Neat Trick

[fido]

Someone just posted a good idea on the Velo LE forum so I thought I would pass it on. Some older bikes have to be modified to add a brake light as this was a retrospective change to the legal requirements. The chap on the LE site has suggested a way to keep within the law whilst still only having a single filament bulb in the tail lamp. He changed the bulb for a higher wattage brake light type but fed it from a diode which reduces its brightness. The brake light switch is connected directly to the bulb without the diode so when the brake is applied the light comes on at full brightness. 

[LJ.]

Brilliant idea! I'd imagine that it would be a resister rather than a diode though? and of course a single 21 watt filament bulb would be much cheaper than a twin one. Is anyone able to work out what rate resister/colour code would be needed to bring down voltage to around 5 watt if we were using a 6 volt system?

[fido]

A resistor would need to be quite a high wattage one so I would guess a rectifier diode with a suitable forward internal resistance was chosen.  In practice this will involve trial & error. At 6 volts a brake light bulb has a resistance of about 2 ohms whereas a tail lamp bulb has a resistance of about 9 ohms. It won't be as simple as using a 7 ohm resistor but it probably won't need to be any more than 7 ohms.

[Dynamo Regulators Mike]

I would estimate you need about a 2 ohm resistor to drop the power to about 6W. Dissipation of resistor is also about 6W. Simplistic application of Ohm's law does not work here as filament resistance is not at constant temperature, and by a long way. Colour temp of light will also vary being much redder.
A series diode prevents speedo light coming on when brake applied but would be a useful warning light if you could see it in daylight, so perhaps not.
These bulbs are very cheap, the fitting may not be.
If going a custom route here I would fit an LED replacement, using nice efficient devices with broad angle illumination. Not easy to find in an LED bulb on the market AFIK.