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Re: Where can I get a 12V timer switch?
Quote:
Med, as far as ohms law and conductors an amp is exactly one amp. Ohms law tells you the voltage drop across a resistance due to current. V=I*R where V is the voltage drop. Here is the calculation for voltage drop across a 100' 14awg wire carrying 15A.
14AWG wire is 2.52 ohms per 1000' so 100' would be 0.252 ohms
0.252*15A=3.78V drop
So a 12V run drops to 8.22V
A 120V run drops to 116.2V
See what's happening here? The voltage drop is exactly the same but you just can't tolerate a 4V drop on a 12V bus. On 120V line you just don't care - 116V is just fine. The wire in both cases is dissipating exactly 3.78V*15A=56.7W which it can do because that 56W is being dissipated evenly over 100'.
"Is this correct? Is there a way to convert from the 120V safe rating to a 12V safe amp rating?"
It's up to switch and relay companies to rate their devices at different voltages but I gave you a reference point in the first post and I'm surprised by this 4A number. That said it seems that you're pretty safely into the external relay territory if you're drawing 20A.
This makes sense. The part I'm worried about is the possibility low conductivity parts inside the switch. After all, it's 120V right? Who cares about a little resistance. If the contacts are small and the resistance causes you to dissapate that 56.7W in a 1/8" piece of metal inside the switch you could be seeing some ...
One experiment is to just hook it up and watch it with an IR thermometer.
So forgive my electrical ignorance. How does one go about setting up a relay? (I have a feeling Bjones's egg timer is going to suddenly look more attractive...).
MedSailor
I have a sauna on my boat, therefore I win.
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