Ext Temp Sensor on Boiler

[1974stephen]

Is the external temperature sensor connection on a boiler effectively just a variable resistance reader, or voltage sensor, or do they vary with manufacturer?

I have a Lamborghini Extrema 38C combi (yeah yeah - crap I know, but you telling me isn't going to make me throw it away and buy a proper one), and can't find any details any where for them, and Ferroli are support is useless, as I think they just buy them in.

I've worked on some control systems and they have stats that can have ext temp sensors to avoid having the stat in the room, which is effectively a 0 - 10k resistance across the appropriate input. I'm wondering if I can just get one of them across the input? The manual doesn't suggest whether it's voltage or resistance related.

Any suggestions?

 

[Dan Robinson]

They are ntc's, usually 10 or 12k. Nothing special.

 

[matt1e]

They are ntc's, usually 10 or 12k. Nothing special.

As dan has said,they are thermistors more often than not
Some are more sophisticated but that's usually commercial installs and again only certain manufacturers

Matt

 

[Agile]

The external temp sensor is an NTC thermistor. Usually about 10k or 12k.

The circuit applies a voltage through a series resistor to the NTC. The NTC only varies between about 16k to 8k with UK outdoor temps.

The resulting voltage across the NTC is then read by an ADC and used to compute the boiler flow temperature required to maintain the desired internal temperature.

Because of this way they are used, its possible to add an offset voltage by adding a diode in series with the NTC. A diode has a fixed forward voltage drop of about 0.6v I will be demonstrating that at the curry!

Tony

 

[1974stephen]

Cheers guys.

Is there somewhere on-line where I can get on of these thermistors that is likely to be a suitable generic part? Typing 'thermistor' into Google brings up a large variety of different parts, some of which it appears are for temperature flow in DHW etc.

In the control systems I've worked on there has always been a differential setting+/- for calibrating to take into account cable run etc. I'm not too bothered about that particularly, as I can adjust on the curve as this is less critical, but would a suitable cable just be some two core 0.75mm flex? Cable run is likely to be about 10m.