On 22 Mar 2008 09:06:21 GMT, (Andrew
Gabriel) wrote:

A common design uses a low voltage DC motor, in series with the
element. The motor therefore requires a bridge rectifier.
A common design fault with this scheme is the use of a 50V bridge
rectifier on the basis that the motor is only something like 10-20V.
Motor commutators are not perfect, and there will be momentary
periods when it's open circuit, resulting in 240VAC across the
bridge, which can blow it. I've replace 3 such bridge rectifiers
over the last ~20 years, and always replace with mains ones which
then don't blow again.

Speed/power control is done either by having a tap on the element,
or by using a diode to half wave rectify (or some combination of
both). You can't just have a cold fan in these designs though, as
the heater is required to drop the voltage for the fan.

Thanks for the informative reply. I did see diodes so they could be
for either of the purposes you describe but I didn't have a proper
study to find out more.

Thanks, Stephen.