Single thermistor as input for 2 different devices

[hermhart]

I am reading temperature using a 10K thermistor. I am feeding this thermistor as the input to a solar hot water controller which uses a microcontroller in there somewhere. I would also like to feed this same thermistor as input to an A/D converter board for monitoring purposes. Actually I have 5 thermistors and only one at a time is used by the controller. I would like to read the status of all of them using the A/D. I could use a rotary switch to choose which one at any given moment goes to the controller, while the rest go to the A/D. That's sort a cheesy work-around... kind of amateurish in my opinion. Can I just run parallel leads... one to each device? If I do, theoretically one device will load the other to some extent. I presume both devices have high impedance inputs, so that loading should be minimal. But tech support for one of the devices tells me not to do it. I know he has to think of liability. But what is the right way to do this? I suppose some sort of buffer device exists... one in to many out. Or does it? Or is there an easy simple way to do this?

 

[55pilot]

A thermistor requires some sort of driver, which is located on their board. If you monitor it, you need to know a lot about their driver circuit, including access to their reference if you want high accuracy. You also need to make sure that your monitor is REALLY high impedance as you may be dealing with only a few micro-amps per degree C flowing in the thermistor.

I am also confused about the 5 thermistors. How are the 5 currently set up. Do you select between them using a rotary switch or do the switch between them automatically using a mux in the device?

If you switch between them manually, then you can connect the 4 unused ones to your own circuit, your driver and your monitor. The selection switch will be rather cumbersome, but it can be done. If the switching is happening on their device, then I would agree with their tech that you should not be messing with their stuff.

---55p