Driving Light Relays?

Submitted: Monday, Aug 29, 2005 at 23:07
ThreadID: 25975 Views:3685 Replies:4 FollowUps:2
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Just curious as to what exactly the relay on the spotlights does? I have wired up my spotties and they work fine but wondering how come the high beam wires dont burn out. Does the relay work by only using the high beam to switch the relay then all the power comes from the main battery wire through the relay to the accessory(87) pins. Unsure about this as the high beam wires and the wires to the switch then to the relay must be only about 3-4mm or 15amps but the wires from the relay to the lights are about 6mm/25amps. The spotlights pull about 17 amps?

Cheers.
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Reply By: Willb - Monday, Aug 29, 2005 at 23:51

Monday, Aug 29, 2005 at 23:51
Hi Matt.D, the relay acts like a switch, the #30 terminal is connected to the battery supply, #87 to you driving lights, these are rated higher then the other two terminals, #86 (switching power), #85 earth. The relay requires a lower voltage supply to switch on and off, so you don't need such a high load wire for those terminals.
I would use 20 amp wire from the relay to the spotties, and the same from the battery to the relay.
I am sure some one out there could probably explain it a bit better then me, there are some clever people on this site. ( and thats a genuine comment not a smart @ss one).
Will
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AnswerID: 127383

Follow Up By: Member - John C (QLD) - Tuesday, Aug 30, 2005 at 08:14

Tuesday, Aug 30, 2005 at 08:14
Good explanation. No complaints about that.

See if I can add a little more without creating confusion :-)

The relay is an electronic switch, switched by a small ampage current. Ampages less than 1 amp can switch the relay, but does need to be around 12 volt. (Sometimes 10 volts will switch a relay.)
Using a relay means you can put the main switch (relay) near the spots and have less distance for the high ampage wire to travel, and less voltage drop. Instead of the high ampage wire travelling to the dash and where ever else, a low ampage wire and switch is mounted on the dash. And the relay makes it easier to put a low ampage controlling connection into the high beam circuit.

Just in case you don't know the difference between ampage and volts. Simple explanation. Think of a water pump. Voltage is the pressure put out by the pump. Ampage is the volume of water pumped. You can have high pressure down a small pipe, (high voltage, low ampage), or low pressure and high volume large diameter pipe (low voltage, high ampage). I know, a bit simple for some, and it is more applicable to DC, but lets not get into that. We are talking 12v car systems :-) Hope I havn't bored everyone with that.

regards John
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FollowupID: 381893

Reply By: joc45 - Monday, Aug 29, 2005 at 23:54

Monday, Aug 29, 2005 at 23:54
Hi Matt,
Generally, mfr's only use wire just heavy enough to run the standard headlights. A small amount of voltage is dropped along the wire, and maybe also thru the headlight switch, but this is calculated to be within allowable limits. Adding addional lights _Affordable_Storage_Drawers.aspx more current, and more voltage is dropped on the wiring, resulting in both diminished light output, and heating of the wiring. The headlight switch may also overheat. By running the spotties thru a relay means that the original wiring carries no extra current; the relay and its direct wiring from the battery carries all the extra current to the spotties.
On some vehicles, even the original wiring drops too much voltage to the lights - this is why firms like Pirahna make wiring looms to run the headlights off a heavier cable and give more light output.
Hope this helps,
Gerry
AnswerID: 127384

Reply By: fisho64 - Tuesday, Aug 30, 2005 at 02:08

Tuesday, Aug 30, 2005 at 02:08
relay also protects the switches by lowering the switching current (not the voltage as mentioned earlier)
AnswerID: 127391

Follow Up By: MATT.D(WA) - Tuesday, Aug 30, 2005 at 07:31

Tuesday, Aug 30, 2005 at 07:31
Thanx for the replies thought this was the case but wasnt sure.
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FollowupID: 381890

Reply By: tex1972 - Wednesday, Aug 31, 2005 at 20:41

Wednesday, Aug 31, 2005 at 20:41
also halogen globe don't like voltage drop so getting the power supply from the battery/alternator via a relay keeps the supply voltage up around 14 volts

." The rated output of an automotive lamp is figured at 13.5 volts (the output of a typical alternator), not at 12 volts as you might expect. If you loose 5% of the voltage due to voltage drop through small wiring or corroded connections, which gets you to 12.8 volts, you are only putting out 80% of the rated luminous intensity - for a 100 watt lamp that’s only 80 watts equivalent!! "
AnswerID: 127672

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