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Posted by David Sherman [24.32.202.83] on Thursday, January 28, 2010 at 19:11:58 :
In Reply to: Re: Amp draw on Elec Wipers posted by Ron in San Mateo [75.18.201.86] on Thursday, January 28, 2010 at 18:24:20 :
Sorry, I keep forgetting to put my whole first name in so people don't think I'm Dennis. I think your approach would be good. If you have an ana.log amp meter to put in series with the whole contraption, or even a volt-meter to put across the motor, you'll see the voltage/current fluctuate quite a lot as the wipers wipe. Grab hold of the arm to simulate pushing snow off and you'll see the drain go up even more.
That's why I think the best approaches are either to get technical about it and build an actual voltage regulator that puts out a constant voltage (maybe 7 volts) regardless of load, or just try various resistors until you find one that seems to make it work about right. 1.5 ohms is in the right range to start with. You can put two in parallel if one by itself drops too much, or put two in series if a single one doesn't drop the voltage enough. Too much voltage will definitely burn out a wiper motor, so once you get it going, let it run a while and see if the motor is getting hot, or measure the voltage across it and make sure it's not more than 8 volts or so.
No matter what resistance you use in series with the motor, the bottom line is that with resistance in series with the motor, its speed regulation will suffer, compared to hooking it straight to a constant voltage source (battery). It will speed up more under light loads and and bog down more under heavy loads. In my experience, a simple resistor of the right value is good enough, and is simple and reliable, but it's not as good as having a motor that matches the battery voltage.
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