beginner: wiggle room on the rated coil current of a relay

Please could someone advise me on a semi-electro-tech issue? I don't know how much wiggle room I get, for the rated coil current of a relay.

See the attached sketch of a small project in the planning stage.

My plan is to use a common-or-garden thermostat to switch a 12VDC circuit on and off. The spec for the thermostat says it will take max 30 volts, DC or AC. It's usually powered by 24VAC. I'm not sure if it's important, but the thermostat "cycles" at 1.5 degrees Fahrenheit.

For Re1, the relay will only be used to switch from 12VDC, for the thermostat, to 115V (I'm in the US), for a motor. I'm thinking of using a relay called G5Q-1A-DC12. Its rated coil current is 16.7 mA; its rated coil resistance is 720 ohms.

The distance between the thermostat and the relay will be 50 feet (15 meters).

I believe I need 12/.0167 = 720 ohms resistance. So, three (and a half) questions.

Do I need to take the resistance of 100 feet of 20 AWG (30 meters of 0.5 mm) copper wire into consideration?
Or use something like an LM217 as a current regulator?
Or would a simple 750 ohm half-watt resistor suffice?

Any other obvious suggestions?

Thanks for your help.

Most inexpensive home wall thermostats run off of at pair of AA batteries, they use a latching relay to save power and they will work fine with your 12V devices.

If you can buy it from somewhere with a return policy if it's not right, open it up and read the relay specs, it may handle a much higher voltage & current (but watch out for the snubbers on the relay)

You don't need any added resistance. The "rated coil resistance" is it's actual resistance. Thus 12V across the coil will give you the desired 16.7mA.

The resistance of 100 feet 20AWG wire is only about 2 ohms total (both wires round trip) so is of no consequence.

Are the thermostat contacts rated to carry 16.7mA? I assume so, but it's good to check.

Most home thermostat relays should handle at least 1-2A @ 24VAC

crutschow said:

You don't need any added resistance. The "rated coil resistance" is it's actual resistance. Thus 12V across the coil will give you the desired 16.7mA.

The resistance of 100 feet 20AWG wire is only about 2 ohms total (both wires round trip) so is of no consequence.

Are the thermostat contacts rated to carry 16.7mA? I assume so, but it's good to check.

Click to expand...

Thank you. That is exactly the information I was missing. If I present 12V to the relay, the relay will take care of everything else.

Confirming what a later contributor said, the thermostat I'm using can carry 1.25 A.

Wow! You make this easy. Thanks again.