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peteyg

How hot should a relay get?

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Just finished rewiring the headlamps with thicker cable and new relays after the previous stuff melted but I'm concerned that the relays are getting too hot. I've wired one up to 4 dipped beam and the other for 4 main beam. They are 30amp relays.

Pete


Pete '79 S2

LEW Miss September 2009

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Upgrade today to remove Google ads and support TLF.

Chattering relays (with a bad connection) will get hot - but you'll be able to hear that.

Just put a long screwdriver on the relay, with the other end in you ear and if you hear it vibrating...

Edited by s4simon

Simon  (94 S4)      My Esprit will be for sale in late 2017

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Relays are new and there's no chattering.

Pete


Pete '79 S2

LEW Miss September 2009

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Work out the current being drawn by the lamps, for example a 100w lamp draws 8.3A, so four of them will draw 33.2A. ( Current = power divided by volts ) You may need relays rated at 40A or higher, or use two relays wired in parrallel to share the current. If you pull 33A through a 30 amp relay it will get warm.


Too many Toys are never enough !

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Thanks Steve. I'm guessing the lamps are 55W so by my calculations, with all 4 on thay should only be drawing 18.3A. This is why I'm concerned. I've cleaned all the earths. Not sure what else to do. Guess I might have to bite the bullet and get an autosparky to have a look. Just feel a bit disappointed I couldn't have done it myself.

Pete


Pete '79 S2

LEW Miss September 2009

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Dig out your test meter and measure the current through one lamp. Then you'll KNOW what you have...multiply it by 4 and that's the total current. Measure the volts across the lamp and that's the voltage...multiply VxI and you have watts. At least you should be able to check out the sums against reality...also try having the relay operated, with the load from the lights removed...then you'll see how hot it gets from just the current through the relay coil. After that, you'll be in a position to KNOW what's making the things get hot....

If you haven't got a test meter, buy one!! Digital autoranging meters are cheap these days...just be careful on current ranges!

Edited by molemot

Scientists investigate that which already is; Engineers create that which has never been." - Albert Einstein

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Great advice John but now I'm getting confused again.

I checked the bulbs. They are 55/60W.

The Voltage across a bulb is 9.9V.

The Voltages across the 3 points on the relay are between 10.4 - 10.6.

I'm taking the power direct from the starter solenoid to a fusebox and then onto the relays which are also fused. The cables aren't getting hot, just the relays.

Pete


Pete '79 S2

LEW Miss September 2009

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Any other ideas guys. Getting a bit bored being limited to daylight driving!

Pete


Pete '79 S2

LEW Miss September 2009

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Is there a fuse on the feed if so how big is it?

If not add a 25A fuse, if anything goes wrong it'll pop.

Relays should not get hot, the only bits carrying current are the contacts which should have no resistance.

Heat will come from the coils which should not be drawing much current. If they are then it implies a problem with the relays, they aren't 6v relays are they?


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The safest thing is to separate the circuits. 1 relay for each headlight operation - LH Low, LH High, RH Low and RH High. It's a bit overkill I know but the lights and cooling fans draw the most and are best protected the most. Alternatively just upgrade the relays you have to 40amp or 50amp. I have 50 amp relays on each of my 3 fans protected by 15 amp fuses. If you used the fuse panel we discussed the power feeds to the relays are already protected so your are in no danger of doing anything wrong. As Rich says, the fuse will just go pop.

Gavin


Cheers, Gavin

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Thanks for the ideas guys!

Cable running from starter solenoid is 42A into the junction box (same as yours Gavin) which has a 30A fuse. The cables from fuse box to relays are 25A and both 12V relays have 30A fuses (that came with them).

Pete


Pete '79 S2

LEW Miss September 2009

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It's the coil circuit I'd be concerned about rather than the heavy power cables.


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Pete,

I would change the wires from the fuse panel to the relays to 30amp. Your wire should be an equal amp rate for what the circuit is fused to, 30amp fuse = 30amp wire. If your circuit is pulling 30amps and you only have a 25amp wire, the wire will warm up. If it was me I would upgrade to 40amp wiring from the fuse panel to 40amp relays. Leave the fuses in the panel at 30 amps, which I think is the max per branch on that panel anyway, then you know your are well and truly covered and protected.

Gavin

Edited by GavinS1

Cheers, Gavin

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OK. To save messing about with the wiring again, I'll stick a 25A fuse selection in and get hold of some 40A relays. Anyone know a supplier for the relays as I've not noticed 40A ones from the supplier I've used before? Do they need to be fused?

Thanks for the advice,

Pete

Edited by peteyg

Pete '79 S2

LEW Miss September 2009

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Still don't see why a relay would get hot from over-rated rated fuses or wiring... The only reason for a hot relay is a fault in the relay or one that is incorrectly rated on the coil side.

A 6V relay on 12V for example, or a cheap relay that can't cope with continuous operation. There is nothing else to get hot in a simple relay.

Agreed your fuses should be lower rated than the wiring, that way the fuse melts before the wire, but this will not address the problem.

Also be aware that the fuse rating is the minimum amp rating, it will sustain significantly higher currents for short periods without failure.

Rich


Lotus Esprit S4 - Work in progress

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Great advice John but now I'm getting confused again.

I checked the bulbs. They are 55/60W.

The Voltage across a bulb is 9.9V.

The Voltages across the 3 points on the relay are between 10.4 - 10.6.

I'm taking the power direct from the starter solenoid to a fusebox and then onto the relays which are also fused. The cables aren't getting hot, just the relays.

Pete

If you have such voltages across the bulbs and the relay, you have significant volts drop - unwanted resistance - somewhere in the circuit. Not wanting to teach Granny eggsucking, but are you sure you have the right sort of relay and the correct wires connected to the right relay connections? Find the connections to the relay coil and ensure you don't have something weird going on, like feeding the lights through the coil....this might give you the volts drop you have plus the relay heating. Trace out the wiring, check the wiring diagram, measure volts at the relay connections and the lamps, draw out a circuit diagram of what you have...then you ought to be able to see what's going on.


Scientists investigate that which already is; Engineers create that which has never been." - Albert Einstein

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Good point. If you have the wiring switched over then putting all that current through the coil will heat it up pretty damn quick!


Lotus Esprit S4 - Work in progress

Porsche 924 Turbo - Parts chaser

Smart Roadster Coupe - Hers

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Thanks guys. I'll try and do a diagram when I get a moment.

Pete


Pete '79 S2

LEW Miss September 2009

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Hope someone can decipher this. Was more interesting in blowing things up in chemistry than making the light bulb come on in physics!

There were 2 disconnected wires on one side, one blue/red & one blue/white which is why there are only 3 lamps on each side in the diagram.

I’ve tried to wire 4 dipped beam through one relay and 4 main through the other. It works but I still don’t understand why the relays are heating up.

Pete

LotusLightWiring.jpg


Pete '79 S2

LEW Miss September 2009

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Leftmost BR switches on the other 2 BRs

Centreal BW switches on the other 2 BWs

Nothing should get hot there.

How hot are they getting? Are they too hot to touch?


Lotus Esprit S4 - Work in progress

Porsche 924 Turbo - Parts chaser

Smart Roadster Coupe - Hers

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They get too hot to hold after a couple of mins.

Not got hold of higher amp relays yet.

Pete


Pete '79 S2

LEW Miss September 2009

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