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Posted by D. Sherman [24.32.202.83] on Thursday, January 28, 2010 at 17:16:19 :
In Reply to: Amp draw on Elec Wipers posted by Ron in San Mateo [75.18.201.86] on Thursday, January 28, 2010 at 16:35:43 :
The amount of current that wipers draw varies all over the place throughout the wiping cycle and depending on the conditions (temperature, friction, snow, etc). Somebody might tell you they draw 10 amps, just to pick a random number, but that's probably their peak draw and most of the time they only draw a couple amps. The best I've ever been able to do, regardless of rated voltages, type of motor, battery voltage, etc is try various high-power resistors, or a big rheostat of 10 ohms or so and futz with it until I get what seems to be satisfactory operation. They'll work much better if you build an actual voltage regulator, rather than just a dropping resistor, and then it's easy to incorporate a speed control as well. I did that on one of the sno-cats, with the aluminum dash as the heat sink, and it works fine.
The reason a constant-voltage supply is better than a "dropping resistor" setup is that with a constant voltage supply, like the battery it was designed to run on, the motor can automatically draw more current and increase its torque when it starts to bog down. With just a dropping resistor, when it tries to do that, the voltage drops as the current goes up and so it doesn't pick up any extra torque and it just bogs down more.
Be aware that no matter what value resistance you end up with, the "voltage reducer" is going to dissipate as much power as the wipers themselves use, which means you need something that can dissipate a fair amount of wattage. I don't think much of those ceramic "voltage reducer" units that are really just a couple of resistors. Because they're just resistors, the amount of voltage that comes out of them varies all over the place with load. That's fine if the load is small and constant, like a fuel gauge, or even large and constant like a heater blower, but it's not good if the current varies as much as it does with windshield wipers. If building a true voltage regulator is beyond what you want to do, I've found that an ignition ballast resistor, which is around 1 ohm, is pretty close to being about right for making a 6 volt wipers more or less work on 12 volt, or for making a "slow speed" position for 12 volt wipers. You can try it and measure the voltage at the wiper motor to see what you're getting. If the motor doesn't get hot to the touch after running a while, and if it goes fast enough to wipe the windshield, you might just call it good.
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