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Electronic rheostat

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technogeek

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Basically, what I need is a way to turn a cheap potentiometer into a large wattage rheostat. The difficulty here is that there is no separate power supply. It must take power from the circuit. (e.g. as if it were a rheostat with 2 pins)

The "rheostat" needs to be able to handle 20 watts, and vary from about 0-20 ohms.

One requirement is that there can't be much drift. If it is set at something, it should stay there +/- 15% at most. Even with changes in temperature in any active elements.

Suggestions on how to proceed?
 
I would not want to pretend to know the best solution, but this sounds like you could use a power BJT in the linear region.

If you connect the gate to the power trough a resistor in series with your pot, then it should act nearly as a larger pot with a bit of a dead zone at the bottom. Well, and also only work for on current direction.

**broken link removed**

Please wait for someone more knowledgeable than me to confirm though, as I am still slightly unconfortable with BJTs.
As a side remark you will never get a perfect 0 with any transistor.
 
SuperFrog posted his while I was working on mine.

All plotted vs resistance of R1 (the pot)

Blue=Dissipation in the Pot (i.e 1W Pot)
Lt Blue=Effective resistance of the network
Purple=Dissipation in R3
Green=Voltage across the network
Red=Dissipation in the 3055
 

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Thanks!

I'd like to get the potentiometer into the 250mw range or less though. Can I use a darlington to decrease the base current?

And how stable are BJT's with respect to temperature? If I set this circuit to "10 ohms" when it's room temp (25C) and everything heats up to 80C, how will that effect the circuit?
 
And how stable are BJT's with respect to temperature? If I set this circuit to "10 ohms" when it's room temp (25C) and everything heats up to 80C, how will that effect the circuit?
The gain of a BJT varies significantly with temperature (typically >30% from 25C to 125C), thus you would need a large heat-sink to minimize the transistor temperature change, for MikeMl's circuit to work within your limits.
 
I would have thought a power MOSFET, voltage controlled at the gate by the bias of the potentiometer and a series resistor, current limited at the D-S, would be a good solution.

Do you guys care to expand on why the BJT is better? Pros, cons.
 
The gain of a BJT varies significantly with temperature (typically >30% from 25C to 125C), thus you would need a large heat-sink to minimize the transistor temperature change, for MikeMl's circuit to work within your limits.

I thought so. Same problem with mosfets I believe...

Other than an opamp with a separate supply, can this problem even be solved?
 
The main problem with a MOSFET is that they typical take several volts to start turning on. That means your resistor wouldn't start conducting until the voltage reached at least the MOSFET gate threshold voltage.

A bipolar transistor only takes about 0.65V to start turning on so it would start conducting at a much lower voltage than a MOSFET.
 
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If you are just dimming a 12V lamp, then a 2 wire PWM dimmer can be built like this:
**broken link removed**
Not a very good image but you can find a better one if that is what you are after. What exactly are you trying to "control" with the rheostat?
 
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