A plain resistor doesn't work very well


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Posted by D Sherman [72.47.9.228] on Friday, January 06, 2012 at 16:12:32 :

In Reply to: Considering adding a Resistor to Drop Voltate posted by Will (in IL) [98.215.26.138] on Friday, January 06, 2012 at 09:29:43 :

I've tried that method. The trouble is that the current the motor draws varies hugely throughout the arc of wiper motion and depending on if there's any snow on the window. The motor is wound so as to try to give a constant speed with a constant voltage supply. When it needs more torque it tries to draw a lot more current. If you have a resistor sized to keep it at the right speed with a light load, as soon as there's much load, it drops the voltage too much and the wiper stalls. Simple dropping resistors are fine for fans and blowers where the load is steady and predictable, but not for when the load varies a lot. They'll sort of maybe work some of the time, but in my experience, not well enough. Better to switch to 12V motors or come up with some sort of stiff voltage regulator.

If I was designing one from scratch, I'd make it a simple chopper that produces a 12 volt square wave at 50% duty cycle. The motor wouldn't care, so long a it averages out to 6 volts. It would be nice to use a 3-terminal linear regulator like a 7806, but they're only good for 1 amp. There are higher-power versions, but you still need a heat sink to cool them and the circuitry gets more elaborate. I did use a 3-terminal regulator to make a true variable-voltage (not variable-resistance) speed control for the single wiper motor on one of my sno-cats. It works very well and maintains its steady set speed unless there's so much snow that it just can't move it even at full voltage. I used an LM317 and simply bolted it to the back of the aluminum dashboard for a heat sink. To control it I used a radio-type knob that pulls out to turn it on and then twists to adjust the speed.



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