Measuring Voltage of a Thermistor Circuit

[AdamLee]

Good whatever-time-of-day-it-is, kind reader!

We're trying to hook up a thermistor circuit for leak detection to an 8/8/8 interface kit and I'm wondering what the best way to go about this is. Essentially what we want to measure is voltage. The trick is that the thermistor needs to be heated with a 12V DC source. I, with my infinite lack of electrical knowledge, tried to measure it like an analog input but I saw the voltage go straight to 5V (knew it must actually be the full 12V) upon which I immediately disconnected before doing any damage.

Basically the circuit is just a 12V DC source with a thermistor and 22 ohm resistor in series.

Is there a good way to read this with the interface kit? I'd really prefer not to use a VINT hub if possible as we've already got one and it's maxed out.

[mparadis]

I would start be checking the data sheet for the sensor to confirm what the output voltage is. There are plenty of sensors that are powered with 12V but still output 0-5V. I would also check to see if it is able to accept a lower supply voltage.

If it really does output 0-12V, you could measure it with a voltage sensor like the 1135, although because this is a ±30V sensor you'll be losing some precision.

Alternatively, there are many other ways to measure temperature with Phidgets that may be easier and cheaper to implement.

[AdamLee]

We know the circuit outputs < 5V under all conditions. We're not 100% on the voltage range but we've tested it thoroughly and are confident it will fall within the limits of the board.

What wound up working was putting the data line in parallel on the grounded side of the thermistor.

Code: Select all

                             --- (-)
(+) --- 22ohm --- thermistor
                             --- data (voltage input channel)

Hopefully that drawing makes sense. What we don't understand is why this is working the way it is, or at all really. When we measure with a multimeter across the 22ohm resister we see the voltage that the voltage input is reading. Originally, as I mentioned, we expected the voltage input to work like a multimeter and wired the data and ground across the 22ohm resistor, which again was reading the full 12V.

This temperature measurement is actually done as part of a leak detection system which needs to be quick to response and submersible in corrosive liquids. We did quite a bit of research to settle on this circuit and at this point it's what we need to go with.

None of us are electrical engineers so really we're just trying to understand what's going on here. Does the data line eventually make its way to the ground on the board?

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