[Amps] Switching Power Supplies

[David Lisney]

We had some issues with a UHF PMR base station being "heard" 50KHz away, due to residual ripple from a switch-mode. No time for experiments with it so it just got it's power source shifted to a linear supply.

Undoubtedly the switch-mode supply has much going for it as a power source for heaters, built in soft start (usually) and insensitivity to widely varying line voltage.

Obviously the degree of filtering you will have to do to keep the spurs manageable is up for debate... with respect to mechanical resonances of the filament structure I suspect these would be an issue if the filament was driven from AC at some frequencies, I suspect a little residual ripple of perhaps a few mV superimposed on a dc supply would have minimal effect.

Sadly I have in the past done mechanical resonance experiments on lamp filaments, at the time I worked for GEC Osram, a very conventional strip type lamp suffered an inexplicable short life in many applications. Running one on a vibrating table at swept frequency showed a filament resonance at a low harmonic of our mains frequency, It seems that the filament buzzed itself to death in the earths magnetic field when powered at 50Hz. I suspect that a tube/valve heater being more robust and operated at a much lower temperature could have a mechanical resonance somewhere in the audio spectrum as suggested by another contributor, I doubt it would be up at the typical switch-mode frequency though.