Relay query

[tonyk]

Hi
This might sound a bit simplistic. I'm going wire up my headlights and new (! Christmas present) driving lights, 55W each, through relays. I was wondering whether the relays can be wired directly to the spare terminal on the alternator, 45 amp. Would the current not be stable or is it OK to do this to save long lengths of wire running to the solenoid junction? How many amps should the relays be rated for?

I've been through the recent thread

https://www.britishcarforum.com/bcf...removal-and-Moss-headlight-relay-installation

but this wasn't mentioned.

 

[Bob_Spidell]

If the 'spare terminal' is an output terminal--same as the lead going to the solenoid--you can wire the lights up there (if you're not sure just connect to the same lead that goes to the starter solenoid). The alternator, battery and all current sinks (the 'load') are one circuit--the 'signal' will be the same no matter where you tap it (assuming no unwarranted resistances; e.g. a bad connection). The output of the alternator is rectified by 3 (usually) diodes and smoothed by the battery, which acts as a very large capacitor.

Two 55W lights is 110W total. A '12V' system will generally run at about 14V no load, but to add some margin calculate the load as 110W/12V = 9.2A. You want an overcapacity of 1.5-2 times so I'd use a (single) relay rated for at least 15A, preferably 20A. If you use two relays you'll probably have to use 10A relays (usually, components are rated in units of 5).

Note you'll want at least 21AWG--preferably 20AWG--wire going to each relay and light: https://www.powerstream.com/Wire_Size.htm

The relay specs should quote the load needed to switch the relay--same considerations apply.

 

[tonyk]

Thanks for that. You'll gather that I don't know much about relays. I've got an electric fan as well, not yet installed. This comes with an installation kit. So I assume the same considerations apply. I think it might be safer to connect to the solenoid. Not sure what the spare alternator terminal does. I'll get some 20G wire.

It might be easier to buy a kit, but your information is excellent for making sure the components can handle the current.

 

[Keoke]

It might be easier to buy a kit,---NAW !

Go here using Google

and you will know all you want to know about relays ,ratings and how to wire them up---Keoke
:semi-twins:

 

[Fury5]

There is also a very good write up on installing relays for automotive lights (complete with a good, simple wiring diagram) here:

https://www.danielsternlighting.com/tech/relays/relays.html

Stern's website has a wealth of info on vehicle lighting issues and it is worth reading all of his tech articles. For the relay conversion Stern suggests using a minimum of 14AWG, but recommends using 12AWG, wire for the power circuit and does recommend pulling power directly from the output terminal on the back of the alternator. Voltage drops in the wiring harness and connectors are the biggest enemies of light output so larger guage wire and properly soldered and sealed connections will be your best defense. I have done this on a number of my older vehicles (but not my Healey, yet) and the improvement is quite noticeable.

Cheers.

Richard Owens

 

[tonyk]

Thanks. Good stuff. I'll be an expert in no time??