On 2/18/2016 4:49 PM, Don Y wrote:

You can also design something to sense power being APPLIED
(from the contacts) and engage a relay coil letting the
relay contacts engage the heater load. E.g.,

C
O + --+---||----- to relay coil
N |
T |
A +---||----- to motor circuit
C
T - -------------- to relay coil and motor circuit
S


I should have explained this, just in case... :

The role of the diodes is to prevent power from the batteries
ON THE "MOTOR CIRCUIT" from energizing the relay when there
is NO power on the "contacts". The second diode blocks
the current from flowing right to left (battery being connected
on the right side of it) and into the relay coil -- which would
turn the relay on all the time (as long as the batteries had power!).
We only want the relay to turn on when power is available at "contacts".

[The first diode *could* be omitted but there are some cases where
it can be of use]