Dynamo Lights

That's my learned thing for the day. I can go to bed now :)

See also: http://blog.dutchbikebits.com/2012/01/selecting-and-installing-dynamo.html

Q: Why can't you use just a dynamo rear light without a front light ?
A: The simple answer is that a rear dynamo light will always be damaged if used without a front light. The explanation of why is quite long and technical, but you'll find it below:

It is perfectly safe to use any of our dynamo front lights with our dynamos without also connecting a rear light. This will not damage the headlight. However, you must never use a dynamo to power a rear light without a headlight also being connected as this is quite likely to cause damage due to driving the rear light with too high a voltage. The reason why is as follows:

Sources of electrical power can approximate either a constant voltage or a constant current.

Constant voltage: A battery is an example of a constant voltage source, and it will attempt to supply the same voltage whatever load is attached. If you short-circuit a battery, then the battery will attempt to discharge at an infinite current and this is why lithium batteries, which are capable of very high currents, can cause fires.

Constant current: A bicycle dynamo is an example of a constant current source. Bicycle dynamos are rated at 6 V 3 W but what this really means is that they are current sources which attempt to deliver half an amp across whatever load is connected. The expected load of a rear and front light together is the equivalent of 12 ohms. A dynamo tries to supply sufficient voltage always to deliver the same current. With a short circuit, the dynamo will still deliver about 0.5 A at zero volts, but with an open circuit the dynamo will produce so high a voltage as it can in an attempt to deliver 0.5 amps.

You don't need to know any maths to safely connect dynamo lights. Simply pick a front and (optional) rear light, and wire them in parallel to a dynamo. However, if you want to know why you should never use a rear light on its own, read on:

The 12 ohm equivalent resistance for front and rear lights together comes from the front and rear lights being connected in parallel. The front light has an equivalent resistance of 15 ohms while the rear light is the equivalent of 60 ohms. To combine the two we use 1 / (1 / 15 + 1 / 60) = 12. Now that we know the equivalent resistance of each light we can calculate the current and power for each if used on their own.

For a front light alone, V=IR, V=0.5 * 15 = 7.5 V. P = IV, P = 0.5 * 7.5 = 3.75 W. This is a little over the rated value of 2.4 W, sufficient to damage older front lights with bulbs unless the bulb is changed to 3 W. However, modern LED based front lights can cope.

For a rear light, V=IR, V=0.5 * 60 = 30 V. P = IV, P = 0.5 * 30 = 15 W. This is a long way over the rated values of 0.6 W and 6 V and this is why dynamo rear lights do not last long if connected on their own to a dynamo. The front light is required to keep the voltage down to the correct level for the rear light.