13-09-2014, 08:44 AM
(12-09-2014, 08:10 PM)Old Sparky Wrote: Having fitted a 1N4007, with a 100 ohm surge limiting resistor in series, the no load HT voltage is 325v, measured with my DMM, and with 240vac from the mains supply. I therefore need to reduce the HT to somewhere around 200-220v, which, if I've calculated correctly, will need a 5watt 1800 or 2200 ohm resistor, the stated HT current being around 45mA according to the service sheet. No doubt a capacitor dropper would be better, or, of course a new rectifier valve, if one is available anywhere!
That R value looks a bit high to me, are you confusing peak (no load) with RMS ?
Subtract the working HT voltage from the nominal AC mains supply voltage, divide that by the working current to get R, the internal resistance of the silicon diode can be ignored, the resultant R should be around what Alan Beckett has suggested.
Having said that you need to make sure the res. capacitor is up to it, you can always incorporate a NTC R into the series limiting if you want a "soft start"
Lawrence.
