RTD Simulator

I'm looking for a device/circuit that will take a 4-20mA from one PLC that is being used as a device simulator and convert the 4-20 to the
appropriate RTD signal that will be the input to a second PLC under test.
PLC (4-20mA Output) ---> ???? ----> (RTD Input) PLC
I want to dynamically simulate a RTD input into the second PLC for testing purposes.
Thanks for any help.
John
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snipped-for-privacy@sawyersnet.com wrote:

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.
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On 7 Oct 2005 14:06:32 -0700, the renowned snipped-for-privacy@sawyersnet.com wrote:

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
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snipped-for-privacy@sawyersnet.com wrote:

RTDs are pretty linear. You can come close with a resistor (AKA voltage-to-current converter and something to offset the (effect of the) 4 mA baseline.
Jerry
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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?
1) Negative Offset pot to take care of the 4 ma baseline. Set one time.
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 4-20 ma input.
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:

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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 +-20 millivolt range, then the output and input divider in parallel make up a grounded active variable resistance whose value is set by the current passed through its gain control terminal. I leave it to you to come up with a circuit that scales and offsets the 4-20 mA control current to the less than 1 mA needed by the amplifier.
http://cgi.ebay.com/800V-6-3V-at-700VA-Toroid-Tube-Power-Amp-Transformer_W0QQitemZ7509374156QQ http://www.national.com/ds/LM/LM13700.pdf
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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...
http://www.btc-net.com/RTD_SIM.bmp
I created the circuit on a breadboard and interfaced it to a Moore Industries RIY RTD to 4-20mA converter that was setup to accept a 100 ohm Pt RTD. The RIY has digital display that will convert the input of the RTD to a temp. Here are the numbers that I got...
0mA --> -48 degF 4mA --> 139 degF 12mA --> 522 degF 20ma --> 937 degF
I was quite pleased with the resolution and the range of the circuit. Since the PLC that I am using (www.automationdirect.com) to source the 4-20mA will do 0mA to 20mA, the circuit is perfect for my needs.
Take care and thanks for everyone's input.
John
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