I have a piece of historic equipment that HAS to have an incandescent bulb.* The preferred-voltage bulb (64V) is no longer available, so I'm forced to use 32V bulbs. This one wants 8.4 amps at full brightness/spec, and about 4 to 5 amps at "dim" setting.

This is giving me two big problems. The second one I'll cover right now, which is that the power supply (nominally 75V) is sagging as low as 60V, and that's dimming the light. I'm hoping to eliminate that problem.

Originally, this used a dropping resistor to produce bright and dim brightness, which made sense for a 64V bulb and 75V supply. But we had to re-factor the resistor to make it work on 32V, and it's positively enormous, is overloading the supply, and you can cook eggs on the panel :)

So I want to install the original/historic resistor, and behind it conceal an electronic switching power supply of some kind. I'm hoping the bucking nature of a switching power supply will take care of the voltage dip problem.

However, the LED revolution has had a very distorting influence on the supply chain. Given my oddball voltages (on both sides), the most procurable way** I've found to do this is actually using constant-current power supplies. And I'm super comfortable with driving an incandescent light constant-current.

What's readily available in appropriate input voltages and output currents (and voltages) is a 4-amp CC power supply (driver).

  • 4 amps is very workable for the "dim" setting.
  • 8 amps is acceptable for the "bright" setting - it's 9% below target brightness, but that's better than the voltage dips are doing already.

So my hope is to have one such supply/driver be turned on at "dim" setting, and both of them turned on at "bright" setting. This will mean paralleling two CC supplies/drivers.

Is this a very foolish idea, or does this make sense? If we were in a constant-voltage world, I'd be skittish, but these supplies will inherently current-limit. What could go wrong?


* So this is not an XY problem in that sense, I'm not asking "how do I get over my fear of LED" - LED is off the table.

** This is also not a "Shop-for-me" question; I'm not looking for someone to locate a 75 to 35V 300W constant-voltage power supply, though I wouldn't mind lol... I just haven't found any at non-nosebleed prices.


1 Answer 1


I would guess it should work okay. Paralleling constant current supplies is analogous to putting constant voltage supplies in series.

I hope you've considered that putting a constant current into a positive temperature coefficient load could conceivably run away. It's probably okay, but..


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