30-07-2015, 07:46 PM
Without doing the sums my instincts are against using a pot like this. It's simple and that's good. However there's going to be a lot of heat which may or may not matter. What's really worrying me is that the output voltage is going to be very dependent on the load from the valve filament. The only safe way to use a pot is to start at zero and wind it up until the measured filament/heater voltage is correct. This is made more difficult by the fact that a cold filament has a much lower resistance and will take more current until it is warm.
Alan Beckett's warning about part of the track is valid. It's simpler to think about the current rating of the track which doesn't change. For example a 20 ohm 50 watt pot will carry about 1.6 amps. If that current is passed through the whole winding the dissipation will indeed be 50W. Pass it through half the winding and 25W will be disspated. As Alan said, what you can't do is dissipate 50W in half the winding.
Incidentally Alan's warning applies equally to variacs. A 2A variac will supply 2A safely. Whether that's at 200V or 2V. At 200V it's supplying 400W, at 2V just 4W.
Alan Beckett's warning about part of the track is valid. It's simpler to think about the current rating of the track which doesn't change. For example a 20 ohm 50 watt pot will carry about 1.6 amps. If that current is passed through the whole winding the dissipation will indeed be 50W. Pass it through half the winding and 25W will be disspated. As Alan said, what you can't do is dissipate 50W in half the winding.
Incidentally Alan's warning applies equally to variacs. A 2A variac will supply 2A safely. Whether that's at 200V or 2V. At 200V it's supplying 400W, at 2V just 4W.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
