I have a car windshield wiper motor that I plan to use in a Halloween prop. My power supply is a 12v DC battery charger. Can anyone think of a cheap way for me to slow down the speed of the motor when connected to the power supply? Would a potentiometer or ceiling fan dimmer switch work?

 

A dimmer switch won't work, as those are meant for AC. A potentiometer would work, but you would need a pretty big one, in terms of power capacity. Those are hard to find (and expensive) these days.

What you want is a low resistance, high wattage power resistor (or a selection of them). Probably the best place to get that would be from wherever you got the wiper motor (scrap yard?). I haven't ripped one of those out of a car, but I would imagine that they use resistors for variable speed (although some newer ones might be electronic).

The other part of a car I would look in is for two and three speed radiator fan motors. Those use resistors as well. Automotive HVAC fan resistors are common, but they are designed to be cooled by forced air (in the duct), so they might burn out too easily.

I will warn you to be very careful with battery chargers. A lot of them are of very poor quality and catch fires quite easily. Also, some of them sense battery charge level and automatically adjust their output, so it might give you a bit of a problem.

 

Like Mr Griffin has said, a power resistor in series with the motor should work, its not variable but if you can calculate what you want you don't need it to be. It’s probably good to start with something that should

Another way is to use a 120V <-> 12V transformer with bridge rectifier on the output, and drive the transformer with a Variac or light dimmer control. I've never done this but it should work. You could probably scavenge these parts or make a trip to Radio Shack, or online surplus stores. Just make sure your transformer is rated for enough current to drive the wiper motor with a safety factor.

KEJR

 

The light dimmer or variac plus transformer plus bridge rectifier combination that Ken Emmons Jr. mentioned should work and it eliminates the need for the battery charger and power resistors. His post reminded me that I have seen this done before to drive 12VDC motors.

You should be able to get all these parts through an industrial electrical distributor or else some place like Electrosonic.

Make sure the transformer is over-sized by enough to handle the motor starting current (this will be several times the full load current). If the transformer is too small, the motor may not have enough starting torque for whatever your load is.

If you are wiring this up, make sure that you know what you are doing and that you don't electrocute yourself or anyone else. If you don't know what you are doing when it comes to wiring, get someone else who does to put it together for you.

 

Yea from what i remember (which isn't much). the way those light dimmers work is there a little pot that changes the resistance of the circuit that charges a cap to gate a SRC by varying the charge time you can change the amplitude of the AC Sine wave. but will only work with AC cause you have to have that zero cross to open the SRC

 

In the early days of SCRs we used to make our own speed controllers. I couldn't remember the circuit but it was very simple just an RC setup.
I found this really comprehensive link

http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF

If you look on page 98 you will see a DC motor powered from AC. Page 127 shows how to use DC motor with pulse width modulation.

Roy