Would one 40-amp relay be okay for a pair of 55-watt lights? I would think so because if wired in parallel, both lights would receive the 40-amps, but only 6-7 volts. 1 Watt = 1 Volt x 1 Amp. So 6 Volts x 40 Amps = 240 Watts capability. I am rewiring my fog lights from being through the switch to having the switch activate a relay. I am also adding some smaller reverse lights to mount inside the rear bumper the same way with the reverse lights activating the relay. I am considering using a On-Off-On switch so that I can have the lights on the rear either be On no matter the gear, On in Reverse, or Off in all together. I am going to check into the laws to see if auxiliary reverse lights are illegal first. Although I doubt these are any brighter than the lights on some newer cars. I bought one of the project boxes at Radioshack to mount my auxiliary fuse box and the front relay inside of. The rear will be mounted inside behind the trim somewhere.

 

One forty-amp relay will be plenty for 55Wx2 lamps.

However, you're working your maths in the wrong direction. Amperes = Watts/Volts (use 12V nominal) Therefore:

55Wx2 = 110W
110W/12VDC = 9-1/6 Amperes

Use a relay that is rated for a minimum of 10A (should pose no trouble) and a 10A fuse to protect the lamps.

 

What's the point in a relay?
I understand it to be just a bigger fuse.

If you have a light system w/ a 20 RELAY and one w/ a 20 INLINE FUSE is there any difference in the lights? Brightness, intensity, bulb life?

OR does it just cut cost since you use less heavy duty wire?

I would appreciate a knowledgeable answer since I don't have any electrician friends. thanks

 

OK im not a good explainer, but correct me if I screwed up: All a relay does is act as a switch. you use them when the current is too much to run through the switch you use. It also can simplify wiring. It uses a seperate "signal" circuit to switch the relay on and off. That way you could have huge 00 guage wire for something, while only using 18 guage wire for the switch in the cabin itself. Its like a super simple form of "remote control"

 

Yup, just think starter relay. Saves your ignition switch

 

A relay is used so the full voltage is not run directly to the switch. shorts/fires can happen a lot easier if no relay is used.

 

Alright. I'm doing this because the PO has it running through the switch and it burnt up. Stopped working and smelled like burnt plastic. Also, for the one that was asking the purpose of a relay, you can mount the relay as close as you want to the powered object and therefore have less voltage drop through the wire. I'm having trouble finding a 3-position rocker switch. I checked the parts store and Radioshack. Dont think theres any other place local I could look.

I dont know if the maths in the wrong direction but the formula should work out the same either way. I also had the voltage backwards. Voltage is constant in a parallel circuit and current is constant in a series circuit. Its been a couple years since I had my circuits classes so i'm a little rusty. Now I need to do some Googling to see if the 18ga wire I bought will be enough to run both lights from relay.

 

A relay does a few things - none of them directly involve circuit protection.

- A relay allows a main power feed lead to be much shorter (install so that the relay is physically between the battery and the power consumer. You won't have to run a full-gage power lead from the battery to the switch to the consumer - which increaes losses and results in increased wire size as well.)

- A relay is little more than a "switch-controlled switch," meaning that you're able to use pretty much any convenient switch to control a much larger load than you would normally be able to (the solid-state equivalent, by the by, would be the transistor.) A DIN relay typically has a "trip" current requirement of 9VDC @ 100-150mA, but that allows you to control (typically) up to 30VDC @ 30-40A, depending on relay spec. 30A switches are very large - even a small pushbutton or magnetic reed switch will control a relay handily.

- The ability to use a much smaller switch will give you more flexibility in switch selection and mounting.

- Since the "trip" current is so low, you can use very small wire to control it. I know of people who use Cat5 data lead to control relay banks - while it does work (it's well within the specs of UTP Cat5,) I don't like using solid wire in automotive applications (less resistant to vibration.) Using the 8x24AWG solid wire can lead to conductor failure due to vibration. What do I use? Stranded wire - typically 18/2 (for a single relay,) or 18/more (18/3, 18/4, sometimes 18/5 or 18/6 if I can find it) for banks. I suggest the SJOOW jacket spec, since that's water-, oil-, and grease-resistant (I've been using SJOOW underhood for a number of years without incident.) SJOOW 18/2 and 18/3 stranded wire can be had in bulk at most hardware stores. (NB: While, as stated, Cat5 can and do work for a while; neither grade of Cat5 has a jacket specificed for use in oily/greasy environments. And, I'm inclined to think that petroleum liquids, solvents, and vapours are all antagonistic to the common grade of Cat5 - the plenum grade may hold up better, but it's more difficult to find.)

- A relay can also allow for a "remote circuit" when setting up something akin to a kill switch. Hide the relay underhood for, say, the fuel pump; then have the "trip" lead somewhere convenient but not readily visible. Problem solved (for instance, I have a pressure switch in my seat keyed to my weight - most kids don't weigh enough to close the switch. I also ran a rocker to a DIN relay to bypass the starter relay underhood - you can crank the key to START all day and it won't do anything. Even if you do weigh enough to close the switch... This has already come in handy and saved my truck. People tell me I'm paranoid. I'm not paranoid, I just think of everything.)

A relay does not and will not ever replace a fuse, so a fuse will still be necessary! However, there are a number of advantages involved in actually using a relay in a circuit like this, so there's really no sound reason to not use a relay when presented with a choice! And, since most of the relays in the XJ/MJ are DIN relays, you can save yourself a headache if you just use DIN relays in your wiring projects. (Later models use Mini-DIN relays - they're about half the size, and typically rated for 20A or so. No worries, use Mini-DIN relays if the design will allow it.)