17-08-2021, 12:01 PM
For Amie's case of a diode dropper the output is half cycles of AC, either above or below 0V, depending on diode polarity. If you connect an ordianary "true rms" DVM across the heater what will it see. The coupling capacitor will put the zero line part way up the waveform and the meter will measure the resulting AC. I think this will be the correct answer but I'm not entirely sure.
If I can be bothered I'll try it with my Fluke 87.
A while ago I needed to measure the CRT heater voltage in a modified video monitor. It was sitting on DC and either 15625Hz or 10125Hz. With a spiky form factor as it was derived from line scan. My Fluke 87 certainly wouldn't give me a reliable answer at those frequencies and that form factor. In the end somebody (Mike Hennessey?) suggested I use my Rigol 1054Z scope which can compute the RMS voltage of a waveform. Until then I didn't know it could do that. I don't remember whether I floated the monitor or the scope. I did it the nice way by using an isolating transformer, not by lifting the earth.
If I can be bothered I'll try it with my Fluke 87.
A while ago I needed to measure the CRT heater voltage in a modified video monitor. It was sitting on DC and either 15625Hz or 10125Hz. With a spiky form factor as it was derived from line scan. My Fluke 87 certainly wouldn't give me a reliable answer at those frequencies and that form factor. In the end somebody (Mike Hennessey?) suggested I use my Rigol 1054Z scope which can compute the RMS voltage of a waveform. Until then I didn't know it could do that. I don't remember whether I floated the monitor or the scope. I did it the nice way by using an isolating transformer, not by lifting the earth.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
