The bender used a 450mm length of curtain wire expanded to 500mm. Reducing the length
of the curtain wire but still expanding it to the same length will surprisingly increase
the power dissipated. This can be proven using Ohms Law.
If a smaller bender is all that is required the wire maybe too short to limit the
current sufficiently and the wire may become too hot or the power supply overloaded
unless a lower voltage supply is available. If a lower voltage supply is not available
a dropper resistance can be made using the same curtain wire. Using two lengths in
parallel will avoid the wire reaching such a high temperature. Even so, it may well
get hot enough to cause a burn so having established the dropper required by trial
and error, ensure that the final assembly is adequately protected with a cover.
As an alternative to the battery charger as a power source a car battery could be
used, this would certainly be capable of much higher current output. In this respect
I did find that when applying my 12V charger to the full length heater that the time
it took to make a bend was rather long. To overcome this when bending shorter lengths
I used a crocodile clip and made one connection part way along the heater, as a result
increasing the current and therefore the resulting temperature. The situation is
as can be seen therefore rather more complex than would first seem to be the case.
Trial and error are as a result the order of the day!
There is no reason why the power source has to be dc and if a suitable transformer
is available, (probably around 150va say 15 amps and 10 volts) then this could be
used. The transformer must be double wound, DO NOT USE AN AUTO TRANSFORMER OR A VARIAC.
A variac connected onto the output from a double wound transformer would though be
an ideal way of adjusting the voltage applied to the heater. This of course would
increase the cost appreciably.
Viewers who have electrical resistance wire available around 0.8mm to 1.2mm may be
tempted to try this with a single strand. Whilst this will work it will require a
low volt high current source, typically 2 volts at 50 amps. This because its resistance
will be much lower than the coiled heater. Do not forget though that you will need
a spring at one end to take up the changes in length as the wire heats and cools.
Winding it into a spring form will though be necessary to achieve sufficient resistance
to work on a higher voltage lower current power source.