# Can resistors be used to bring a heating element within range?

I have a replacement 24V/60W desoldering gun I would like to modify for use on my Aoyue Int958A+ soldering station. My 968A+ has a smoke absorber which has enough suction to double as a solder vacuum. Mapping the pins is relatively straightforward.

During my measurements, however, I found the desoldering gun's heat sensor resistance was significantly lower than my soldering iron's (see table below). When I briefly heated the tip of the gun with a torch lighter, I noticed the resistance steadily climb to around 6 ohms. Could this be a thermistor instead of a thermocouple? Also, is it possible to correct the resistance range by adding a ~50 ohm resistor in series with the heat sensor?

Desoldering Gun Pinouts

PIN WIRE    PURPOSE
1   Black   Heat Sensor
2   Blue    Heat Sensor
3   Red     Heating Element
4   Red     Heating Element
5   White   Trigger switch
6   White   Trigger switch
7   CENTER  Vibration sensor (unused)


Resistance Measurements

Pins 1 & 2              1 to 2  Ω
Pins 4 & 5              1 to 2  Ω
Pin 3 & solder tip      Below 2  Ω


Soldering Iron Port Pinouts

PIN WIRE    PURPOSE
1   Blue    Heating element
2   Green   Heating element
3   Yellow  Case ground
4   Red     Heat Sensor
5   Black   Heat Sensor
6   Unused


Resistance Measurements

Pins 1 & 2              2.3 to 3.5  Ω
Pins 4 & 5              43 to 58  Ω
Pin 3 & solder tip      Below 2  Ω


Could this be a thermistor instead of a thermocouple?

Could be either, thermocouples usually put out microvolts. If it's a thermocouple, you'll see the voltage vary by a small amount with temperature.

Also, is it possible to correct the resistance range by adding a ~50 ohm resistor in series with the heat sensor?

It's possible to correct for one temperature point with a resistor, but if your adding a resistor to a thermistor, this will shift the entire curve up and will not be equivalent. This will most likely cause problems for the control circuitry, and the temperature display. Your best bet is to find an equivalent temperature sensor and swap it out.

• Thanks. Unfortunately, the temp sensor is an all in one unit. I am starting to lean towards it being a thermocouple. I now remember seeing small millivolt changes. – user148298 Jan 25 '19 at 0:34
• I did some more tests and discovered the dc voltages rises when I heat the tip. Unfortunately, the resistance also rises. I cant tell if it is a thermocouple or thermistor. Terrified of frying my gun or station. Any nondestructive test I can use to find out? – user148298 Jan 25 '19 at 16:09