Digital indoor outdoor thermometers - Repair experiences - Thermistors

How the sensors work

The sensors consist of thermistors which are temperature sensitive resistors. A lower resistance means higher temperature while a higher resistance means lower temperature. The thermometer periodically sends out a very small current to check the thermistor resistance. This happens every 30 seconds or so.

Patch the outdoor sensor cable

The most common problem with these gadgets is that the outdoor sensor cable gets damaged or completely cut off.

The sensor cable has two wires. It does not matter if the wires are mixed since the sensor is a thermistor - a temperature sensitive resistor, but the wires must not make contact with each other. There is most often no harm if they do, but you get invalid temperature values.

Strip the cable down to the wires, use a soldering iron and solder wires to reconnect them and insulate it with heat shrink tube, hot glue or as a last temporary resort electrical tape. To stop shorts you can make the wires a little bit in different length so the joints get in different places along the cable.

Replace the sensor

This is harder than it first seems because the sensor/thermistor types differ between different thermometers. If you have a spare sensor and a thermometer in need of a sensor then it may be worth trying them together.

But be aware of that the shown temperature might be wrong. Also the scaling might be wrong.

A resistor in series with the sensor might seem to fix the problem with too high temperature shown but then when outdoor temperature changes you might find that the sensor reports wrong temperatures again. The more the temperature deviates from the temperature when the resistor was put in series with the thermistor the more it will differ. This is because the resistor cannot follow the thermistor Beta coefficient curve.

To solve this you need to find a thermistor with a Beta value and a resistance at 25C that matches the thermometer.

To find these values you can connect the thermometer wires to a breadboard and connect a couple of variable resistors of like 10 kOhm each, then adjust them so the thermometer shows 25C, disconnect the thermometer wires and take note of the required resistance. Then reconnect the thermometer and adjust to 50C, disconnect the thermometer and note the required resistance.

You then already have one of the required values to get a replacement thermistor, the resistance at 25C. To get the Beta value you can use the formulas further down in this documentation to calculate it.

If the resistance at 25C is lower than at 50C, then you need a PTC thermistor, if it is higher at 25C than 50C, then you need a NTC, which is more common for digital thermometers.

Share the same sensor

This is a creative idea when you don't have a spare sensor. Simply connect the outdoor sensor wires in parallel with the indoor sensor wires to the indoor sensor.

But this is not simple at all and it does not work because these two measurement currents conflict with each other. One might show the correct temperature while the other one might not, like 0C.

The current from the indoor sensor takes the wrong direction and goes up the outdoor sensor wire instead of into the sensor and vice versa.

A diode or two to stop the currents from taking the wrong ways will not do it either because the reverse leakage through those is too high - at least the ones I tested. I tested with only the outdoor sensor wires and a couple of diodes, In4007 etc. It did not matter in what direction the diode was put, the thermometer sensed its measurement current anyway. It was like if a standard resistor was in the circuit.

Also, the diode will act as a resistor too, 22C was dropped to 16C.

NTC vs PTC thermistor

NTC = Negative temperature coefficient, lower temperature = higher resistance, higher temperature = lower resistance

PTC = Positive temperature coefficient, lower temperature = lower resistance, higher temperature = higher resistance

Most common in thermometers seems to be NTC thermistors.

Thermistor - Calculate Beta

T1 = Temperature 1 in C, preferably 25C
R1 = Resistance at temperature 1

T2 = Temperature 2 in C, preferably 50C
R2 = Resistance at temperature 2

Beta = ln(R1/R2) / (1/(T1 + 273.15) - 1/(T2 + 273.15))


Thermistor - Calculate temperature from resistance

The following is required:
Beta - Thermistor Beta value, check datasheet or calculate
R25 - Thermistor resistance at 25C, check datasheet or measure
Rt - Thermistor resistance to check in C

Formula - different versions, simplifications:
Temperature in C = 1/((ln(Rt / R25) / Beta)+(1/(25+273.15))) - 273.15
Temperature in C = 1/((ln(Rt / R25) / Beta)+(1 / 298,15)) - 273.15

1/((ln(15 / 10) / 2750)+0.003354) - 273.15 = 12,446632 = 12,45C

Note, if doing this in Javascript, you use Math.log instead of ln.
Note, in calculators it needs to be ln (natural logarithmic) not log.

Formula in Javascript:
Temperature in C = 1 / ( (Math.log(Rt / R25) / Beta) + (1 / (25+273.15))) - 273.15
Temperature in C = 1 / ( (Math.log(Rt / R25) / Beta) + (1 / (25+273.15))) - 273.15


Thermistor - Calculate resistance from temperature

The following is required:
Beta - Thermistor Beta value, check datasheet or calculate
T - Thermistor temperature to check in C

Formula - different versions depending on your calculator:
Resistance at temperature T = 10 * exp(Beta * ((1 / (T + 273.15)) - (1 / (273.15 + 25))))

Resistance at temperature T = 10 * e^(Beta * ((1 / (T + 273.15)) - (1 / (273.15 + 25))))
Resistance at temperature T = 10 * e^(Beta * ((1 / (T + 273.15)) - (1 / 298,15)))

Resistance at temperature T = 10 * 2,718282^(Beta * ((1 / (T + 273.15)) - (1 / (273.15 + 25))))
Resistance at temperature T = 10 * 2,718282^(Beta * ((1 / (T + 273.15)) - (1 / 298,15)))

10 * e^(2750 * ((1 / (12.45 + 273.15)) - (1 / 298,15))) = 14,997619 = 15C

Javascript example:
10 * Math.exp(2750 * ((1 / (12.45 + 273.15)) - (1 / (273.15+25))))


Thermometers and their thermistors

Kjell & Company Rubicson 49106 indoor/outdoor thermometer - the outdoor sensor works with an 3950 47K NTC.

This is a personal note. Last updated: 2022-12-01 18:16:07.







Don't forget to pay my friend a visit too. Joakim