Simulating a strain gauge on a wheatstone bridge using JFET as a variable resistor

Thread Starter

fieryfire

Joined Feb 14, 2017
106
Dear All

I am coming to you regarding the possibility to use JFETs as a variable resistor.

I am trying to simulate a JFET to vary between the resistance 348ohm to 350ohm. So as to simulate a quarter bridge wheatstone circuit over 350ohm as shown in the attached image.
I am trying to unbalance the bridge to simulate an output by change the resistance of the JFET branch between 348ohm to 350ohm. But i am a bit lost on how to proceed. Could you please help



Thanks in advance,
 

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Thread Starter

fieryfire

Joined Feb 14, 2017
106
Thank you for the example. Although do we have to use the lm358.mod in this example. I get an error when i execute since i dont have this lib
 

Thread Starter

fieryfire

Joined Feb 14, 2017
106
hi,
This is your modified circuit.

E
Hello eric,

Could you please clarify. Do you mean that i would not have to really use a jfet in this option. but rather simply send a voltage between 0 and 3.6mV inorder to balance the bridge. Wow i am really an idiot. I didnot know it was possible to achieve this so easily.
 

Thread Starter

fieryfire

Joined Feb 14, 2017
106
Hold on i think i got it wrong. You add 2 volts to a resistance value of 348 to bring it to 350. ??? i dont understand how its possible to do that
 

ericgibbs

Joined Jan 29, 2010
9,574
hi,
I thought you were trying to simulate a load being applied to the bridge by using a jFet.
As you know jFet's vary in their parameters, also they are not really linear enough to simulate a bridge resistor.

The R5 [ in my circuit ] it has a starting value of 348R, in LTS it can be varied to say 350R by using a 'voltage' value Vx in the simulation.
It is not really applying a voltage to the bridge its only changing the resistance value from 348R to 350R during the sim run.
E
 

Thread Starter

fieryfire

Joined Feb 14, 2017
106
Ahhh. Actually what i wanted is to be able to make this circuit in practise. For example changing the JFET resistance from 348 ohm to 350ohm by a voltage source. i would control this resistance value using a microcontroller DAC for example. Would you have any leads on how i could proceed?
 

ericgibbs

Joined Jan 29, 2010
9,574
Hold on i think i got it wrong. You add 2 volts to a resistance value of 348 to bring it to 350. ??? i dont understand how its possible to do that

hi,
No,its not adding a voltage.

Say you had a potentiometer that had a range of 348R to 350R, and during the sim run you slowly turned the pot from 348R to 350R over the simulation run, you would get exactly the same Voutput plot as we have shown.

This what LTS does, it uses the rising value of Vx to slowly increase the value of R5 over the sim.

PWL (0,0,1,2) is for the increase in Vx over the sim run cycle.

Do you follow OK.?
E
 

Thread Starter

fieryfire

Joined Feb 14, 2017
106
hi,
No,its not adding a voltage.

Say you had a potentiometer that had a range of 348R to 350R, and during the sim run you slowly turned the pot from 348R to 350R over the simulation run, you would get exactly the same Voutput plot as we have shown.

This what LTS does, it uses the rising value of Vx to slowly increase the value of R5 over the sim.

PWL (0,0,1,2) is for the increase in Vx over the sim run cycle.

Do you follow OK.?
E
Yes i do follow. Thank you eric. But are you aware of any kind of Digital IC. that i can control to swing a resistance between 348 ohm to 350 ohm. in steps of about 0.1ohm.? i saw a few digital potentiometers but they do not come close to this resolution
 

Alec_t

Joined Sep 17, 2013
10,918
As Eric says, the parameters of jfets vary (due to temperature and wide manufacturing tolerances). That goes for mosfets too. So although you could use either to provide a variable resistance, the value of that resistance would not be a predictable function of the control voltage in a practical circuit. You could, however, use a fet or bjt in conjunction with an opamp and a fixed low ohm resistor in a feedback loop to make a voltage-controlled current source of predictable value for one arm of the bridge.
I'm not aware of a digital IC which would meet your requirements.
 
Last edited:

OBW0549

Joined Mar 2, 2015
3,211
I am trying to simulate a JFET to vary between the resistance 348ohm to 350ohm. So as to simulate a quarter bridge wheatstone circuit over 350ohm as shown in the attached image.
I am trying to unbalance the bridge to simulate an output by change the resistance of the JFET branch between 348ohm to 350ohm. But i am a bit lost on how to proceed. Could you please help
The best way to proceed would be to abandon the idea of using a JFET as a voltage-controlled resistor in this application, as it will be very hard to achieve a specific resistance value with any great precision.

You can easily accomplish your objective with the simple circuit shown below. Potentiometer R5 can be either a manual or digital pot or, if you want to eliminate the pot entirely, you can simply apply the output of your DAC directly to the left end of R6.

Either way, this will give you the ability to simulate a strain gage bridge with some precision and predictability.

bridge.png
 

Thread Starter

fieryfire

Joined Feb 14, 2017
106
As Eric says, the parameters of jfets vary (due to temperature and wide manufacturing tolerances). That goes for mosfets too. So although you could use either to provide a variable resistance, the value of that resistance would not be a predictable function of the control voltage in a practical circuit. You could, however, use a fet or bjt in conjunction with an opamp and a fixed low ohm resistor in a feedback loop to make a voltage-controlled current source of predictable value for one arm of the bridge.
I'm not aware of a digital IC which would meet your requirements.
Thank you for the response. but could you please guide me, or provide some reading material i can look further into this topic.
 

Thread Starter

fieryfire

Joined Feb 14, 2017
106
The best way to proceed would be to abandon the idea of using a JFET as a voltage-controlled resistor in this application, as it will be very hard to achieve a specific resistance value with any great precision.

You can easily accomplish your objective with the simple circuit shown below. Potentiometer R5 can be either a manual or digital pot or, if you want to eliminate the pot entirely, you can simply apply the output of your DAC directly to the left end of R6.

Either way, this will give you the ability to simulate a strain gage bridge with some precision and predictability.

View attachment 198430
Very interesting indeed. I am thinking to simultaneosuly control multiple bridges together in the same way. So im thinking maybe a single single to simultaneosuly control several digipots. Im going to take a look at your circuit on LTSpice. Thank you sir :)
 
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