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Posted by Sherman in Idaho [72.47.9.37] on Friday, August 31, 2012 at 02:54:44 :
In Reply to: Re: Fuel gauge Reg, $6. posted by Dave [75.81.249.81] on Thursday, August 30, 2012 at 21:53:43 :
In answer to the reduction in heat, no it won't do much to "steady" the current. The resistance of the wire goes up as it gets hotter. More resistance means less current, which means less voltage at the motor, which means more voltage across the resistor, which means more power dissipated in the resistor, except the current has gone down so it might mean less total power, depending on the voltage/current characteristics of the motor and its load... so the bottom line is "not a heck of a lot of difference either way". It might make the regulation a little better or it might make it a little worse.
This a fairly crude circuit you're building. The fluctuation in motor current based on the mechanical load on the motor as it goes though its pumping cycle, and as the vacuum gets stronger or weaker, will totally overwhelm any minor effects due to the temperature coefficient of the resistor's wire. The real question is will such a crude voltage dropper work at all. With electric wiper motors, a simple series resistor doesn't work, because the load fluctuates too much. The vacuum pump might be just as bad, or it might be almost as smooth as a heater fan, in which case your resistors will be fine.
I'll be curious to hear how well it works.
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