Powering relays

Hello, as some of you may know, I'm in the process of building a custom wiring harness from scratch for my 1984 110 with 200di.
It's almost complete but I've now got to the point where I've got some additional relays that need powering. Load and supply for the circuits relevant is all taken care of but I'm now into what to power them with.
I had considered a circuit with a max 5a fuse that fed the switches, with a 1mm max cable. Then this power would go to the relay as shown below.


My next thought was to just take that 5a supply to all the relays as they're together, then Switch the earth circuit, like this-

Or option three is to jump the main supply into the switch supply then earth again but this is not my preferred method as it leaves something that draws 200ma max with a potential 10a feed going into it. It does keep the switch supply on the same fuse though.

Any help would be greatly appreciated as always.

I wire "extra" stuff using scheme 1. and fuse both the supply to the relay, and the supply to the "light" ( shown in this case ) - If you've got a lot of relays to add, I'd suggest looking for an appropriate fuse box first, or two (etc), and taking it from there.

I've also fitted several fuses in the same circuit in the past to protect it - re-wired a digger a while ago, and fitted two fuses in some circuits, and four in others - sounds insane - but the vibration the loom is subjected to had already caused a fire due to no fuses - and I didn't want the job a second time. Also make fault finding easier - which in turn means you don't get any, IME

Cheers sim, so my original idea then? I'm going to treat myself to a lovely new fuse box, meant to get it before Christmas but changed my mind about a few things.

Yep, the original piccie I'm sure you know this, but there are quite a few suppliers of fuse boxes out there - so have a good look round - and if you can get a spare or three at the same time - future proofs the whole project

Conventionally, the circuit which you've shown in diagram 2 is the way which most are wired with the switch on the earth or ground side.
The coil of a relay draws a very small current, possibly in the range of 100 to 150 mA (0.1 to 0.15 Amps), so your fuse on the low current side should be no more than say, 2 amps which would provide a sensible trade-off between proection and reliability.
The fuse on the high current side should be rated for the load which the relay is switching.

Here's one of my infamous hand drawn circuit diagrams showing how I would wire two relays, but one or three would be easy using the diagram.
I've included the pin numbers on the relays and also how I would wire in the tell-tale indicator bulbs.

Thanks fella's.
How would a standard relay cope with a potential 10a fed to it? On the switching side? I can avoid this and know they only require .1 - .2a but can't find the maximum they can take.

When I last looked at my relays with a meter, I was pleasantly surprised to see that they only drew 70 mA to switch them on.

Where I've added additional circuits to mine, I've gone for an arrangement similar to diagram 1 but with the addition of a fuse in both the 'exciter' supply to the relay and in the 12v supply that the relay switches on and off. For example I've got a few things that I want to come on with the ignition, such as the dash cam and the tracker, so I've put a wire from the back of the ignition switch which goes almost immediately into an in-line fuse, and then on to the driver's side seatbox to flip the relays. Meanwhile the supply for the 'load' comes from my auxiliary fuse box which I've installed behind the driver's seat. That way all the sources of power have a fuse in them before they go through any holes in bulkheads or other sheet metal. Yes of course I've used grommets and plastic trunking where possible, but Land Rover wiring seems prone to chafe and wear through with all the vibration.

Discokids said:

Thanks fella's.
How would a standard relay cope with a potential 10a fed to it? On the switching side? I can avoid this and know they only require .1 - .2a but can't find the maximum they can take.

Click to expand...

The current draw of the relay's exciter circuit is defined by the resistance of the relay itself, so it doesn't really matter. In my example above, Ohm's law suggests that my relays have a resistance of 171.2 Ohms. Obviously, giving this circuit a 60A fuse won't help you much in the event of a short as there's plenty there to start a fire, but the usual 5 or 10 A blade fuses should be fine. I try and use the lowest denomination fuse I can get away with.

Cheers. I'll be fitting plenty of fuses to both circuits, just didn't draw them

My thoughts are with something like the boomslang headlight loom. Standard lr light switch is fed direct from alternator (brown) which then sends power to the headlights at 8a each. When that loom is fitted there doesn't seem to be a restriction of this current to the 'new' relays.
It all just got me thinking.

Discokids said:

Cheers. I'll be fitting plenty of fuses to both circuits, just didn't draw them

My thoughts are with something like the boomslang headlight loom. Standard lr light switch is fed direct from alternator (brown) which then sends power to the headlights at 8a each. When that loom is fitted there doesn't seem to be a restriction of this current to the 'new' relays.
It all just got me thinking.

Click to expand...

Well, the restriction is in the relays themselves. If they're anything like the ones I've added, the exciter coil could have a resistance of around 170 Ohms so the current draw will be tiny. Incidentally, I cracked one of mine open and saw there was actually a tiny coil like a miniature solenoid to pull the switch closed. How quaint. I thought it would be all solid state these days.

Yes, they're interesting inside aren't they?.

Discokids said:

Thanks fella's.
How would a standard relay cope with a potential 10a fed to it? On the switching side? I can avoid this and know they only require .1 - .2a but can't find the maximum they can take.

Click to expand...

It depends on the type of relay you purchase, most standard automotive relays are rated to switch easily up to 20 Amps, for instance the relay on the Td5 engine for switching the supply to the glow plugs is rated at 40 Amps.
I've recently purchased a relay with a current rating of 100 Amps for a little project, but that is a bit specialised I suppose.

Brown said:

Well, the restriction is in the relays themselves. If they're anything like the ones I've added, the exciter coil could have a resistance of around 170 Ohms so the current draw will be tiny. Incidentally, I cracked one of mine open and saw there was actually a tiny coil like a miniature solenoid to pull the switch closed. How quaint. I thought it would be all solid state these days.

Click to expand...

Relays might be "quaint", but the technology is tried and tested and I don't foresee the end of an electrical contact operated by an electromagnet any time soon. They'll just get more reliable, smaller and possibly easier to use.

brian47 said:

It depends on the type of relay you purchase, most standard automotive relays are rated to switch easily up to 20 Amps, for instance the relay on the Td5 engine for switching the supply to the glow plugs is rated at 40 Amps.
I've recently purchased a relay with a current rating of 100 Amps for a little project, but that is a bit specialised I suppose.


Relays might be "quaint", but the technology is tried and tested and I don't foresee the end of an electrical contact operated by an electromagnet any time soon. They'll just get more reliable, smaller and possibly easier to use.

Click to expand...

But what about a relay that switches 20a. If I shove a cable on that comes from a 10a fused supply to the low power (energising coil) side, will the relay melt?

brian47 said:

It depends on the type of relay you purchase, most standard automotive relays are rated to switch easily up to 20 Amps, for instance the relay on the Td5 engine for switching the supply to the glow plugs is rated at 40 Amps.
I've recently purchased a relay with a current rating of 100 Amps for a little project, but that is a bit specialised I suppose.


Relays might be "quaint", but the technology is tried and tested and I don't foresee the end of an electrical contact operated by an electromagnet any time soon. They'll just get more reliable, smaller and possibly easier to use.

Click to expand...

They are still used in the control rooms of nuclear power stations ....

Discokids said:

But what about a relay that switches 20a. If I shove a cable on that comes from a 10a fused supply to the low power (energising coil) side, will the relay melt?

Click to expand...

No, the relay will not melt!
Under normal operating conditions, and on a fault free circuit, the laws developed by that there Mr. Ohm say that the relay coil will only draw the current it requires depending upon its resistance.
When you're talking about a 20 amp supply, that's just its capability, not what it is going to supply, the significant value here is 12 volts. For instance, the 12 volt supply in a vehicle is capable of supplying up to 1000 amps, but that doesn't mean that 1000 amps is being drawn by your sidelights.
If you think of an analogy often quoted, a water pipe. Then the pressure equates nicely to the voltage, while the current (the amps) equates to the flow of the water. Also, if you reduce the size of the pipe, that equates to the resistance.
There's a good write up by @The Mad Hat Man about relays here:-
https://www.landyzone.co.uk/land-rover/basic-relay-info.48117/

Excellent. Mad hat mans guide is thorough.
Just to summarise;
Standard Land Rover light switch takes a feed (Brown unfused usually but I'm going to use a 10a fused supply from a permanent 12v supply (my engine bay fuse box))
Pos 1 sends this full / fused 10a supply of power to the side lights/illumination via red and passes through a 5a fuse. This will be staying.
Pos 2 sends power to the high/low Switch and rear fog circuits via blue. The high / low Switch then directs this power to some 8a fuses via blue/red and blue/white then it's off to the lights. I intend to use these blue/white and blue/red wires to trigger my headlight relays but it's potentially 10a (sidelights draw 4 between them + illumination so does only leave 4-5a) and this is the bit I'm wary of, should I fuse this before it gets to energise the relays, knowing each relay will only 'draw' 150ma max, but there's potentially 10a there If they decide they want it.
Sorry for rambling on

Treat the relay as two different circuits, the coil circuit or low power side and the switched circuit or high power side.
Always put a small fuse in the coil circuit, maybe 1 or 2 amp fuse, that way even if the relay should become faulty the circuit will only draw 2 amps before the fuse blows.
The high power side should be fused to suit the actual load, for instance, for two spot lights, each light will draw just over 4 amps which for the two would then be 8 amps. The fuse should obviously be bigger than 8 amps which suggests that you would need a 10 amp fuse, but you need to build in some reliability too so I would advise a 15Amp fuse for feeding two standard spotlights.
As I said, treat the relay as two completely different circuits and try not to think in terms of a 20 amp supply or a 40 amp supply, you only have a 12 volt supply. The load devices will take what they need from that supply.

brian47 said:

Treat the relay as two different circuits, the coil circuit or low power side and the switched circuit or high power side.
Always put a small fuse in the coil circuit, maybe 1 or 2 amp fuse, that way even if the relay should become faulty the circuit will only draw 2 amps before the fuse blows.
The high power side should be fused to suit the actual load, for instance, for two spot lights, each light will draw just over 4 amps which for the two would then be 8 amps. The fuse should obviously be bigger than 8 amps which suggests that you would need a 10 amp fuse, but you need to build in some reliability too so I would advise a 15Amp fuse for feeding two standard spotlights.
As I said, treat the relay as two completely different circuits and try not to think in terms of a 20 amp supply or a 40 amp supply, you only have a 12 volt supply. The load devices will take what they need from that supply.

Click to expand...

Thanks, I've been referring to the load coil circuit as the switches circuit, no wonder I wasn't making sense of it. What I need is a rest, I've been on this now every single day bar one since before Christmas and it's just getting a bit much, I've even dreamt about it. I'll discount the idea of my third diagram. I've been out all evening laying cables in it, scratching my head. It's almost complete though.

Discokids said:

Thanks, I've been referring to the load coil circuit as the switches circuit, no wonder I wasn't making sense of it. What I need is a rest, I've been on this now every single day bar one since before Christmas and it's just getting a bit much, I've even dreamt about it. I'll discount the idea of my third diagram. I've been out all evening laying cables in it, scratching my head. It's almost complete though.

Click to expand...

Relays can be a bit confusing if you're not used to using them, and then one day it just clicks, if you'll pardon the pun.

sometimes, if it makes it easier, you can get relay sockets that incorporate fuses as well.