F100 - TH1, what is it?

10121781012178 Registered User
edited November 2013 in Misc. Discussion
Hi guys.
I have a problem with an old F100.
On the circuit board there is a cracked component called "TH1". Seems to be a thermistor.
Does someone know if it's a NTC or a PTC and what's the function of? It's soldered directly onto the board.
Maybe it's also possible to replace it by a 5k resistor?


Kind regards,
Mike :)



  • bigdarkspotbigdarkspot Registered User
    edited November 2013
    It appears to function as the cold junction reference for the thermocouple circuit. Without digging in to the details, I couldn't tell you it's temperature coefficient.
  • bigdarkspotbigdarkspot Registered User
    edited November 2013
    The voltage due to current through the series/parallel circuit including the thermistor, it's parallel resistor, and both the setpoint pots is subtracted from the buffered thermocouple voltage (once for each setpoint) to trigger 555 timers for the high and low setpoints.

    The top circuit in the schematic directly controls the heater block, so that it's on if the temp is below the setpoint and off if above.

    The bottom circuit sends a signal to the controller to illuminate the 'ready' light through a 74HC4066 analog switch, and keep the controller from triggering the pump if the ready light isn't on.
  • bigdarkspotbigdarkspot Registered User
    edited November 2013
    I wouldn't replace it with a resistor (just saw that question in your post). If you did, your setpoints will change all over the place based on the temperature of the room you're in, either allowing fluid into the exchanger when it's not hot enough (causing liquid to shoot out the front of the machine), or causing the heat exchanger to get too hot (and trigger the thermal shuttoff attached to the side of the heat exchanger).

    thanks, btw, for this question. it made me think about the circuit some, and pointed to a solution to the problem i'm having with my f100.
  • 10121781012178 Registered User
    edited November 2013
    Hi bigdarkspot.
    Thank you for your explanation.
    So I was thinking right, when I supposed, the function of the thermistor is used for the measurement of the room temperature. I did not place emphasis to this temperature change, so I was thinking about replacing it by a resistor. But it's not only the room temperature, I think the operative point is temperature in the device/case itself.

    Best regards,
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