Author Topic: Is there such a thing as a variable resistor / transistor combo?  (Read 1040 times)

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Offline Plasmateur

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Is there such a thing as a variable resistor / transistor combo?
« on: December 01, 2021, 09:26:27 am »
I'm looking to vary the resistance in a 10 Watt circuit.

Everything on Mouser in the 10 Watt range is called a rheostat and is around $50+ to buy.

Can't I just use a low wattage potentiometer and a transistor to achieve nearly the same effect of a higher wattage variable resistor... minus some linearity?

Is there such a device that is sold as an all-in-one package, or is this usually a build it yourself kind of thing?

EDIT: FOUND THE TERM: Adjustable Voltage Regulator
« Last Edit: December 01, 2021, 08:44:30 pm by Plasmateur »
 

Offline Whales

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Re: Is there such a thing as a variable resistor / transistor combo?
« Reply #1 on: December 01, 2021, 09:36:07 am »
Yes and no.

I use the exact circuit you describe to power the LED strips above my desks.  12V in and an N-channel mosfet controlled by a pot.  Mosfet is wired in common-source arrangement (12V -> LEDs -> Mosfet -> GND).   The non-linearity is actually really useful, because it approximately counter-acts the non-linearity of the LEDs and human eyes.  I make more of the pot's rotation range "useful" by adding some resistors either side of it.

List of things to be wary of when using transistors as variable resistors:

 * Many transistors are not rated for DC resistive use, they're rated for switching (fully off and fully on) instead.  Check the graph on the datasheet called "safe operating area", make sure there is a DC rating on it and see what the max conditions are.  Prepare to be disappointed for most parts.

 * Common source N-mosfet circuits are very nonlinear (input V versus impedence or output V)
 * Common emitter NPN BJT circuits are nonlinear too, but possibly less so?  See their graph of beta/hFE/currentgain on their datasheet (input I versus output I).

 * You might be able to get away with a source-follower (emitter follower) circuit in many circumstances instead.  It's approximately an adjustable voltage source made out of a transistor and a pot.  Linear relationship between input voltage and output voltage, with a bit of load-dependent error.  Really useful to know, especially for controlling small brushed DC motors or lights with in-built resistors.

 * Some jfets can be used very linearly as variable resistors.  Alas Jfets tend to be too small for power usage.

 * As transistors heat up they change gain (thus changing their simulated resistance).  In the worst cases this can cause them to perform thermal runaway; but you can make choices to avoid this happening. 
« Last Edit: December 01, 2021, 09:45:33 am by Whales »
 
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Offline Plasmateur

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Re: Is there such a thing as a variable resistor / transistor combo?
« Reply #2 on: December 01, 2021, 09:48:59 am »
Yes and no.

I use the exact circuit you describe to power the LED strips above my desks.  12V in and an N-channel mosfet controlled by a pot.  Mosfet is wired in common-source arrangement (12V -> LEDs -> Mosfet -> GND).   The non-linearity is actually really useful, because it approximately counter-acts the non-linearity of the LEDs and human eyes.  I make more of the pot's rotation range "useful" by adding some resistors either side of it.

List of things to be wary of when using transistors as variable resistors:

 * Many transistors are not rated for DC resistive use, they're rated for switching (fully off and fully on) instead.  Check the graph on the datasheet called "safe operating area", make sure there is a DC rating on it and see what the max conditions are.  Prepare to be disappointed for most parts.

 * Common source N-mosfet circuits are very nonlinear (input V versus impedence or output V)
 * Common emitter NPN BJT circuits are nonlinear too, but possibly less so?  See their graph of beta/hFE/currentgain on their datasheet (input I versus output I).

 * You might be able to get away with a source-follower (emitter follower) circuit in many circumstances instead.  It's approximately an adjustable voltage source made out of a transistor and a pot.  Linear relationship between input voltage and output voltage, with a bit of load-dependent error.  Really useful to know, especially for controlling small brushed DC motors or lights with in-built resistors.

 * Some jfets can be used very linearly as variable resistors.  Alas Jfets tend to be too small for power usage.

 * As transistors heat up they change gain (thus changing their simulated resistance).  In the worst cases this can cause them to perform thermal runaway; but you can make choices to avoid this happening.

Thank you for all of this information. Luckly, I'm right at 12V, so if this is in your ballpark for something you've already done... hopefully I'll find a few options in the safe operating range that will work for me.
 

Online ledtester

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Re: Is there such a thing as a variable resistor / transistor combo?
« Reply #3 on: December 01, 2021, 11:37:47 am »
...
Can't I just use a low wattage potentiometer and a transistor to achieve nearly the same effect of a higher wattage variable resistor... minus some linearity?
...

What's your application? Sounds like you are looking for a current limiter or constant current circuit -- like with an op-amp, low-wattage pot and a transistor (typically a MOSFET but a BJT could also work) which dissipates the power, e.g.:

https://www.bristolwatch.com/ele3/2a.htm
 

Offline ambrosia heart

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Re: Is there such a thing as a variable resistor / transistor combo?
« Reply #4 on: December 01, 2021, 12:25:39 pm »
I'm looking to vary the resistance in a 10 Watt circuit.

Seek help from taobao  ;) and ask Americans living in Hong
Kong or TaiWan to buy and sent to you  :P . It is 25W.  :popcorn: See photos.
 

Offline tooki

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Re: Is there such a thing as a variable resistor / transistor combo?
« Reply #5 on: December 01, 2021, 12:34:06 pm »
I'm looking to vary the resistance in a 10 Watt circuit.
That’s not an answer to “what’s your application”. It’s your attempted solution.

The “application” means what are you trying to accomplish, not how you’re attempting to accomplish it.

Classic x-y problem: https://xyproblem.info/
 

Offline Plasmateur

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Re: Is there such a thing as a variable resistor / transistor combo?
« Reply #6 on: December 01, 2021, 08:34:20 pm »
...
Can't I just use a low wattage potentiometer and a transistor to achieve nearly the same effect of a higher wattage variable resistor... minus some linearity?
...

What's your application? Sounds like you are looking for a current limiter or constant current circuit -- like with an op-amp, low-wattage pot and a transistor (typically a MOSFET but a BJT could also work) which dissipates the power, e.g.:

https://www.bristolwatch.com/ele3/2a.htm

It's for a particular stage of a HF amplifier in a series of stages.

What I've decided to do, is buy one of the prepackage voltage controllers. Something like this - https://www.amazon.com/UCTRONICS-Adjustable-Converter-Stabilized-Regulator/dp/B07ZSGKTX5

But more bare bones. I just need the dial, not the display. 


EDIT: Adjustable Voltage Regulator is what I am looking for.
« Last Edit: December 01, 2021, 08:44:06 pm by Plasmateur »
 

Offline jpanhalt

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Re: Is there such a thing as a variable resistor / transistor combo?
« Reply #7 on: December 01, 2021, 09:06:55 pm »
Such circuits were used in the 1960's and early 70's to control lighting in aircraft.  You are about 50 years too late.  Great idea, though,
 

Online ledtester

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Re: Is there such a thing as a variable resistor / transistor combo?
« Reply #8 on: December 02, 2021, 04:14:11 am »
It's for a particular stage of a HF amplifier in a series of stages.
...
What I've decided to do, is buy one of the prepackage voltage controllers. Something like this - https://www.amazon.com/UCTRONICS-Adjustable-Converter-Stabilized-Regulator/dp/B07ZSGKTX5
...
EDIT: Adjustable Voltage Regulator is what I am looking for.

A caveat: buck/boost converters are electrically noisy -- they emit their own HF noise on the output rails.

A linear voltage regulator will produce much cleaner power at the expense of requiring more heat to be dissipated (i.e. perhaps you'll need a large heatsink). What range do you want to adjust the voltage through? And what is the current draw of your circuit?
 

Offline Plasmateur

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Re: Is there such a thing as a variable resistor / transistor combo?
« Reply #9 on: December 02, 2021, 04:30:37 am »
It's for a particular stage of a HF amplifier in a series of stages.
...
What I've decided to do, is buy one of the prepackage voltage controllers. Something like this - https://www.amazon.com/UCTRONICS-Adjustable-Converter-Stabilized-Regulator/dp/B07ZSGKTX5
...
EDIT: Adjustable Voltage Regulator is what I am looking for.

A caveat: buck/boost converters are electrically noisy -- they emit their own HF noise on the output rails.

A linear voltage regulator will produce much cleaner power at the expense of requiring more heat to be dissipated (i.e. perhaps you'll need a large heatsink). What range do you want to adjust the voltage through? And what is the current draw of your circuit?

At max power the circuit is about 9.5V and draws roughly 9 Watts. It doesn't have to be completely off, but it would be nice to be able to adjust it down to around 2-3 volts.
 

Online ledtester

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Re: Is there such a thing as a variable resistor / transistor combo?
« Reply #10 on: December 02, 2021, 05:22:41 am »
Here's a good introductory video on linear regulators. It uses the LM317 but almost all linear regulators operate the same way:

LM317 Adjustable Voltage Regulator Tutorial -- Afrotechmods
https://youtu.be/IjJWWGPjc-w

When it comes to linear regulators there are a lot of choices out there with different specs. If you were to use the LM317 here's what you'd have to worry about:

- your circuit appears to draw a max of 1A and that's within the LM317 limits
- to get a regulated output of 9.5V you'll need an input voltage of around 12V
- the lowest voltage attainable by the LM317 is 1.25V so we're good there
- if your circuit draws 1A even at 2V you're looking at having the LM317 dissipate 10W. That's going to require a good size heat sink and perhaps also active cooling (i.e. a fan). If your circuit draws less current at lower voltages then your cooling needs will be less.

There are a bunch of LM317 modules available on Amazon and they implement basically the same design in the Afrotechmods video. Most come with rather puny heat sinks but they would probably work if you just replaced the heat sink with something more substantial.
« Last Edit: December 02, 2021, 05:30:03 am by ledtester »
 
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Offline Plasmateur

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Re: Is there such a thing as a variable resistor / transistor combo?
« Reply #11 on: December 02, 2021, 06:06:58 pm »
Thanks again. I think that's what I ended up buying a few days ago let me check real quick.....

Yep - https://www.ebay.com/itm/191879967600

LM317, they are claiming 2A. lol. It's coming from China.

I was hoping I could get 9.5 from 12V because their description I believe implies 12V give 9V.

9.5V seems to be the sweet spot for my middle stage amp.
 

Offline David Hess

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Re: Is there such a thing as a variable resistor / transistor combo?
« Reply #12 on: December 02, 2021, 09:50:37 pm »
If you want something which acts somewhat like a power resistor, then a bipolar power transistor with a pot across the collector and emitter and the wiper connected to the base can sort of do it.  Some resistance should be added in series with the base to protect the transistor and potentiometer, and a low value of emitter resistance, like 1 amp per 0.68 ohms,  is needed to prevent thermal runway.
 


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