In-Reply-To: Nigel Goodwin said: > I would be interested to see how your maths works in practice, many > years ago a particular range of Ferguson B/W TV's used a capacitor to > feed the heater chain (rather than the usual huge ballast resistor). > Being an inquisitive type I worked out the reactance of the capacitor at > the 50Hz mains frequency - it was nowhere near the required value to set > the current at the required 300mA for the valve (tube for our USA > readers!) heaters. I've always presumed there was more to it than simply > the reactance of the capacitor, but have never really understood why?. The reactance of the capacitor is 90 degrees out of phase with the reactance of the heater chain so they don't simply add together when placed in series. The total reactance of the Heaters in series with the capacitor will be square_root of ( Heater_resistance squared plus capacitor_reactance squared ) Hence to reduce the current the reactance of the capacitor has to be higher than you expected when you thought they simply added. Brian Gregory. briang@cix.co.uk