# Installing aftermarket reverse lights with relay . help please



## Flavo Cadillac (May 7, 2003)

I'm installing aftermarket lights w/ a switch so I can turn them on at will from the cab and also have them come on when the gear selector is in reverse. I keep blowing fuses though. I think my problem may be with fuse and diode sizing. I was told it didn't matter the size of the fuse as long as it was smaller then the diode. The diy I found used a 5 amp fuse and 6 amp diode. The biggest diode I could find was a 3amp though so I used a 2 amp fuse. 

By the way, I haven't even bothered w/ the switch and 2nd diode yet. Just trying to get the simple set up to work. Even just taking the fused hot wire to the aftermarket light and grounding the other wire from the light will get no affect but a blown fuse. Maybe it's possible my ground isn't good enough? I took a bolt off of the truck bed and sanded the painted section, then replaced the bolt w/ the ground. 

Please let me know if it is what I think it is and how to proceed.


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## cuppie (May 4, 2005)

I think you're looking at _everything_ from the wrong angle. 

Obvious question (and, when building any electrical circuit, the most important one): 
what's the load? I'm betting that it's a lot larger than the 28 watts (14.2v*2A) that your 2A fuse will allow. Factory reverse lamps are normally 16-21(ish) watts _each_.  

You're building a circuit. Always start with "how big is my total load?" 
From that, you select the proper size wire for the load, and properly-rated relay(s) and/or switch(es) as required. 
_Then_ you select fuse (and, in your case, diodes) that are appropriate for the application. 


What you're doing, it's not that far removed from something I'm planning on my 'rocco: I want to re-do the sidelights wiring, to enable Euro-style parking lamps. I have an alarm with a single sidelights output on it (internal relay), that needs connected to that. Needs diodes in a Y to connect. So, I asked a forum friend at MaxPC for help. Here's the gist: 


Cup said:


> The question for you:
> at ~14.2v DC, and with a circuit load of 23w (each side), what would be an appropriate diode to use?
> If it helps, forward current would only be applied during locking, unlocking, and alarm triggering.





Ron said:


> I would look at a diode like the 1N5400 or 1N5401. Actually anything between 1N5400 and 1N5408. The only difference being they start with a PIV or 50 volts and go up. They are all 3 amp diodes and the lamps draw 23 / 14.2 or about 1.6 amps give or take.
> 
> You can get them at even Radio Shack for under a few bucks each.


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## Flavo Cadillac (May 7, 2003)

So all I'm really looking at wrong is the size of my fuse and possibly my diode? That fuse seemed way too small. The lights are rated at 55w each. Do you know what size diode or fuse I need? Or tell me how I can figure it out. Does the fuse need to be smaller then the diode?


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## cuppie (May 4, 2005)

A(mps) = W(atts) / V(oltage.) 
So, 55W * 2 (lamps) = 110W. 
110/14.2 (nominal voltage) = 7.7A 
Round up to next size, you need a 10A fuse. 
And, you need a diode that's rated for ~8-10A forward current, at at least 14V (you can always go higher with the voltage rating.) 

Give me a little time, and I can pick thru Radio Shack's site (as well as a few others), and find diodes around what you need.


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## Flavo Cadillac (May 7, 2003)

Thanks so much. My radio shack isn't too tiny but I saw nothing even close to that size. I think I got the biggest diode they had.


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## cuppie (May 4, 2005)

Sorry - was feeling a bit ishtty last night. Fell asleep early! 

Here you go: 
http://search.digikey.com/us/en/products/10A01-T/10A01CT-ND/1922678 
50v PIV; 10A forward current. $0.33 each.


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## dennisgli (May 17, 2003)

Why do you need a 10amp diode to power a relay coil that draws less than 1amp?


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## cuppie (May 4, 2005)

10A _diode_ what you meant to say?  

But, um, yeah... you're right. A 1.5A diode would be more than sufficient for the job. 
Brain got stuck earlier, I guess. :laugh: and :facepalm: 

So, yeah - a pair of IN5400 or IN5401 diodes would be fine. 

And, the original problem (fuse blows) solved by using the right fuse (10A, instead of the original 2A fuse.)


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## rabbitnothopper (Oct 19, 2009)

welcome to electronics 101 

where fuses are examined and understood because of the pile of dead ones we have 

not so bad really 

i blew a few fuses every time i connected fused power to a relay once 
little to my knowledge the inside of the relay had a bad circuit laying from battery to ground and would blow the fuse every time 
luckily a little repair work inside the relay and all was good


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