Hello Peter,

A simple way to do it is to place a diode in series with
each light at the top of the mast. Both lights (with their
diodes in series) are connected across the two wires leading
down the mast. One diode will have its anode toward the
black wire and the other will have its anode toward the
white wire.

Simply reversing the polarity at the base of the mast will
indeed allow you to power either light. The diode must have
a current rating of more than two amperes for a 25 watt
light. (amperes = watts/volts) Since the diodes are cheap, I
would find one with an 8 or 10 ampere rating.

One caution: you will drop about 0.8 volts across the diode
when it is conducting. That will produce a noticeable but
not dramatic effect (if you are looking for it) on an
incandescent lamp, but may have no noticeable effect on a
strobe light.

Good luck!

Chuck

Peter Hendra wrote:
Hi,
I remember seeing somewhere that you could use a diode on each wire of
a pair so that two independent lights could be used by reversing the
positive and negative leads.

I would like to put a white strobe light on top of my mast but as the
mast is wooden box section in construction I cannot run another wire.
I do not wish to run external wiring.

Would someone be so kind as to tell me how and what size diodes to buy
or how to calculate it. My masthead bulb is 25 watt from memory.

Thanks
Peter hendra
NZ yacht Herodotus