# Voltage controlled resistor circuit Help

I have a unit that measures temp with a thermistor. I want to use it to measure voltage.
I am trying to build a circuit to emulate a thermistor. Here are the parameters.

24v control = 37.8K.
1v control change = -1k so there is an inverse relation ship.
Control voltage range = ideal is 6 to 42 volts but can be as tight as 12 to 28 volts.
The 3.5 volts is applied to the thermistor buy the unit.
The unit and control voltage cannot and should not share a common ground.

I have a basic knowledge of electronic but need some help getting started. I was thinking of using some sort of VCR or solid state pot.

Any help is greatly appreciated.

I have a unit that measures temp with a thermistor. I want to use it to measure voltage.
I am trying to build a circuit to emulate a thermistor. Here are the parameters.

24v control = 37.8K.
1v control change = -1k so there is an inverse relation ship.
Control voltage range = ideal is 6 to 42 volts but can be as tight as 12 to 28 volts.
The 3.5 volts is applied to the thermistor buy the unit.
The unit and control voltage cannot and should not share a common ground.

I have a basic knowledge of electronic but need some help getting started. I was thinking of using some sort of VCR or solid state pot.

Any help is greatly appreciated.

After reading that 3 times, I still have no idea what you're doing.

Since you are measuring temperature, would it be safe to assume you're using some kind of microcontroller? Do you have a schematic? A set of instructions you're following?

You'll get much more help if you give people enough information to help you.

berkeman
Mentor
I have a unit that measures temp with a thermistor. I want to use it to measure voltage.
I am trying to build a circuit to emulate a thermistor. Here are the parameters.

24v control = 37.8K.
1v control change = -1k so there is an inverse relation ship.
Control voltage range = ideal is 6 to 42 volts but can be as tight as 12 to 28 volts.
The 3.5 volts is applied to the thermistor buy the unit.
The unit and control voltage cannot and should not share a common ground.

I have a basic knowledge of electronic but need some help getting started. I was thinking of using some sort of VCR or solid state pot.

Any help is greatly appreciated.

Can you use a digital potentiometer as the basis for it?

http://en.wikipedia.org/wiki/Digital_potentiometer

http://www.analog.com/en/digital-to-analog-converters/digital-potentiometers/products/index.html

.

I can't imagine why you would want to emulate a thermistor?
Any circuit to do so would be vastly more complicated and expensive than just using a thermistor.

In saying you want to use your thermistor to measure voltage, do you mean you want to measure the TRUE RMS value of voltage using the heating of the thermistor?

This is a viable proposition.

Or is the use for stabilising the output voltage of an oscillator?

Using the thermistor to measure peak or average values is less so as the response curve is decidedly non-linear.

The unit has a microcontroller. It will measure temp for more than one area. I want to use one of the temp inputs to measure the voltage of a 24v DC battery backup system. I want 24.0 degrees on the unit to equal 24.0 volts in the backup system. Since the thermistor has a value of 37.8K at 24degrees I need the circuit to have the same value of 37.8K when the input voltage is 24V.