[thx1138]

Gentlemen,
I have the new hobbycnc board that's rated for 47vdc max. I also have a old spectralight psu that puts out 48vdc.
Is there a simple way to reduce the output of the spectralight psu to about 43 vdc?

[Torchhead]

If it's regulated there may be a pot on the control card to set output. If it's unregulated or the ferroresonate type then you will have to either reduce the input AC voltage or deal with reducing the volts in the DC section. Regulated supplies with active components (pass transistors) will have a voltage divider across the output to feedback a portion of the voltage. Sometimes ther eis a pot in the chain but if not you might can replace one of the resistors with a pot of the same or higher value to trim the setting. It's hard to say without seeing the unit or a schematic.

Since you just need to shed 5 volts (might want to make it 7 or 8 in case the line voltage goes up) and the total current is not too high there are several alternatives but we need to know a little more about the supply.

[NC Cams]

I'd try to get a shematic for the Spectralight PSU. Once you get it, scan and post it and the electronics whiz's here can probably tell/show you what to change.

As Torchead says, there are pass element controls that can probably be tweaked to drop the voltage as you want.
HOWEVER, in doing so, the power has to go someplace and that could result in the pass elements running hotter.
I ran into this problem when I dropped the output of a 5v p/s to 3.5 to drive some small Christmas decoration lights.

Thus, you may open yourself up to other issues but it shouldn't be too bad to go from 47 to 43vdc.

[Al_The_Man]

A cheap try is to use three rectifiers in series on the output, This way is fairly dynamic in that the volt drop should be fairly constant with load.
Al.

[NC Cams]

"Rectifiers" are also known in some circles as "diodes".

Just make sure you use ones that are big enough to carry the current that will be drawn.

Depending on type of diode and current draw, you can figure on about a 0.75volt per diode voltage drop for each one you pass thru.