camelot-forum.co.uk

Anyone know if I can 'share' a solar pt1000 sensor between two solar controllers? It would save a lot of work trying to get another tank sensor operational.

A Pt1000 is just a temperature sensitive resistor 1000 ohms at 0 degrees C rising about 4 ohms per degree (tere are tables to give accuarate values). How would you share it without interefering wih the resistive value?

As I would assume very little current flows through the sensor, I am guessing that the apparent resistance of the other unit connected to the sensor would be quite high, and so the sensed resistance seen by the second controller would not be greatly affected. But that is purely a guess. Just wondered if anyone had ever done it and whether it works. Also, with digital controllers, I imagine they poll the sensor once every 30seconds or so?

This is relevant (well down) even though it suggests thermistor turns out he meant Pt1000.

https://forums.electricimp.com/t/monito ... es/2021/18

Looks like its probably not worth the effort nless you are well into elecronics, in which case I would guess you would not be asking

I'd imagine each controller would put a fixed current through it and measure the voltage. Two controllers = twice the current = twice the voltage. That's going to give completely the wrong measurement.

Is the sensor poked into a pocket in the tank? If so, I'd just poke another one into the same hole.
EDIT: I just looked it up and saw it's in a s/s tube with a fairly thick cable so that'll only work if the pocket is wide enough for two of them.

Not sure I understood that much of what they were saying in that thread! I did wonder whether the solar controller might be polling the sensor intermittently - as the temperature takes at least several seconds to register any difference. Could be that one A-to-D converter reads sensor 1, then disconnects and reads sensor 2, disconnects and reads sensor 3 then repeats? But then again, they may well be applying voltage to the sensor constantly. throughout the process. I'm not going to make the change for a few weeks, but when I do, I'll have a play around and see what happens. Two controllers each putting 10mA through a sensor shouldn't half the value, as only 1x current flows through each controller measuring circuit. However, the question is - what happens if you connect another controller to the first one - does current flow through that? In which case, it will 'see' the second controller as a sensor reading, or in my case an adjustment to the reading on a real sensor. It could also blow both controllers! Hope not.

Here is a typical circuit interfacing to a PT1000 temperature sensor...




As indicated, the circuit works by feeding a current through the sensor. In this case it happens to be 0.3mA.
If you then connect 2 of these circuits to the same sensor, the current would be 0.6mA and the 'apparent' value of the resistance would double as far as each circuit is concerned, leading each circuit to 'see' a temperature of approaching 300°C.