RTD Simulator

Probably a circuit using a J-FET as the resistive element would do the trick for you. There are also some voltage controlled potentiometers about that may also be suitable.

Of course, if you want realism, you could use a real RTD with a heater element wrapped around it.

The RTD instrument works by putting a current through the sensor (and the compensation leadwires). Usually in the range of a few hundred uA to a few mA and often (but not always) DC.

Ohm's law is E = I * R

So, to simulate a resistance R you need an active circuit which will present a voltage at its terminals proportional to the product of the applied current and the desired (simulated) resistance.

Does that give you some ideas as to how it might be accomplished?

Best regards, Spehro Pefhany

RTDs are pretty linear. You can come close with a resistor (AKA voltage-to-current converter and something to offset the (effect of the)

Jerry

Feed two signals into a 2 channel op amp summing amp via voltage followers or gain 1 inverting op amp for each channel, as needed, on each input. Use a 3 sections of quad op amp?

2) Use the 4-20 ma signal to feed a voltage divider resistor string, scaling the voltage output to match the needed RTD signal range over the

Could you use a voltage output configuration from the first PLC instead? Feed a voltage divider to scale as needed. Many PLC A/D I/O can be configured to do this, by channel or card.

There are also din mount signal converter mods to convert 4-20 ma to other V or I signal ranges, but not to RTD that I know as these are inputs.

Dennis

snipped-for-privacy@sawyersnet.com wrote:

Perhaps the concept you are looking for is that if the transresistor (voltage or current dependent resistor).

I think you might make something like this from a transconductance amplifier, like the CA3080 or its descendants. It delivers an output current proportional to the difference voltage applied to its two inputs, and also proportional to the current applied to its gain control input. If you ground the + differential input and connect the

- input to its output through a voltage divider (with a total resistance higher than the highest RTD resistance you might ever want to simulate), since the differential input is linear over only about a

Hello All,

Thanks to everyone who responded to this post. It got this software guy pointed down the right path to make this happen. What I ended up doing is a voltage divider circuit with three resistors to give me what I wanted. Take a look at the circuit and please let me know what you think...

I was quite pleased with the resolution and the range of the circuit. Since the PLC that I am using

Take care and thanks for everyone's input.

John