Full current switch vs relay?

[dman93]

This isn't exactly a power question but i figured I'd get some good input from the electrical gurus here. I have a portable ARB compressor (single) which I'm finally going to mount onboard, under the hood. I'm going to run tankless (slow, I know) and so will only use it when stopped, hood open. Since wiring runs will be short with the unit mounted near the battery, is there any reason not to keep it simple, and just use a 40A switch inline with the leads to the battery, instead of a relay? I see what look like decent quality toggle switches rated for this current @ 12v for around $20 US, and a sealed welder on/off switch rated at 40A @ 600V for just a bit more. ARB spec is 32A max under load and startup draw may be briefly higher, perhaps, but a 40 switch seems sufficient, especially if I'm careful to turn it on before loading it. Thoughts?

 

[Ducky's Dad]

Keep it simple, forget the relay. I'm doing something similar with an ARB single on my '08 Tundra. Planning to fab a mount to hold the compressor to the core supports in front of the radiator, then wire directly to the battery with heavy cable and a Blue Sea Systems rotary switch. Total cable run will be less than three feet. My ARb is the "portable", so it already has its own switch, but I'm more comfortable using the Blue Sea switch inline so that I have a fail-safe disconnect between the battery and the compressor.

My Dodge has a big Warn compressor mounted the same way, same rotary switch. Been in there for six or seven years with no problems. Warn recommended the positive disconnect, even though their compressor also has its own switch.

 

[plh]

Personally I do the relay to keep the high current wires as short as possible.

 

[thethePete]

Yes. Relay, always.

Limit the distance your high current wires are running, you can get nicer switches, and your big power isn't running through as many bulkheads on its way to the compressor. It's not that much extra work to add a relay and it's the proper way to do it.

 

[Ducky's Dad]

He's putting the compressor in the engine compartment. There will be no bulkheads to go through. Sounds like he plans to switch it on/off from under the hood. The compressor has its own power switch and the ARB cables can be ring-terminaled right to the battery. Hell, mine will be within about 18 inches of the battery. Relay is a waste of time and money and wire and space.

 

[thethePete]

I missed the part where the switch was staying in the engine bay. In that case, yeah, just get some big ugly 70A toggle and rock it.

 

[dman93]

He's putting the compressor in the engine compartment. There will be no bulkheads to go through. Sounds like he plans to switch it on/off from under the hood. The compressor has its own power switch and the ARB cables can be ring-terminaled right to the battery. Hell, mine will be within about 18 inches of the battery. Relay is a waste of time and money and wire and space.

Exactly. However, I do have a follow-on switch question. I've seen some 40-50A toggle switches that are quite inexpensive, but there are also rotary battery disconnect switches that have very high current ratings. Is this rating just for the steady-state load after it's switched, or can the contacts in this type of switch actually handle high current when switching under load (say 30A) without arcing etc? Specifically I'm looking at a Blue Sea which is rated at 300A continuous, 500A intermittent at 48V. By the way, my portable ARB has no on/off switch, just the pressure switch, and turns on as soon as you connect the alligator clips to 12V.

 

[Ducky's Dad]

Is this rating just for the steady-state load after it's switched, or can the contacts in this type of switch actually handle high current when switching under load (say 30A) without arcing etc? Specifically I'm looking at a Blue Sea which is rated at 300A continuous, 500A intermittent at 48V. By the way, my portable ARB has no on/off switch, just the pressure switch, and turns on as soon as you connect the alligator clips to 12V.

Your ARB portable is different than mine, which is less than a year old. In your case, just get a Blue Sea rotary switch and put it inline wherever it's convenient. If your compressor is not fused by ARB, get an appropriate inline fuse holder, too. Blue Sea tech support can confirm switching under load, but I think these are all non-arcing for marine use. I switch mine sometimes without any issues.

 

[dman93]

Thanks everyone. I'm a mechanical engineer whose electrical skills are pretty much limited to understanding Ohm's Law. But I also believe in the KISS principle. Worst thing that could happen is the switch contacts overheat and weld themselves closed then I'll just pull the fuse.

 

[Ducky's Dad]

I suggest you fit an explosive disconnect (much like some BMWs ) to your battery connection.

And always carry a titanium spork.

 

[Semi-Hex]

And always carry a titanium spork.

With one edge sharpened.

 

[AaronK]

Problem is here at Expo, If one thing is made very simple, Something else must be overly complex.
( I know its stupid, But rules are rules...)
I suggest you fit an explosive disconnect (much like some BMWs ) to your battery connection.

I'd go a step farther and build a rocket propelled ejection system for the compressor

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