Need help with relay selection.
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From: Port Arthur, TX
Need help with relay selection. I have an application for a relay which I do not know how to select.
I have display lights (LEDs) on my car and Halo Headlights. The Halos are currently wired to the parking lamps.
I want to make the halos come on with the display lights, but I do not want to turn the display lights on while driving and I do not want the parking lamps to 'backfeed' and turn on the display lights. Therefore I need a relay. I've drawn a small schematic in the attached picture.
I have display lights (LEDs) on my car and Halo Headlights. The Halos are currently wired to the parking lamps.
I want to make the halos come on with the display lights, but I do not want to turn the display lights on while driving and I do not want the parking lamps to 'backfeed' and turn on the display lights. Therefore I need a relay. I've drawn a small schematic in the attached picture.
Joined: Nov 2003
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From: Jacksonville, FL (originally from Toronto Canada)
Wouldn't it be easier to use double pole switches for S1 and S2? That way you could wire L3 to one pole on both switches, L1 to the other pole on S1, and L2 to the other pole on S2.
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From: Port Arthur, TX
That really won't work. 'S2' in the diag is really the parking light circuit which comes on with DRL, headlights etc. I am not really using a switch for that side.
Hoping to automate the function, not put in an extra switch.
Thanks for the thought.
Hoping to automate the function, not put in an extra switch.
Thanks for the thought.
Joined: Nov 2003
Posts: 11,332
Likes: 347
From: Jacksonville, FL (originally from Toronto Canada)
You can do it with an ordinary SPDT Bosch-style (5-pin) automotive relay but you'll have to provide an additional power source and ground besides the lamp circuits themselves.
Assuming S2 is the factory headlight/parking light switch and S1 is a separate switch you provide:
30 - output to L3
85 - chassis ground
86 - 12V+ input from S1
87 - 12V+ from a fused source
87A - 12V+ input from S2 (parking lights)
Since 87A is connected to 30 when the relay is at rest, the lights will go on and off with the parking lights. When you turn S1 on, the relay energizes connecting 87 to 30 and powering the lights from your fused power source.
This has the advantage of running very little current through your switch S1 (just enough to trip the relay) giving you lots of options for using a small switch.
Assuming S2 is the factory headlight/parking light switch and S1 is a separate switch you provide:
30 - output to L3
85 - chassis ground
86 - 12V+ input from S1
87 - 12V+ from a fused source
87A - 12V+ input from S2 (parking lights)
Since 87A is connected to 30 when the relay is at rest, the lights will go on and off with the parking lights. When you turn S1 on, the relay energizes connecting 87 to 30 and powering the lights from your fused power source.
This has the advantage of running very little current through your switch S1 (just enough to trip the relay) giving you lots of options for using a small switch.
