The AM Forum

I would like to use some 28V 40mA lamps and run them on 20V, at which they use 33mA and are bright enough.

These are wanted as pilot lamps because they are on hand and also of the highest quality manufacture.

The sockets include a pushbutton and I would like to use that as a lamp test feature.

I see no easy way with a regular hookup using a 3K resistor to drop 120V down to 20V for the lamps because using the switch would seem to back-feed 120VAC to the monitored circuit, even if it is only a few mA I don't want that, but if I use a rectifier and size the filter cap correctly it seems like it should work because the RMS current is the same, and indeed the RMS volts dropped across the resistor is 100V, which is what is wanted.

This would let me use diodes to isolate the voltage to be watched from the 'always on' source of voltage for testing the lamps.

Does it seem reasonable? Is the waveform going to be an issue? after all a sine wave has the same peak size per RMS volts.

Otherwise a possibility might be to run 20VAC to the pushbuttons and to the lamp direct but that is another transformer.