Coils were originally CB for ground and SW for ignition i.e. negative and
positive respectively, these have to be reversed if changing the polarity of
the car. Later coils with - and + markings are self-explanatory.
There is no point putting a diode bridge between the voltage supply and the
sockets, you would still have to decide which way round you want the polarity
i.e. the output of the bridge connected to the sockets, you might just as well
connect the 12v and ground that way round directly. And aren't the sockets
red and black any way i.e. +ve and -ve? To do anything else would be
confusing. In order to allow a device to be connected to either polarity the
diode bridge has to be with the *device*.
----- Original Message -----
There is a polarity for coils. Originally, when pos earth, the CB
terminal of coil went to Contact Breaker (points) terminal. Replacement
coils are usually + and -.
... you can make up an
electrical bridge of diodes to use in the little dash panel sockets that
will ensure that the polarity is correct for the device you want to power
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