how to wire a relay for lights... the 'RIGHT' way

E

ericfx1984

NAXJA Forum User
Location
Kansas City Area
(NOTE: if anyone is interested I could make a youtube video and/or LIVE pictures)

they say a picture is worth a thousand words... here it goes...

another thing to note is that the output from the switch to the relay pin 85 can be ANY gauge wire as it carries almost NO load... also this wire can be split to activate as many relays as you want...

alternatively you can connect pin 87 on the first relay to pin 86 any other relays (just make sure that those relays have pin 85 connected to ground)

relay.jpg
 
while this subject is near the top. What Ga wire should be used in what amp ratings. Im with ericfx with the desire to do it right, not trying to hyjack, the subject is just up.
 
I recommend never putting more than 2 lights on one relay. Better to have more capacity than is needed. I see too many systems wired up with too much load on the relays and they get too hot in use. As for using ground to trigger the coil in the relay, I feel that it is best to have shorter runs of hot wires in the system, so I run power to the switch and then use the positive to trigger the relay.
 
John, a lot of automotive systems use ground side switching (not going to debate whether right or wrong, I'm not an EE.)
 
jeeperjohn said:
I recommend never putting more than 2 lights on one relay. Better to have more capacity than is needed. I see too many systems wired up with too much load on the relays and they get too hot in use. As for using ground to trigger the coil in the relay, I feel that it is best to have shorter runs of hot wires in the system, so I run power to the switch and then use the positive to trigger the relay.
Click to expand...

Wouldn't that mean you have more that way becasue then you have to run a hot wire into the cab to the switch? I like to make a jumper from 30 to 86 and ground side switch 85, simple, no hot wire in tha cab and one less wire.
 
I'm waiting on a PDF from http://www.parts-express.com. I've got a hard copy of it at home, and PE has granted permission to post it here. Me, I like to get 12v stuff from PE, I just got a hundred dollar order in, relays, bases, and weather sealing butt splices. If you order relays from PE, make sure to add p/n 500-001 for a hard copy.

The idea behind a relay is to use a low current and/or low voltage circuit to switch a high current/high voltage current. A starter relay or solenoid is a good example of this. You don't have a 100a circuit through the key switch, yet that's what it takes to turn over a starter.
 
There are good reasons to sometimes use ground side switching. The potential problem with this is the fact that if a short to ground or switch failure occurs, the relay is energized and the controlled component (lights, motor, etc.) will then run until something melts, catches on fire or the battery runs down. If the short occurs while switching a properly fused hot wire, the fuse blows and that is the end of it.
 
whitexj98 said:
Wouldn't that mean you have more that way becasue then you have to run a hot wire into the cab to the switch? I like to make a jumper from 30 to 86 and ground side switch 85, simple, no hot wire in tha cab and one less wire.
Click to expand...
Depends on how you decide to wire it. Since the coil requires very little current to activate I would get power from the fuse block at a hot terminal.
 
Ground triggering is often cheaper. Less wires,
If the ECU was + triggering relays. The main power lead into the cabin would need to carry just that much more load. Maybe needing to be bigger. The ECU would need to carry and distribute this power. As is, it's a short and simple path to the ground.
Also Computers are generally ground triggered internally. It's a lot faster/easyer to pull something low then to build a charge in it.
I figured computer mind bender build the ECU and all the little gismos that it runs so they are going with what they know works best in there world. Including neg triggering.
 
Last edited:
jeeperjohn said:
Depends on how you decide to wire it. Since the coil requires very little current to activate I would get power from the fuse block at a hot terminal.
Click to expand...

that is doable as well... lots of options... you notice that I said the 'RIGHT' way, being careful to use quotes, as to show that it is my opinion basically
 
Switching the ground terminal can get tricky sometimes, you have to be carefull when controlling the ground on some loads as the ground terminal isn't always isolated, but usually is the load has a seperate load wire/terminal it's most likely isolated, but not guaranteed.

Also when switching the ground for a relay the power will/can actually back feed through the coil when the ground switch is open. Not really a big deal for a simple circuit like you have drawn up, but can cause complications when doing more intricate circuits. Their are relays available with built in diodes to prevent this though.

There's more than one way to skin a cat.... errr, I mean wire a circuit
 
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