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

[The Car Stereo Company]

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.

this part right here. which leg of the relay is not being used in your wiring? because if i were to wire it up, i would be using a spdt relay and no diode.

 

[Oldfart]

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!).

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!!!!

 

[GordoJay]

Thanks for the clarification.

It should be 15+ amps, correct ?

I think we're talking at cross purposes. For what you want to do, you need a relay. The coil of the relay hooks to the cathode of the diode and to the upfitter switch. The anode of the diode hooks to the high beam wire. The output of the relay turns your light bar on and off. The diode only needs to conduct the activation current for the relay, which should be as low as possible because the BCM monitors high beam current and if it's out of spec for too long it shuts itself down and you are screwed. So even a high activation current relay is a problem, much less trying to pull the current needed to run your light bar from the high beam wire. By the same token, the upfitter switch doesn't need to conduct much current, far less than an amp, if it's activating the relay. 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.

 

[Oldfart]

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.

I agree, after reading FTO's replies to me last year, and to several others since then, I don't think it's worth risking your BCM for the small convenience of having them tied to the high beam. Having off road aux lights tied into your high beams is also against many states inspection laws.

 

[sixshooter_45]

Doesn't LVJ, if I've that correct, sell a unit with 6 relays & fuses with diodes and states that somehow this unit can be tied into the Highbeams?

 

[The Car Stereo Company]

this is getting way overcomplicated. simply put, a diode is not needed.

use your lvt to trigger a spdt relay.
wire relay as follows.
85 power from lvt
86 ground
87 battery power
87a aux switch power
30 to the light

 

[bryanb]

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!!!!

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!

 

[The Car Stereo Company]

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!

thanks a lot jerk.......

 

[GordoJay]

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

Oh, sure, throw us all under the bus. Although I must admit that I belong there.

 

[Oldfart]

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!

I was actually laughing to myself when I was reading your reply, about how far over my head it was. There's definitely other guys in here that really know that stuff, that I'm sure totally understood your post. I got into electronics lightly in my physics classes in college, but that was a LONG time ago. My dad is a mechanical engineer from Lehigh. He helped me a lot over the years with help fabricating stuff for racing. His knowledge of metallurgy is scary.