variable 100-110vdc power supply from 110v mains

Thread Starter


Joined May 17, 2011
alright, I have a DC gearmotor, rated 90v coil (maximum rating 130v, goes faster with 110 rather than 90, but don't want to push it close to 130) with a 0.60amp requirement. at the time I got it, I read a schematic that called for line voltage, a rectifier (yanked from old PC power supply) two smoothing caps (also from PC power supply) and a dimmer switch before it all to vary the power. well, I think it was a stupid idea to do it that way. it works, but only after I turn the dimmer up a bit. I know, safety first and isolation transformers, but it is built into the grounded PC power supply case. but I can't in my right mind allow this to go this way because I feel its unsafe and unstable.

I need to take 110v house current, and if possible find a (1:1?) isolation transformer, rectify to DC, smooth out with more reliable or better suited caps, regulate the power to something cleaner than the jagged power I am getting now (sometimes when I start it up the motor bucks, and on lower power it moves, stops, moves, stops, doing that until I increase power). I would also like to trade the dimmer switch for something a bit cleaner if possible. for the life of me cant find a good schematic and or parts to do so.

any good advice short of buying a commercial motor control or power supply?


Joined Feb 11, 2008
Harbour Freight in USA have a complete assembled "variable speed motor controller" for about $39.95 if I remember right. You could connect a 400v 15A bridge rectifier on the output of that.

It's going to be difficult to get anything like low speed power from that motor without a regulated voltage DC supply, or even a closed loop motor speed controller in the worst case.

What is the load? What does the motor drive?


Joined Jul 26, 2010
and a dimmer switch before it all to vary the power

Dimmers work by chopping AC waveforms, I don't think they'd work on something that was rectified and filtered. I suppose you could use it before the rectifier portion of the circuit but remember this is going to create all sorts of switching harmonics on the line so you'd want to use some ferrites on the input and output lines of the dimmer.

Ideally you'd get a 0 - 140 VAC Variac but those can be very expensive.


Joined Apr 20, 2004
One way out, if that 600 ma figure is good, is to grab two 36 volt transformers and use the secondaries in series for a voltage boost. That will give close to 100 VDC after rectification and filtering. PWM (many references on the site) is then the btter speed control.

Mouser has Hammond transformers - 36 volt @ 820 ma - for around $24.