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ac to dc Help

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New Member
My son and I are building a small pump and I would like to use a car wiper motor. I understand how to convert ac to dc using a rectifier and compaciter but my problem is the wiper motor is 12vdc but at 30amps. I need some help converting the ac to dc but getting the required amps.

Yes I am a newbie.


New Member
Heya Hollywood!

I have a few questions for you:

1. Are you using a transformer for your AC to DC supply?
2. Are the wiper motor values that you provided, the max values. For instance, the wiper motor may state 12VDC 30 Amp max. Or are the values that you provided, what the motor absolutely needs to turn. For instance, the motor absolutely needs 12VDC and 30amps to rotate.

A possibility is to build yourself a little variable power supply, using a transformer. You can probably build a power supply to deliver 12VDC and a range of amperage (adjustable) from 30 amps and lower. This way, you can control the RPM's of the motor, by feeding it an adjustable amperage. You can start with a transformer that will deliver 12VDC at 30 amps, then add in circuity to enable you to adjust the amperage, and thus be able to control the RPM's of the motor.

Here is a page, with power supply circuits that may fit your needs.
look under power supply.

Good luck!


New Member
It needs 12vdc at 30amps to run. I am assuming I can use a light dimmer switch wired in before converting to dc as speed control. (adjusting the speed works with a dimmer switch using a rectifier and capaciter on a 90vcd motor).
rated current

There is no way that the wiper motor is going to draw 30 amps, ever. That would represent about 360 watts- a huge amount of power, given that we are talking about a simple wiper motor. I am guessing that the amperage is actually '3.0' amps, not '30' amps.

Any 12 to 14 volt DC power supply that can put out 2 or 3 amps should do the job nicely. Probably best to err on the side of caution, and go with a three amp supply.

A simple power supply for this can be built from:

- 110 to 12VAC transformer, rated for 3 or 4 A;
- single diode to half-wave recify the transformer output, rated 50V and 10A. the higer current rating is for momentary surge current.

Connect the primary side of the transformer to 110AC ( a fuse is a good isea in line with one of the wires).

Connect one wire of the motor to one wire of the secondary winding. Connnect the other wire of the secondary to the remaining motor wire, with the diode placed inline.

You don't need a capacitor if you are just ruinning the motor- we're not talking Mars Rover here-, as the motor can handle the fluctuating DC.

Your local library will have acopy of 'The ARRL Handbook', which has diagrams and excellent explanations for building all kinds of power supplies.

Hope this helps
dimmer switch

Just saw your dimmer switch post- watch out!

Those are designed for incandescent or resistive loads only. Will not work for long or properly, or at all on any kind of inductive ( i.e. coil type) load. The dimmer switches work by chopping the AC sine wave fed to the load, and indcutive loads like motors start to beahve very strangely and dangerously when this happens. Mostly they throw back a lot of power to the dimmer and fry the Dimmer's SCR or triac.

If you want variable speed control your best bet may be to just get a variable bench supply and use that for speed control, as proper speed control via PWM involves lots of tasty and expensive electronic components.


New Member
Dingo said:
If you want variable speed control your best bet may be to just get a variable bench supply and use that for speed control, as proper speed control via PWM involves lots of tasty and expensive electronic components.
Hi, I just asked this in a different thread. Do you have any examples of these tasty designs and components?
Yes, Pwm is nice and tasty, and can be a headache sometimes depending on the size of load driven. Large loads mean larger driver electronics.

Basic PWM can be done on the Basic stamp or PIC, then sent to driver electronics to amplify the signal. If you are driving a small motor, you could use the L298 chip for forward/backward and speed control. They have it here, with the manual for download:

If you use that board, you need to replace the diodes with 'fast recovery' diodes, i.e schottky rectifiers.

Good luck.
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