When to use a relay??

[Danrex Junior Member Find all posts View Profile Close Danrex]

Hi All,
Bit confused (and red faced) in when to use a relay? ops:
And what does the relay do?
Done a bit of electric work on the old girl and hear or see different ways to wire things up. For instance a compressor, why put a relay in when you hard wire it and when left portable you just wack it on with the alligator clips?
Things I have on the car,(do any of these need a relay?)
Compressor
Rear light with dash switch
3 Power outlets in rear
2 power outlets in mid
Two way
Led lights under bonnet with switch
Spotties (got relay)

Any help or guidance to another forum (searched but no direct answers).
Cheers Danrex

[leachy_9 Advanced Member Find all posts View Profile Close leachy_9]

Re: When to use a relay??

Danrex,

Small dash mounted switches are only capable of switching relatively low current (power), where as relays are able to safely switch higher current. So when you want to switch devices that have power rating higher than the switch can handle a relay is required. Using a relay also allows you to run smaller gauge wiring up to the switch, with the larger gauge, power wiring, only needed to from the power source to device being powered.
Since space is limited on most dashes the switches used are generally small and only capable a switching low current , usually <2Amps. So any device drawing more current than this will require a relay. As a general rule any lights (possibly not LEDs) and anything with a motor will need a relay. As will a outlets as they are usually rated, and presumably wired and fused to deliver up to 10A

Compressor - relay needed
Rear light with dash switch - relay needed
3 Power outlets in rear - relay needed
2 power outlets in mid - relay needed
Two way - relay not required
Led lights under bonnet with switch - Depends on the rating of the LEDs and switch
Spotties - relay needed

Leachy

[Danrex Junior Member Find all posts View Profile Close Danrex]

Re: When to use a relay??

Cheers Leachy,
Can see thru the mud a bit better now. Just a few more questions if ok?

What size/type of relay will i use for these different applications ? Does it matter!

My compressor has a built in switch on the pump itself and I dont plan to put one in the cab as it is easier to flick the switch where it is, do I still need a relay (going by your explaination I'll say no.)?

The two power outlets in the back seat area is from a Weaco kit ( came all as one just connect red/red and black/black) By memory I think It has a cut out built in, does it need a relay, (was not stated in the instructions)?

Thanks for your help going to track down some wiring diagrams I have seen on PP to get the wiring sorted on the car( remember seeing them but where ???) crack out the search button and crack out the tinnie!
Cheers

[MickL Addicted PP Member Find all posts View Profile Close MickL]

Re: When to use a relay??

What size/type of relay will i use for these different applications ? Does it matter!

I prefer Bosch or Hella relays. Most off the shelf, automotive relays are a copy of the Bosch relay. For the most part the contacts are rated to 30Amps which will suit most applications.
Use a relay with diode protection across the coil to prevent spiking.

My compressor has a built in switch on the pump itself and I dont plan to put one in the cab as it is easier to flick the switch where it is, do I still need a relay (going by your explaination I'll say no.)?

No relay required if using the wiring supplied with a portable compressor, the manufacturer will have sized the switch and wiring to suit the current draw.

The two power outlets in the back seat area is from a Weaco kit ( came all as one just connect red/red and black/black) By memory I think It has a cut out built in, does it need a relay, (was not stated in the instructions)?

Assuming this is a fused run from the battery, no relay required unless you want to switch the supply on/off with the ignition.

going to track down some wiring diagrams I have seen on PP to get the wiring sorted on the car( remember seeing them but where ???)

The wiring diagram in this post can be easily modified to run any accessory, just delete the feed from the headlight and substitute with an earth and a switched accessories/ignition supply.

Spotlight wiring diagram

Mick

[leachy_9 Advanced Member Find all posts View Profile Close leachy_9]

Re: When to use a relay??

The two power outlets in the back seat area is from a Weaco kit ( came all as one just connect red/red and black/black) By memory I think It has a cut out built in, does it need a relay, (was not stated in the instructions)?

Assuming this is a fused run from the battery, no relay required unless you want to switch the supply on/off with the ignition.

Mick

Relays are only required where you want to turn the outlet on and off. Since they are for a fridge I assume you will want them live all the time. So as Mick says no switching i.e no relay is needed. Just make sure the wiring used is a large enough gauge not only to handle to current but to minimise voltage drop. Also make sure a correct size fuse (fuse should be sized to protect the cable) is installed as close to the source as possible.

Leachy

[Danrex Junior Member Find all posts View Profile Close Danrex]

Re: When to use a relay??

Cheers Guys

All seems pretty straight forward now ( Der :P ). So got a bit more tinkering to do on the bus (bugger )
The rear light I will have to chuck in a relay.Not so hard just have find where I ran the wires under the dash.
The compressor and mid power outlets are fine, both got 30amp fuses near the battery and 6mm daul core wire( checked the drop in amps today bugger all 0.3amps)
The rear outlets, well could be fun.
They run from the battery thru a 30amp fuse along a 6mm wire to the back. Where I have set up a power outlet board consisting of 2 x outlets, 2 x switches for them and 2 X 20amp fuses. ( Picture posted hope turns out first time!). So I am thinking a relay is required for both of these. Only low voltage appliances are used on these outlets and occassionally the fridge.Also the switches are rated to 20amps. Will I still need the relays?

A big thanksI have found the info and users on PP are way above other forums I have looked at.Thanks again and keep it up!

[Danrex Junior Member Find all posts View Profile Close Danrex]

Re: When to use a relay??

They run from the battery thru a 30amp fuse along a 6mm wire to the back. Where I have set up a power outlet board consisting of 2 x outlets, 2 x switches for them and 2 X 20amp fuses. ( Picture posted hope turns out first time!). So I am thinking a relay is required for both of these. Only low voltage appliances are used on these outlets and occassionally the fridge.Also the switches are rated to 20amps. Will I still need the relays?

No, you won't need a relay, how you have is fine if all you want is to be able to switch it on and off next to the outlet.

I'd probably drop those 20 A fuses down to 15A though as it's always best to have a safety margin (i won't get all techincal on fuses and their actual ratings). Also, what rating are those outlets? As the fuse should be rated lower then those also.

Trojan
Thanks for the info.I can see how a relay works now and when to use one.
Was thinking the relays would be an over kill for the power outlets(glad as well, there is enough spagetti behind there already and nice to have a second opinion).And will probably give the relay a miss on the rear light as well, since it is all working fine and minimal drop in power. Thanks for the info guys I think i opened a can of worms here :lol:

[ozwapet2 Advanced Member Find all posts View Profile Close ozwapet2]

Re: When to use a relay??

Cheers Guys

All seems pretty straight forward now ( Der :P ). So got a bit more tinkering to do on the bus (bugger )
The rear light I will have to chuck in a relay.Not so hard just have find where I ran the wires under the dash.
The compressor and mid power outlets are fine, both got 30amp fuses near the battery and 6mm daul core wire( checked the drop in amps today bugger all 0.3amps)
The rear outlets, well could be fun.
They run from the battery thru a 30amp fuse along a 6mm wire to the back. Where I have set up a power outlet board consisting of 2 x outlets, 2 x switches for them and 2 X 20amp fuses. ( Picture posted hope turns out first time!). So I am thinking a relay is required for both of these. Only low voltage appliances are used on these outlets and occassionally the fridge.Also the switches are rated to 20amps. Will I still need the relays?

A big thanksI have found the info and users on PP are way above other forums I have looked at.Thanks again and keep it up!

Good work !

Just a small technical point - you don't get a drop in amperage, you get a drop in voltage.
So if you measured the voltage across the terminals at the battery, and then again at your device and the difference is 0.3 V, that's a 0.3 volt drop. You must have the device (s) running ( i.e. under load ) for this to be meaningful. It's an excellent, simple method for determining if your wiring is thick enough.
If on the other hand you put your meter in series with the positive lead, and turned it to amps mode you would be reading Amps current draw. An air compressor running is likely to draw more than 10 amps, a fridge with it's compressor running maybe 2 amps. Careful though - the cheaper multi meters have a limited ability to measure higher currents.