Relays for Dipping Headlights

[Colin]

I am putting two relays in to control my main and dipped beam headlights. I can understand how to wire up the coils of the relays to the light switch an dip switch and earth but I am not sure where the power for the lights will come from. I am thinking that I will need to take it from one of the A connections on the controller (voltage regulator) Am I correct and if so which A connection should I use?
Thanks, as always, for any help...Colin.

[luli]

You can see the modified / non-modified circuits here: http://wp.me/pXLKy-1xp
Note the wires coulor code.

[chris dancey]

Hi Colin,

I would recommend running the cable from A4 on the VR . A4 is the fused accessory terminal that is controlled by the ignition switch. One could also use A2 which is current fed via the ammeter, and fused, but would mean that it would remain a live feed.

I like the idea of running a separate fused circuit to each wired accessory, this can make problem solving much easier, also if one accessory shorts, then one does not lose all the accessories on the same fused circuit branch.

Chris

[luli]

Running the high lights through the ignition switch is not necessarily a good idea – unless this switch is supported by a relay. The reason is that these old switches have relatively high resistance – anything between 0.1 to 0.3 ohm. The combined lights current (back, side, high) is ~ 7.5 Amperes. That means that about 0.75 to 2.25 volts will be dropped on the switch. Since the light intensity is proportional to the square of the voltage on the bulbs – this means a reduction of 15 – 40 percent in the light intensity!
Moreover, some 5.5 – 16.5 watts will be developed on the copper contacts inside the switch, which will gradually be oxidized and get into a spiral destruction.
The solution is to use a main switch relay, that removes the load from the switch – this can be seen in the wiring diagrams I had published in the link above.

[chris dancey]

I do advocate fitting relays to ALL switched circuits, not only those controlled by the ignition switch. In my experience of driving pre-war vehicles for over 50 years ( not only Rovers ) I have found that on the whole vintage lighting was pretty basic at best, and that whether wiring through the ignition switch or otherwise made little difference to the amount of light emitted. I studied and understand your comments on switch resistance, I studied electronics to degree level, but on physical trial I found little, if any, difference to the luminance when wired either way.

[Colin]

Thanks both for your help and advise. I'll let you know how I get on...