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About Lamps and Relays When is a spot lamp not a spot lamp? When the distance between the ground and the bottom of the illuminated area of the lamp is less than 500 millimetres - in which case it is classed as a FOG LAMP - regardless of what it said on the box! So, if you mounted your lamps too low, they may be used ONLY in fog, or falling snow.

Is it permitted to switch headlamps on and off alternately? I don't think so. The Road Vehicles Lighting Regulations 1971 specifically excluded this (paragraph 19. (2) and I suspect any Statutory Instrument (legal document) which superseded this will say the same - possibly with provisions for emergency vehicles to use alternating lamps. However, it *might* not be illegal to flash alternately additional spot lights fitted to the vehicle.

What sort of wire should I use to connect additional lamps? It depends on the bulb rating. Supposing you want to fit lamps which use 55 Watt bulbs. Two bulbs will use 110 Watts of power. If the battery voltage with the engine running is 13.5 volts (typical) then the current required by the bulbs together will be Amps = Watts/Volts = 110/13.5 = 8.15 Amps So the minimum current rating of a wire feeding both lamps should be 10 Amps. Since wire has a small but significant resistance, there will be a tiny voltage drop across it, so using thicker wire (say 20 Amp rated) might make the bulbs a fraction brighter. In any case, the turn-on surge will be about 20 Amps because a bulb filament has a lower resistance when it is cold. Its resistance increases when it becomes white hot and it then takes less current. So, to cope with the turn-on surge while still protecting the cable in the event of a short-circuit, there should be a fuse rated at 20 Amps next to the battery. This will be the feed to the relay needed by the additional lamps.

What's the purpose of a relay? A relay is a high-current switch operated by a low-current magnetic coil. You can use a low-current switch to feed the relay coil. The relay contacts are large and can carry heavy currents. The relay can be mounted close to the battery. This system allows you to control a big current without the need to run a thick, high-current wire to the dashboard switch and back again. Only a thin, insulated wire is needed between the dashboard switch and the relay coil. In addition, if one side of the relay coil is connected to the battery, then the dashboard switch simply needs to connect the other side of the relay coil to chassis in order to complete the circuit. This makes wiring a lot easier since you don't need to find a voltage supply behind the dashboard.

The fuse MUST be close to the battery in order to protect the car from fire in the event of a short-circuit. The tell-tale warning bulb on the dashboard is a legal requirement. It has to be chosen so that it allows enough current to flow to operate the relay. Since it is sharing the battery voltage with the relay, it will be dim. If the bulb fails, the relay can not be turned on. As an alternative to selecting a suitable bulb, you can use a Red, Amber or Green LED, together with three diodes and a resistor. The LED is unlikely to fail and may be easier to fit than a bulb. In addition, it doesn't become hot.

This is simple to make. All you need is an LED, a 100 Ohm resistor and three 1N4001 diodes. Solder them together as shown. Test it. Insulate the wires. Note: the "anvil" shape inside the LED must be the end which is connected to chassis. Note: the diodes have their bands at the left hand ends. Use this combination to replace the bulb in the diagram above. Do not use it for any other purpose unless you understand precisely how it works!

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