LM335 thermistor with 4 to 20 mA output at 70-90 degrees F

[Ron Hansen]

Can anybody enlighten me as to how to construct a circuit using a LM335 thermistor to output
4 to 20mA at a specific temperature range of between 70 degrees (4mA) and 90 degrees (20mA) Fahrenheit. For now, that is all this circuit need do.


Thx!

 

[MikeMl]

Breaking it into two parts: an opamp to amplify 70 degF to X volts and 90 degF to Y volts. Then a second opamp as a current source where X volts in makes 4mA and Y volts makes 20mA. The supply voltage should be 24V so that the current loop has a compliance of ~20V.

It might be possible to combine both functions into a single stage, but since most opamps come 2 or 4 per package, why bother.

 

[MikeMl]

Here is a hack at it. I assumed that the 24V supply is very precise. It is used as the reference for the circuit. The LM335 model reacts to temperatures in degC, so I convert degF to degC inside voltage source V1. This source controls the simulation sweeping from 70degF (0s) to 90degF (10s).

I solved for R4, R5 and R6 algebraically. You will have to figure out how to buy these resistors or use trimpots. The opamps must be capable of pulling their outputs pretty close to 24V, so you must use rail-to-rail output opamps. Another idea is to power the opamps on a voltage like 30V, and then to derive the 24.00V using a regulator. That will let you use common 30V opamps. Also check the opamp's common-mode input range.