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DC Power control device

 A variably conductive pressurized tube for controlling dc current
  (-2)
(-2) 		  [vote for,
against]

I haven't heard of anything like this but it's simple enough that it seems like if it could be done easily it would have already... The idea is to control electric power efficiently by having a tube filled with a conductive gas such as mercury vapor used in fluorescent lighting, that can be varied in pressure producing different resistances, which could be used as the transmission in an electric car, etc. Has this been done before?
—tabarin, May 10 2004

Explanation of PWM motor control http://www.4qdtec.com/pwm-01.html
A much better solution [scad mientist, Oct 04 2004]

[link]






       how would this be efficient? you're still controlling current via variable resistance, thereby turning excess power into heat.
—Freefall, May 10 2004
   

       My thought was that the efficiency would be high because the resistance would be caused by the vacuum, therefor not losing the unswitched power as heat like a variable resistor
—tabarin, May 10 2004
   

       I like the automated switching solutions. While your idea has merit, [tabarin], on the one hand I think this is not an appropriate use of pressure switches for your current source, and on the other hand it seems unresponsive and inefficient.
—dpsyplc, May 10 2004
   

       Perhaps if you use a.c. then the arc will only re-strike when the appropriate voltage is reached, thereby producing a PWM type result.
—Ling, May 12 2004
   

       Of course that would just be phase angle control, and that is probably easier to do with triacs.
—brewer, May 12 2004
   

       Argh ... Brain hurts ...must ....find... childish and immature... halfbakery ideas.
—PainOCommonSense, May 12 2004
   

       Allen Bradley was selling "Compression Rheostats" back in 1903 to do this. Used plates of carbon instead of gas though, and was used to control electric crane motors.
—ServoMan314, Jul 07 2008
   

       //the resistance would be caused by the vacuum, therefor not losing the unswitched power as heat//   

       V = I * R   

       and   

       P = V * I   

       therefore either   

       P = V^2 / R   

       or   

       P = I^2 * R   

       Due to conservation of energy, the lost power has to go somewhere, and due to the laws of thermodynamics, it ends up appearing as heat at some point.   

       PWM systems avoid this by switching the resistance rapidly on and off. Either R is very small, in which case P is very small (I^2 * R) or R is very large, making P very small (V^2 / R). PWM systems only get warm during the very, very brief moment that the resistance is changing - this is what makes them efficient.
—Wrongfellow, Jul 07 2008
   

       My vote is for PWM. "They" solved the efficient DC motor control issue ages ago with PWM. It got even better with the advent of MOSFET's - which also make it compact.   

       Also pretty good for dimming DC lights <LED only>. Get the frequency above a few kHz and it's perfect.
—Custardguts, Jul 07 2008
   
      
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