Will this small FET work instead of a relay?

I have a new Carrier Infinity 5 stage heat pump system - best there is. Turns out in their wisdom they will not let the AC function below stage 2 if it is between 60 and 55° and at less than stage 4 if it is between 55 and 40° outside temps. This totally makes this system unusable in my new home that has 4 zones one of which is interior and requires cooling. Unfortunately it took me 2 months to figure this out as it is an "unpublished specification" that is inconsistent with their published specification.

So I now need to "trick" the control board so it is never between these two temperatures. I have a circuit that I want to switch in a parallel resistor (R5) to the outside temperature thermistor (R4) with a small FET (Q1) so it stays about the 60° range. This will allow the unit to run as it should and it does indeed work well when it does.

I am not a circuit guru and even less in code but I built a thermometer a while back and dug all that up. All I need to do is switch in this resistor (I think) and I would like to do it with a small signal N channel FET. I would like to stay away from a relay if this will work and use the old thermometer board with the modification and some additional code that I will post later if this will work electrically. I also don't have any relays. There is no power on the circuit and the HP simply reads the thermistor value and uses a look-up I assume for the corresponding temperature vs. resistance.

Please have a look and give your advice. The display and 7219 work correctly now.

Thanks very much

It is risky to wire a commercial unit to anything that could inadvertently send current into it and ruin it. (Despite the care you take to think you're simply switching in a resistor). You might get the modification to work for you, but will the next owner of the house know what to do with it? And how to fix it?

I followed a clever hint to turn a plain mercury switch thermostat into a programmable type. I mounted a 7W light bulb to the wall below it. An ordinary plug-in timer turned the bulb on. Heated air rose up to the thermostat to 'fool' it. It stayed off during the night although room air went down to 60 or 65 degrees.

I guess that won't create the same effect you want.
Here is a way to achieve some degree of isolation. Suppose you were install a CdS photoresistor? Use your thermometer circuit to light an led at the needed temperatures. It will reduce the CdS sensor resistance. A lot of experimentation may be needed.

Hi,

Your MOSFET circuit can't work.

A Mosfet is controlled with it's V_GS(look into the datasheet). V_GS means: Voltage from gate to source.
You control the gate voltage referenced to your system GND. But the Mosfet doesn't know anything about the GND.
Since your Mosfet_source is floating referenced to system GND .... you will never know when/how to switch the Mosfet ON or OFF.

The next thing is that gate to source voltage is undefined and it will pick up any potential around.
The Mosfet inside gate to source (and gate to drain) isolation barrier is very sensitive and any voltage beyond the "absolute max gate to source voltage" specification may immediately kill the Mosfet.
Mind: a Mosfet is no isolating device.

An optocoupler (for DC) or a photomos relay (for AC and DC) may be the better choice.

Klaus

Thanks for the replies.

I considered the gate voltage issue and "assumed" it was OK.
This board would be powered from the main control board in the heat pump so the ground is common.

This unit was going to be a small board I would add by the heat pump. It is done in a fail safe manner so it would not disrupt normal operation if it failed. The equipment would not have any modification either.

This board would be powered from the main control board in the heat pump so the ground is common.

Does that change anything?

Hi,

so the ground is common.

Click to expand...

Please understand that GND is not your reference to control the Mosfet.
--> You need to define Mosfet_source voltage.

Klaus