Say is it possible to simply connect a light dimmer to a rectifier bridege to output a certain DC level voltage? It is to drive a 100~150Watt DC motor.

 

Light dimmers are designed to drive purely resistive loads (incandescent lamps). A DC motor is a reactive load (inductive). You'd likely fry the dimmer pretty quickly.

 

I think we had this question a few days ago...must have been a different load.

I ask, would a large filter capacitor on the output of the bridge rectifier save the dimmer from the harmful effects of the inductive reactance? If so, can the OP can procede with his experiment?

 

A light dimmer is normally AC line operated; playing with its output will run afoul of AAC rules-no?

 

Let's get some more input from the OP. What might that "certain DC level" be?

A small DC motor can be controlled by other means.

 

Bernard's right - a dimmer won't provide isolation from mains power.

On this board, the mains supply must be galvanically isolated from the project power supply using inductive coupling (ie: a transformer). Otherwise, it cannot be made safe, and life/property are at risk.

 

Agreed - the dimmer is out, but what is the motor and the degree of control desired.

 

If you're trying to "dim" the mains for something like that, go with a variac.
http://en.wikipedia.org/wiki/Autotransformer

But were not supposed to talk about those here.

 

Unfortunately, variacs do not provide galvanic isolation either. They are quite lethal unless handled competently.

 

Thanks for the clarification. So the main reason this cannot be implemented is because of the following:

1. Type of load must be RESISTIVE? A DC motor would be reactive
Suggestion: Why not add a cap then? And perhaps a Diode on the bridge output.

2. It isn't a very small motor. It's a 150W~200W by estimate. Voltage level might vary from 12V to 60V. The motor has no nameplate so I might just add a fuse for safety. I don't also feel like having it rewinded since I find it very troublesome to uninstall. I estimate the maximum current to be 2A judging by its wires.

3. PWM is out of the question since I tried this. The motor just vibrates. It doesn't like the signal chopped or something. Pure 36VDC make the motor run ok, 36VDC chopped it doesn't like, 5Khz. It also makes funny sounds.

4. Isolation? You mean the mains from the output of the DC rectifier? I could isolate it with an isolation-transformer.

5. A Variac would be nice. Unfortunately I have other plans for my variac. Plus I want to limit my output voltage @ 60V and not go above.

6. With the triac I think it is possible to limit it to 50~55VAC max.

The output must be variable from 12V~60V -> Linear


I saw some DC servo drives use the same principle of SCR. The firing angles are just a bit delayed to give the desired output DC level. I don't see why this can't be done for Triacs as well.

 

Well, you could start off using a transformer that would cut the 120VAC mains down to 60VAC.

Using just a cap on the output side of the dimmer would result in very high current for a very short period of time. If there were an inductor (it would need to be a large value inductor) between the output side of the dimmer and the cap, that might work - however, you might still wind up with reverse-EMF spikes when the TRIAC in the dimmer turned off at the zero crossing; I'm not certain how that would work at the moment.

 

Well, you could start off using a transformer that would cut the 120VAC mains down to 60VAC.

Using just a cap on the output side of the dimmer would result in very high current for a very short period of time. If there were an inductor (it would need to be a large value inductor) between the output side of the dimmer and the cap, that might work - however, you might still wind up with reverse-EMF spikes when the TRIAC in the dimmer turned off at the zero crossing; I'm not certain how that would work at the moment.

Stepdown trafo from 220VAC to 60VAC is also what I had in mind. Then Triac going to bridge. Output from Bridge has diode, cap and inductor. Exactly like you describe I saw on a DC servo board.

How about a diode on the output? That would solve the reverse EMF spikes won't it?

I think a fast recov or regular diode oughta be sufficient for back emf. (When back voltage occurs, diode will short)

What I need is a variable DC power supply from 12V -60V @ ~5A. This is the only approach I can see next to variac + rectifier bridge. Though triac approach seem more elegant and lighter in weight.

 

The dimmer would have a very weird response when driving a bridge rectifier, as these things only take current for a small slice of time very near the peak of the AC wave.

So at least the first half of the dimmer's pot (100 to 50%) would do absolutely nothing. Any control would be pushed into the last half.

The cap should isolate the dimmer from anything untoward the motor generates. It should all work, provided a sweet spot is found.

My only objection about direct to line is it violates the TOS of this board, as we would like to keep the new guys safe and sound.

After all, if this was an AC motor no one would expect it to be isolated from the AC line.

 

Unfortunately, variacs do not provide galvanic isolation either. They are quite lethal unless handled competently.

That's why I said we can't talk about them, and linked to wiki. I'll mention it next time.

My only objection about direct to line is it violates the TOS of this board, as we would like to keep the new guys safe and sound.

After all, if this was an AC motor no one would expect it to be isolated from the AC line.

All hail the TOS

 

What I need is a variable DC power supply from 12V -60V @ ~5A. This is the only approach I can see next to variac + rectifier bridge. Though triac approach seem more elegant and lighter in weight.

That was my impression -- why don't you just buy one? Or, get a low-cost switching power supply and rig up a MOSFET to control the current to the motor. For about $100 US you could get two of these which, I'd assume, could be put in series to get 96 V at 5 A.

Getting a triac-controlled light dimmer to behave as a DC supply over the voltage and power levels you're talking about would take one hell of a capacitor for filtering...

 

That was my impression -- why don't you just buy one? Or, get a low-cost switching power supply and rig up a MOSFET to control the current to the motor. For about $100 US you could get two of these which, I'd assume, could be put in series to get 96 V at 5 A.

Getting a triac-controlled light dimmer to behave as a DC supply over the voltage and power levels you're talking about would take one hell of a capacitor for filtering...

It's what I'm talking about. The motor doesn't like PWM. I tried PWM with different standard frequencies and the ones you simply in hardware stores. The motor runs but does erratic vibrations.. Though when I couple multiple power supplies I get a stable torque and no vibrations. The problems is.. This motor would be nice if it were stepless.

It's original power supply is a 30kG power supply that just went to smoke. The board exploded. A lot of components damaged and caps. The thing was 35 years old.

I was thinking now with the advent of Triac and Thyristors the unit could be made smaller and lighter.

My last resort would be to use my Variac.

 

I have a light dimmer that I use to vary the speed on an electric leaf blower (universal motor). I use it as a forced draft blower for my burn barrel when burning tree trimmings and lawn waste. Been using it for years.