Thanks for the correction, Adrian. I knew I'd seen them somewhere.
The extra power may present some concern if you do a lot of driving in heavy
stop-and-go traffic at night, or make only short trips at night and still
have the stock Lucas generator. Especially when combined with other loads
(brake lights, wipers, heater), the generator won't keep up at low engine
rpm and you may end your drive with a battery that is somewhat low. But it
shouldn't be a problem as long as your next drive includes enough faster
driving to recharge the battery. When I had problems, I was working 2nd
shift and so drove in the dark both ways (in winter) plus out to dinner and
back, 5 days a week.
But the wiring and so on is heavy enough, no worries about burning it up.
That was with 55 watt halogens on the stock TR3A wiring (never did get
around to adding relays to that car, although the 60 amp alternator was a
marvelous improvement).
The main reason for adding relays IMO would be to get the maximum voltage to
the bulbs, for the maximum light output. I assume that more light is your
reason for wanting halogen bulbs (I heartily approve BTW), so might as well
make the most of them. The stock wiring, switch contacts, etc. will lose
0.5 volts or more (I measured 0.7 on my TR3) while with some care you should
be able to get down to 0.1 with relays and direct wiring. Not a huge
difference, but enough to see in a side-by-side comparison. I also rather
like the idea of reducing the load on the original switches.
After some thought (and bad experiences), I wound up with 4 relays, one for
each filament, mounted to the top of the headlight buckets. On a TR3 that
makes them totally invisible but still fairly accessible (by removing the
bucket from the apron). It also lets me run a separate self-resetting
circuit breaker for each side so that I still have light if one breaker (or
one relay) fails for some reason. Modern plastic relays are cheap and I'd
gladly pay $20 to avoid having all the headlights go out at once again.
More than a few years ago, I was driving down a mountain at midnight and
rather more than the posted speed limit when a fuse failed and killed both
headlights. Only took a moment to find the dimmer switch, but still not an
experience I look forward to repeating!
In case you are wondering, I eventually traced the fuse failure to nearly
invisible corrosion on the fuse holder contacts. Under prolonged operation
at fairly high current (one fuse for both 90 watt bulbs) the holder would
eventually get hot enough to melt the solder inside the fuse cap. The fuse
element was still visibly fine, but you could see where the solder had run
away from the connection. Of course I didn't figure that out until after it
had happened again, fortunately under more benign circumstances. Took a
Scotch-Brite (mild abrasive) pad to the contacts and never had the problem
again.
Randall
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