Help on SPST relay to connect light bar to high beams

[Johnkn]

I'm also interested. Would like to run the 3 new lights on each side of the lower front valence through outfitter switches to enable them, then trigger only when the high beams are on.

 

[Kahuna]

I'm also interested. Would like to run the 3 new lights on each side of the lower front valence through outfitter switches to enable them, then trigger only when the high beams are on.

Then I would just power the switch side of the relay with the upfitters and the coils on 3 relays with the headlight tap. It's a lot easier if you don't need to control the lights independently of the headlight switch with the upfitters.

 

[Kahuna]

I posted in the other thread; not knowing what's on the high beam's circuit except the LED which points the wrong way, I think it is good idea to use a flyback diode for any relay it's controlling. Also, I'm putting a diode between the headlight tap and the relay it's controlling to ensure feedback is quashed.

You've lost me on the flyback diode. What could that be needed for exactly? Headlight tap diode is easy enough to add.

 

[mezger]

The relay is controlled by a coil; when you have your high beams on, current will flow from the headlight tap through the coil to ground.

A coil resists time rate changes in current similar to the way a mass resists acceleration.

If you suddenly open a circuit with a coil in it (flip to low beams), if the coil doesn't have an alternate path to flow current, the coil will produce a very big voltage across its terminals attempting to keep the current going (in particular, it's trying to maintain its magnetic flux), and in this case the voltage on the headlight tap side will be negative. Depending on the circuit, this could exceed 100V; if it were connected to a switch, it'll cause arcing across switches' terminals. A flyback diode give the coil a path to gracefully dump its energy into the coil's internal resistance.

The mass analog would be using your brakes instead of hitting a wall.

I don't currently know what's on the headlight side of the circuit; there could be a path for the coil to dissipate its energy gracefully.

Given that I don't know what's on the other side and given the cheapness of diodes I'm taking a careful route until I either get a schematic or hear from someone knowledgeable indicating a reason why it's a non-issue to run without spike suppression.

It is notable that there are some disadvantages to running a flyback diode; in particular, the relay will be a little slower to open and it's harder on the relay's contacts.

 

[Gordini]

Agreed. I used Delcity 73990 NC/NO Change Over Relay w/Diode, 30/50A, 12V DC, about $9 ea. Hi-tech, low maintenance as they say...

 

[Kahuna]

In the micro relay format would something like this do the trick?

https://theelectricaldepot.com/rela...cro-relay-24v-spdt-10-15-amps-diode-br-1-each

 

[dewalt]

UH 24 volt? That would be really complicating a simple thing

 

[Kahuna]

Errr, yeah, sorry I was reading the PDF on that page which lists a 12v version. My bad.

https://theelectricaldepot.com/pdf/Relays/Micro ISO Relays.pdf

 

[dewalt]

Use a 5 pin relay and 2 switches and you can have automatic on with high beam or manual on

 

[mezger]

FWIW, I've had mine wired up for a couple weeks. Pulled a lead from my driver's side headlight. The setup works great with the flyback diode, i.e. any additional delay on their turn off is going to be imperceptible to most.

Spent some extra $$$ and bought the Motorcraft WPT1321 and WPT1279 pigtails I saw recommended on FRF so I didn't have to cut into the headlamp wires. Those worked excellent except I had to trim one plastic orientation tab on one of the pigtails, once trimmed, fit was perfect and they came with all needed waterproof heat shrink. I pulled my lead from the driver's side headlight instead of the passenger side, made for trivial wiring routing to where I placed my relay box. R&R of the headlight was straightforward after watching a vid on what tabs need to be popped off.