Wiring Relays

When I wired in a relay for my QI headlights, I put the relay on the fender well and made a short connection to the starter relay. Between the relay and the tap off the starter relay I put a 30A modular thermal circuit breaker. You need to put extra protection on the high-current line that feeds the relay. The original wiring should only energize the relay coil, and the high-current feed should come from another low resistance source. If there is a short downstream from the relay, the original circuit won't sense it, because its only connected to the relay coil.

Was that what you were asking?

 

I see what you mean. You're using the relay to protect the switch contacts from the load. I don't see that there's much advantage to having relays in that setup, at least in the stock configuration, because the original wiring and the switches will be sized to the load you expect. I would reserve relays for a situation where the accessories will draw more current than the original circuit can handle, and in that case it's kind of pointless to use the original wiring to feed the high-current side of the circuit.

 

I understand your question. The one fuse is fine, the additional current that the relay will draw is insignificant compared to the headlamps. If either one shorts out, the fuse will blow properly. You could add the fuse, lets say a 1.5 amp or whatever is slightly higher than the relay current draw, but in reality if something does go wrong with the relay (IE. A rare short), it will probably draw 40++ amps and that would've blown the bigger fuse. Or if it doesn't, it just burns up the relay, and you still buy a new relay. I think the biggest mistake people make is not installing big enough wiring when the current draw is high.

 

JMO - if an aftermarket toggle won't handle the current, the factory switch won't either. Usually the headlight relay (if one is used) is under the hood, where the resistive losses due to the wire length (use fat wire too) can be minimized. Also, many aftermarket headlight bulbs draw too much current for the factory switch and wiring - these are typically fitted with a relay and a short, low resistance high current feed.

 

I just looked at the SMP marine products catalog. The typical pull switch there is rated at 10-12A/12VDC. Their standard toggle switch is rated 20A/12VDC. I think this is typical - if you buy quality parts, the manufacturer will provide a current rating for the part. This current rating will be accurate for an ordinary resistive load, like headlights. How much current do you expect the circuit to draw?

http://www.smpcorp.com/web_app/catalog/smpmarinecatalogpublic.asp

 

Mike, I think my answer was "none of the above." :? If the switch and relay are both downstream from their fusing device, it won't matter which shorts out - they'll both be protected. However, I think that, unless you're using a sub-mini toggle, you don't need a relay. You can get a 50A toggle from SMP (DS-191) - the race cars used to run the big Cibie' lights directly off of a switch like that, with no relay. 300W at 12V = 25A ... no problem.

 

Yes Mike, tap the switchline on the headlight side of the circuit breaker. It's not likely to need fuse protection, but if it shorts you will be protected both sides of the relay. You're very likely to pop a 30A circuit breaker for a short in the switchline, with no fuse at all it will burn til the wire burns off the connectors, or the Jeep catches fire. It's a long story, but I've seen 16 guage wire completely burn it's insulation off recently, and it's not pretty. And as for the relay question, I would put relays on everything that draws more than a few amps. I have some of my Jeep done, and will continue to add more.

Paul

 

As long as there is some sort of fuse before it somewhere, you should be OK.

 

Just please tell me you have a key or something someplace... your 6 is too nice to have it come up missing if they figure out the 'right' switch combination while your in the store or something, not that they are hard to hotwire i guess...

 

I'd skip the relay- install a suitable (current rating) toggle and not fuse the supply.

As someone metioned, I would only add a relay where I was adding a component that had a higher current draw than the original wire could handle.

Just my "after the fact " opinion...

MM

 

You do realize if any part of that circuit or device shorted it would catch fire right? :?

 

Yes- I should have been more clear. I was making the assumption that the original fuse would be used and should have stated that no additional fuse would be required.

Thanks for pointing that out.

MM