|
|
Posted by Sherman in Idaho [72.47.9.37] on Thursday, August 30, 2012 at 15:57:57 :
In Reply to: Re: Fuel gauge Reg, $6. posted by Dave [75.81.249.81] on Thursday, August 30, 2012 at 13:14:21 :
Hard to say ezactly what it draws and how steady the draw, but if it was mine, I would just experiment with power resistors in the range of an ohm or so until it seemed to work right and didn't get hot. 6 amps at 6 volts is one ohm, which happens to be the same as a typical ignition ballast resistor, so I'd start there. 6 amps at 6 volts is also 36 watts, which is how much power the motor is using but it's also how much heat the dropping resistor has to dissipate. That's a significant amount of heat.
The trouble with using a simple dropping resistor is that they have poor voltage regulation and they dissipate a lot of heat. The good thing about using a simple dropping resistor is they are about impossible to destroy, unlike fancy electronic gizmos that will likely fail if somebody disconnects the battery while the engine is running, or hooks jumper cables up backward. You can also make your own power resistors by taking the coil out of a burned-out electric heater, figuring out which length gives you the resistance you need (use alligator clips or jumper cables), and then cut out that amount of heater coil and mount it in some way that it can get hot without hurting anything.
With something like what you're doing, I'm a big believer in experimenting until it seems to work pretty well, rather than attempting to calculate things precisely and building something elaborate. My first step would be to try a ballast resistor. If it pumps vacuum okay, and if a voltmeter across the motor shows something in the 5 to 8 volt range, and the ballast resistor doesn't go up in smoke, I'd call it good. If the voltage is way too high, try two ballast resistors in series. If way too low, try two in parallel. One of those combinations will be close enough for government work.
Follow Ups: