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December 09, 2019, 02:26:52 02:26


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Author Topic: NTC simulator  (Read 1459 times)
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GroundPlaneLoop
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« on: October 29, 2019, 04:57:24 16:57 »

Hi,

do you have an idea what would be the best idea to design an NTC simulator? I have no circuit how the NTC signal is processed, but need to extend the cabling for some hundred meters in a noisy environment. So basically i need a convertor from 0-10V or 4-20mA or digital line to emulate the resistance. Ideas: resistor board with relays - NO. Digital pot - no experience if it works floating.
Its a silly idea to use a heater and heat the same NTC? That would work only above ambient temp. So maybe peltier for coooling, which i had extreme bad experience due to instability due to different parameters when heating and cooling.

Any idea?

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reksbg
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« Reply #1 on: October 29, 2019, 05:15:59 17:15 »

Why don't you just use simple potentiometer or resistor decade box to simulate the NTC resistance at different temperatures?
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GroundPlaneLoop
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« Reply #2 on: October 29, 2019, 05:17:26 17:17 »

Its a remote located sensor for a industrial process. Any other inputs accept 0-10V, only this one does not.

I need a 100% sure solution, becaause the NTC is 10k and about 300m in a heavy EMC polluted environment. Im afraid it could cause problems with the control unit.
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bigtoy
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« Reply #3 on: October 29, 2019, 11:59:10 23:59 »

You could certainly use a digital pot. You might need more than one in series (eg a higher-value pot in series with a lower-value pot) to obtain the resolution you need. Isolation would indeed be required, but so long as you use an isolated power supply to allow the pot (and it's associated circuitry) to float to the NTC input, it should be OK.

Just a warning about digital pots - check carefully for accuracy. I've looked at them for building programmable-gain op-amp circuits, and found that most aren't very accurate. Just make sure first that the pots have the accuracy you need, including stability over temperature.
« Last Edit: October 30, 2019, 09:38:57 09:38 by bigtoy » Logged
reksbg
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« Reply #4 on: October 31, 2019, 06:21:41 06:21 »

Now I see what you mean by simulation - you want to measure the temperature locally where the sensor is located and then based on the temperature simulate the NTC value at the input of the controller. I though you just want to simulate the sensor to check how the circuit works. The input is a NTC but actually it will accept voltage because most likely there is another resistor from the input to VCC. So build the same circuit locally where the sensor should be and then just voltage repeater to the input of the controller. Or even better - convert the voltage to 4-20mA and then convert back to voltage at the controller's input.
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Sideshow Bob
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« Reply #5 on: October 31, 2019, 09:15:54 09:15 »

I think the more corret word will be emulate and not simulate. In order to emulate you will need to know more about the input stage on your controller. Like is it a bridge configuration or some other configuration. As a crash course look at this https://www.tdk-electronics.tdk.com/download/531110/5608e4b12153bb12af2808fbedc5a55b/pdf-applicationnotes.pdf
Also remeber that NTC resistor are not linear vs temperature. Also the resitance can be found using a formula.The formula will depend on the NTC type. From what I see maybe start using a digital to analog converter and say a lookup table will be the viable viable alternative. But again it will depend on the input stage
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« Reply #6 on: November 06, 2019, 12:22:52 12:22 »

AWG 17 ~= 1mm2  => ~5 Ohm / 300 m
So if additional 10 Ohm is not a big dial for you (0.1% from 10 kOhm) then simple capacitor on receiver side perhaps together with RC snubber is an effective solution against noise. Even more - I expect some filtration already exists in corresponding input of industrial PLC.

So although I see a nice solution for NTC emulator I hold myself from sharing now because it seems redundant. I'd start from careful reading of manual (RTFM) for given controller regarding features and limitations.
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