Steve
Maas
Long Beach, California, USA
January, 2007
My 1960 Austin Healey Sprite has extra electrical components: a radiator fan and an electric fuel pump. As time goes on, I may add other things, as well. The pump and fan together draw about 8 amps, putting an extra load on the ignition switch, which was not designed to cut a lot of extra DC current. Still, these components obviously need to be switched; you don't want the fan or fuel pump to be powered when the ignition is off. So, I decided to add a separate circuit for them, with a relay to handle the switching.
The figure below shows the circuit, and the next figure shows the installation. It's all pretty simple. The "switched" circuit is controlled by the ignition switch; the "unswitched" circuit is the part connected directly to the battery (via the regulator, of course), so it's always powered. The relay is a Panasonic automotive relay from Digi-Key. It has a 12V coil and 40-amp, normally open contacts. It powers a barrier strip, which allows connections of any new components that I might want to install. The relay coil is 100 ohms, so the relay draws only 120 mA. The current for the additional components now comes directly from the battery, switched by the relay.
With this arrangement, each additional component must have its own fuse. I don't think that's a terribly bad thing--the two-fuse setup in the stock Sprite isn't very well conceived, in any case. Many parts of the electrical system, such as the headlights, are not fused at all (eventually I'll probably fix that!), and the fused, "always-on" part of the system powers only the horn. This doesn't make a lot of sense. I hope that these modifications may represent the beginning of an improved electrical design for my Sprite.
I left the stock arrangement for powering switched parts of the circuit unmodified. It would become an additional problem, requiring extra fusing and making sure all the wiring sizing would still be OK, and I saw no need to change it. The only addition to this part of the circuit is my electronic ignition, which probably does not take much additional current.
I don't like saying this, but I suppose it's necessary, now that the lawyers have taken over American society. If you choose to do this, or something similar, but don't know enough about automobiles or electronics to be comfortable with it, get some help. In any case, I'm not forcing anyone to make this modification, so if you choose to attempt it, you take full responsibility for the results. This is just a report on my experience with these modifications. It is not intended to be a set of instructions for duplicating my work or a recommendation to do it. You're on your own.