Do I Really Need a Relay?

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12vdeej

Member - Posts: 26

Joined: July 04, 2006

Location: United Kingdom

Posted: July 04, 2006 at 5:20 PM / IP Logged

I'm helping a mate of mine install four halogen floodlights on the front of his truck, which come in pairs with one relay per pair of lights. I can wire them up using the supplied relays, but it would be much simpler if i could use a high current switch to run them directly. The lights are only 30W each equating to 10A total current draw, and there are plenty of switches that will handle this much current. I can't see this as being a problem but im not particularly experienced with auto-electrics so maybe im missing sumthing. Any help would be much appreciated. Thanks

captainzab

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Joined: February 09, 2005

Location: United States

Posted: July 04, 2006 at 9:30 PM / IP Logged

A relay is just a switch, but seeing as though you have a high current switch, that should work too.

(those relay supplied is for use if you have a regular switch thats is like rated for a few amp, but if you know your newer switch can handle it then your fine)

KPierson

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Joined: April 14, 2005

Location: Ohio, United States

Posted: July 05, 2006 at 12:39 AM / IP Logged

I believe one reason they do this is because of the voltage drop associated with running wires longer then necesarry in an automotive environment.

If you run the wires directly from the battery to the fog lights thats a fairly short run. But, if you run the wire from the battery, in to the passenger compartment, to the switch, then back out to fog lights you have much more wire, you also have a more complicated wiring setup now that the same wire is running in and out of the passenger compartment. Also, since this is the feed wire it must be capable of carrying 10A+ of current, so it will be a thicker, less convenient wire.

With a relay, you can use small wire, you can switch the ground inside the passenger compartment and then run one small guage wire in to the engine compartment from the switch to the relay. Hook pin 30 and pin 86 directly to the battery and minimize the length of the power cable, and actually simplify the installation.

Kevin Pierson

12vdeej

Member - Posts: 26

Joined: July 04, 2006

Location: United Kingdom

Posted: July 05, 2006 at 3:22 AM / IP Logged

Thats a fair point about the cabling, but i think with only 10A its still not much of a problem as i can use 2.5mm^2 wire which really isnt that big. Also, considering how the cabling has to go up to the roof of the cab, the cable run isnt going to be particularly short either way. Thanks for the info guys, if anyone else can think of any other reasons that i should be using a relay that would be great.

ff-mike

Copper - Posts: 199

Joined: November 15, 2004

Location: United States

Posted: July 05, 2006 at 10:43 AM / IP Logged

Are you sure on that wire size? 2.5 square mm translates to .9 mm diameter, which equates to 19 guage for the US. That is way too small in my opinion- 2.5 mm diameter would be fine though (~ 10 guage).

Remember this: For every 1/10th of a volt lost, you will lose 5% of the lamps brightness.

12vdeej

Member - Posts: 26

Joined: July 04, 2006

Location: United Kingdom

Posted: July 06, 2006 at 2:01 AM / IP Logged

I know its a small guage wire but 2.5mm^2 is the standard wiring thickness for 16A plugs, with 1.5mm^2 for 10A IEC leads (In the UK) so I can't see it being a problem. Does the fact that auto electrics are low voltage DC rather than the high voltage AC for the mains cabling affect the current carrying specifications? If DC requires lower resistance cable than AC then i will need to reconsider my choice, and go for thicker guage wire.

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Do I Really Need a Relay?

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