Relay Question?

[dodge68charger]

Just Wondering Where And When i Use a Relay?

Im Running Front Fog Lights To a 16 amp Push Button Switch. Do I Use a Relay?

Also Running A Under Dash Heater To A 16 amp Switch, The Lead From The Fuse Box Is a 20 amp Circuit.

Do I Need a Relay Before The Switch?

Thanks

 

[Crackedback]

JMO, depends on run length of your wiring.

Fogs, I'd run a relay to shorten the actual power path from source to lights. Voltage drop issues.

The dash heater is a short run from fuse block to heater. Might be OK.

I general I like relays so the trigger/switch is not taxed for full load.

You want some sort of protection in the circuit on your power side.

 

[Shorthorse]

Most will recommend relays. I prefer to keep it simple. Common sense still applies to either solution.
Know the amp draw needed for your devices and design from there. Use the proper gauge wire. Fuse valve should be the manufacturers recommend size. Select a compatible amp rated switch that will handle amps higher than the device draw. Good connections are a must. Strain relief for the wires is critical, never allow a connection to bare the weight of the wire. When routing wire, protect against sharp edges of any metal.

 

[kiwigtx]

Just Wondering Where And When i Use a Relay?

Im Running Front Fog Lights To a 16 amp Push Button Switch. Do I Use a Relay?

The rule of thumb is you use a relay whenever you need to switch a load.

The control circuit is fine to 'drive' the relay....but the load - whether it be lighting, horn, compressor etc should always have the main current draw run through a relay contact.
This protects the switch contacts and gives them longevity and reliability.

Always select the cable supplying the load (horn, lights etc) to be suitable for the expected current draw.
Always select the appropriate size fuse to protect that load. Next size up from the expected maximum load is usually sufficient.

Never put a 30 Amp fuse in to protect a 5 Amp load that is constantly blowing the fuse - if a fuse blows, you have a problem.

 

[Nacho-RT74]

ON THIS CASE, using relays will give the chance to use smaller switches and wires to/front the switch on cab, and keep the load just on engine bay, aside save from voltage decay on the distance. BUT. Couple of standard fog lights are not really to be scared. Factory used 14 gauge wires and regular switches are able to hold the regular load. Cars with 4 beams has been able to handle 4 high beams without relays ( that's about 16-20 amps with all 4 lighting on, assuming the high beam is the standard 60 watts rate ), but of course that's with new wiring, plugs and switches everywhere. Time makes its work to the copper thought, specially on abused cars. Now of you are using high power fog lights, use relays will be better, just like when upgrading headlights to more powerfull beams. But using standard fog lights ( usually they are not more powerfull than high beams ) relays are not really mandatory.

Please, if your car still gets the ammeter working and attached, be sure to NOT feed them from batt post... either with relay or not.

Accesories source on fuse box, if comes from the fused provision, is tipically on 20 amps rate, feeded with 16 gauge wire and is sourced from the correct side of the charging system. Standard rate fog lights use to suck 5-8 amps. Will need to take care of what other accesories do you have sourcing from it. I can't recall from where is sourced the E body fog light option thought to take it as an example.

Now if you are thinking on get ot from a diff provision, like some splice into the acc network before fuse box, like straight from ign switch ACC wire ( black ), rate the in line fuse accordingly, don't "over fuse" ( as mentioned ) it because it won't serve its purpouse of protect before a bigger failure. The goal of the fuse is protect the system, so make the maths

 

[Nacho-RT74]

The control circuit is fine to 'drive' the relay....but the load - whether it be lighting, horn, compressor etc should always have the main current draw run through a relay contact.
This protects the switch contacts and gives them longevity and reliability.

The horn relay case is quite diff than the load itself at least for the STOCK system. The relay for the horn is actually to be able to trigger them using ground/negative side of the power to still activate the positive side to make them work. But if they would be triggered with positive from steering wheel, relay wouldn't be mandatory.

Similar stage is the electrical fans using a ground trigger senders, althought on these will serve dual purpouse... reliability for the power and using the ground as a trigger.

Using blowing horns, will serve also dual purpouse if bypassing the stock relay.

 

[Nacho-RT74]

Do I Need a Relay Before The Switch?

Actually the relays use to be AFTER the switch being triggered by it, and get it fused on the power side of the relay. If using a 4 or 5 pins Bosch kind relay, fuse should be before the #30 prong.

Aside that, you are mentioning "16 amps switches" which doesn't mean the accesories ( heater and fog lights ) actually suck that, just that the swithes are ABLE to handle up to 16 amps. Need to know how much suck each accesory no matter the switch capability.

Stock wiring use a dedicated fuse for the heater blower sharing with some other device on car which requires less load. Is not usually on the accesories fuse. Check the owners manual for each fuse function. Once again, comparing with stock system, blowers are on 8-15 amps ( depending on speed ). Modern blowers are maybe less suckers. I already mentioned the fog lights rate, depending on wattage