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  #111  
Old 02-17-2012, 09:59 AM
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Originally Posted by subzali View Post
Can someone school me on relays for belt-driven compressors? Marco gave me some input yesterday, but still not convinced I need a relay...
Relay for what, the pressure switch or clutch or something? I would use one just to avoid a sensor switch fusing closed. Ergo, with a relay you can wire important actuators as fail safe and reduce current on the primary switch, prolonging its life.
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  #112  
Old 02-17-2012, 10:09 AM
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Well here's what I was originally thinking: +12V --> 5A fuse --> dash switch --> pressure switch (rated for 30A) --> compressor clutch. Does that clutch really draw enough current to warrant a relay? Does the clutch draw more current if it has to engage/hold against higher backpressure (higher load)? Or is it just on/off?
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  #113  
Old 02-17-2012, 10:18 AM
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The clutch is just a coil, basically big version of a solenoid. It takes a certain amount of current at a minimum threshold voltage to hold the clutch engaged and it won't matter the load as such, it's either on or off.

If the voltage drops below the threshold the clutch will disengage regardless of the current. Your circuit could easily have a couple volts of drop, where as a relay right at the clutch could be a volt or less.

Think about it, a fuse + dash switch + 10 feet of wire + pressure switch, that could be 0.5 ohm, which at 5A is 2.5V. You'd have to use heavy wire to keep the drops low and it still might be 0.25ohm at 5A, which is 1.25V. That's before your contacts corrode from the FJ40 being oh so weather tight.

If you use that control circuit to switch the coil of a relay, there might be 250mA flowing, so maybe 0.125V of drop. You could wire that with cheap-o 18AWG and route it where ever the heck you want. Then a 3 foot length of 14AWG from the battery and you're done.
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  #114  
Old 02-17-2012, 10:19 AM
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I was thinking since the pressure switch is rated for 30A, just use a nice industrial like toggle switch and avoid more wiring than neccesary.

Nice thing about a relay though is that you can tie it all to a circuit that is normally on and off with the ignition.. then you avoid killing your battery by accidentally leaving it on.

A compressor clutch will draw a battery down to absolutely nothing if you leave it turned on ..
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  #115  
Old 02-17-2012, 10:25 AM
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Ideally my dash switch would get power only when ignition is "ON" - still have to work that angle though. I think I'll give it a whirl without a relay for now and see how it does. The compressor turns on and off now, so I know I'm not currently getting too much voltage drop, but that's a good point Dave that I'll keep in mind.
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  #116  
Old 02-17-2012, 10:33 AM
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Originally Posted by DaveInDenver View Post
Why the reluctance to use a relay? :-/
I'm not a sparky. I'm scared. This is how I deal with electrical problems:

Actually I'll give it a little more thought today. So if I use a relay, I need fused power for the switch side of things (in case there's a short at some point) and fused power for the clutch? So 2 separate fused circuits? I've never used a relay before.
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  #117  
Old 02-17-2012, 10:37 AM
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Quote:
Originally Posted by subzali View Post
I'm not a sparky. I'm scared. This is how I deal with electrical problems:

Actually I'll give it a little more thought today. So if I use a relay, I need fused power for the switch side of things (in case there's a short at some point) and fused power for the clutch? So 2 separate fused circuits? I've never used a relay before.
Not necessarily, with a relay you can tap just about any existing hot or ignition-on circuit for the control side. It's very low current. You hang a fuse from the battery to the relay high side, put the relay right by the battery and a decent wire to the clutch.

This don't take a rocket doctor to figure out, trust me you can do it. IMVVHO doing it this way helps avoid electrical gremlins. The fewer devices in the high current path the better. OTOH, if it works the way you have it, then not bothering is a valid way. But remember that OEMs use an A/C clutch relay for a reason.
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  #118  
Old 02-17-2012, 11:13 AM
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If yo uare going ot use a dash circuit, you should use a relay......
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  #119  
Old 02-17-2012, 11:29 AM
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Originally Posted by rover67 View Post
If yo uare going ot use a dash circuit, you should use a relay......
Or........you know you will need it anyway.....go ahead and put in a accesory fuse box now! That way your rock lights, air comp, fridge and LED array are taken care of!



:evilgrin:
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  #120  
Old 02-17-2012, 11:32 AM
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Or........you know you will need it anyway.....go ahead and put in a accesory fuse box now! That way your rock lights, air comp, fridge and LED array are taken care of!



:evilgrin:
that's actually exactly what I did in my 40. it has one bank that is switched with the ignition... the other is hot all the time.

I use one of the switched circuits for my heated seats
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