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Old 11-30-2012, 09:16 AM   #1
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Electric current draw,

Got a question for all you wizz minds;; When I plug my electric heater in(or light bulb) and I turn the reostat. to a lower setting Say medium; And the current Splits and the reostat sees 1/2 the current the heater sees 1/2 on the current. To lower the setting/heatoutput.. Now Just how much does the Electrical meter see what is the Applied power ? Used power? Actual Working Power?? One would/could consider a reostar/fixed resistor as a perasite draw... Lets keep it simple and civel. And don't worrie about the grammerma.
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Old 11-30-2012, 09:53 AM   #2
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Well, when I have mine on high, my coach amp meter says 14amp draw. Medium about 10 and low about 7. The electric meter would see and record the same.
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Old 11-30-2012, 10:28 AM   #3
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One easy way to tell exactly what the current usage is to use a VOM/ammeter and if it is an appliance use a Kill A Watt. A rheostat is a variable resistor and the way it reduces voltage as waste heat. You can use PMW pulse width modulation to as example reduce the speed of a fan that actually reduces the current load with the speed.
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Old 12-01-2012, 09:01 AM   #4
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That all makes good scence;But I was looking at from the point of view Of the reastat; dropping an equal amount of voltage/current As to reduce the amount of current available at the End ;; which could be a heater Or whatever... In Elect. engineering class, We were thought that altho you have some eddy current. As a general rule currnt when supplyed at the power plant, it must be used, it can not be stored, Just food for thought..a 60 W. bulb is Dimmed by means of a resistor/restat to 30 output Does that mean the resistor/reastat used up 30 watts+ 30 in the LB' so the Input would see a 60 W. 1/2 amp draw.. Please do not use this to over tax the mind..
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Old 12-01-2012, 09:37 AM   #5
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A reo does take some power itself just like converters or inverters do in processing power, but its small.
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Old 12-01-2012, 09:59 AM   #6
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In the simplest case, everything you learned in EE is correct. A potentiometer is a voltage divider and the circuit drawing looks like this: In this case a significant amount of the voltage is dropped across R1 so the potentiometer does dissipate a sizable amount of energy.

However, having said all that, I seriously doubt if that many of today's devices use simple potentiometers. Even though my knowledge of electronics is a bit dated, I would assume that some form of gated device would chop the AC waveform limiting current flow by controlling effective voltage without having to dissipate power like a potentiometer would. Essentially, the gate chops the waveform so the circuit is "off" during the higher voltage portion of the sine wave; since the effective voltage is lower the device produces less heat. The "truncated" AC wave has and effective voltage determined by the RMS (root mean square) voltage and the power use (heating) can be calculated from the RMS voltage).

That's a lot more than you probably wanted to know, but it was fun dusting off my EE stuff again.
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Old 12-01-2012, 10:20 AM   #7
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Quote:
Originally Posted by docj View Post
SNIP

However, having said all that, I seriously doubt if that many of today's devices use simple potentiometers. Even though my knowledge of electronics is a bit dated, I would assume that some form of gated device would chop the AC waveform limiting current flow by controlling effective voltage without having to dissipate power like a potentiometer would. Essentially, the gate chops the waveform so the circuit is "off" during the higher voltage portion of the sine wave; since the effective voltage is lower the device produces less heat. The "truncated" AC wave has and effective voltage determined by the RMS (root mean square) voltage and the power use (heating) can be calculated from the RMS voltage).

That's a lot more than you probably wanted to know, but it was fun dusting off my EE stuff again.
You are right. It has been many years since 120 vac light bulbs have been dimmed with a rheostat. Triacs are used instead and since they are not resistive devices there is little loss and little heat generated.

Switched resistors are still used in some auto fan speed control applications though. My Workhorse W20 for example.
I think even there most use PWM circuits. My 1999 Honda uses that type of system.
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