characteristic of the semiconductor type and the latter is the breakdown voltage when the diode is reverse biased (the voltage at which the diode stops behaving like a one-way street and becomes two-way). The forward bias voltage drop (usually ~0.7V for silicon) has nothing to do with either case above (in terms of selecting which diode to use). The reverse bias (or peak inverse voltage) rating is relevant to the first case, but not in a way that is likely to matter for case 1 in automotive since the peak inverse voltage is pretty high for silicon diodes in general (above 12 V). And if you google this topic, you need to understand that there are different diodes not used in either of the two cases above where what I said won't apply in the same way (e.g., Zener diodes).

So tell me which case you are interested in and I'll tell you which diode to use (that doesn't have a 48 week lead time!).

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2020Raptor said:

Thanks for the clarification.

It should be 15+ amps, correct ?

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GordoJay said:

If it was me, I'd skip the high beam connection altogether. That gives you more options as to how much light you're throwing down the road at the cost of being less convenient and it runs no risk of bricking the BCM.

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Oldfart said:

I just wanted to let you know that I don't appreciate you making me feel like I'm severely retarded when I read your reply!!!!

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bryanb said:

I was, however, telling my wife and daughter at the dinner table last night about the exciting diode discussion we were having on FRF. They think you are all nerds like me now. Sorry!

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bryanb said:

They think you are all nerds like me now. Sorry!

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bryanb said:

I'm sorry! I'm an electrical engineer so it's rare I can say anything knowledgeable here. Sometimes I get carried away. I was, however, telling my wife and daughter at the dinner table last night about the exciting diode discussion we were having on FRF. They think you are all nerds like me now. Sorry!

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