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# How do I know what size breaker to use for 12 volt 5 amp/hour SLA ? Answered

I found a 3 amp snap in circuit breaker.  It says 3A-32V.  Does that mean 3 amps up to 32 volts or 3 amps at only 32 volts? The guy I bought it from wasn't sure.  He believed the volts could be anywhere between 1 and 32  and would flip the breaker at 3 amps anywhere in that voltage range.  In other words, 2 volts at 3 amps flips it, 12 volts 3 amps flips it , 24 volts 3 amps flips it, etc.  Is that how breakers work?

Sorry about the backwardness.  I took the pic with my mac.

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To test use ohms law...

V=IR
test voltage of battery
12(ish) = 4Amps * x
12/ 4 = 3 Ohms

Short it out against a 3 ohm resistor and you'll be sure to draw 4 amps
12 volts times 4 Amps = 48 watts, so use a beefy resistor, be sure to do it for only a short time and/or expect it to be a sacrificial resistor...see if that pops it.

If it works the same as every fuse I've seen it senses heat with a bimetal strip, and that would be wattage not just pure amperage.
If it senses current electronically then sure, 3 amps is 3 amps.

Those are maximum figures. It's made for low voltages and relatively large currents.

Your application is probably not as 32 V, so you can compute the power 3A * 32V = 96 Watts. Plug in the voltage and current of your project to figure out your power requirements. If you get something less than 96 W, you can use this breaker.

BTW, if there's a motor or other large inductance in the circuit, you might be in trouble even if you're under the maximum power limit.

well the lower the voltage the higher the amperage unless your using a tranformer so I'd recomend asking a local electronic or home improvement store because three amps is enough to fry you to a crisp

Yea it's the 3A, voltage is irrelevant.

L

3 Amps is 3 Amps.
Its UP to 32V
Steve

That's a good question, In a traditional fuse it was assumed the fuse was for a certain voltage so that amperage going up would heat it sufficiently to fail safely. I guess that's the reason they're not rated in watts.

I hooked it up and it does trip when I short the battery. That's good because I was mainly worried about wreaking the battery. On the other hand, it does not trip when I run something which I know draws 4 amps. I will need to do more testing.

It won't trip for several minutes on a 25% overload, neither will a fuse.
A CB or fuse does NOT limit currrent remember - shorting a battery with it WILL upset the battery if you do it too much.

Steve

To test use ohms law...

V=IR
test voltage of battery
12(ish) = 4Amps * x
12/ 4 = 3 Ohms

Short it out against a 3 ohm resistor and you'll be sure to draw 4 amps
12 volts times 4 Amps = 48 watts, so use a beefy resistor, be sure to do it for only a short time and/or expect it to be a sacrificial resistor...see if that pops it.

If it works the same as every fuse I've seen it senses heat with a bimetal strip, and that would be wattage not just pure amperage.
If it senses current electronically then sure, 3 amps is 3 amps.