Where to find a replacement 35 Mohm NTC / thermistor?

[TheAmmoniacal]

I'm trying to fix a PCR machine / thermocycler, the only fault I can find with it is a broken temperature sensor in the heater block (it reads open / OL ).

The other sensor in the heatblock seems to work and reads 35 Mohm at room temperature and drops to about 25.5 Mohm at 45 C.

Problem is I can't seem to find a comparable part / replacement anywhere? Anyone know?

[ace1903]

35Mhom is quite an unusual value. I would guess that is sensor misidentification. That ohmic range is probably an intrinsic silicon sensor.
Possibly but only if cycling is at extreme temperature ranges.
But PCR is at ~room temperatures I guess.
Give us more info about equipment, pictures, how sensor looks like.
If electronic is simple I think it is 1N4148 used as sensor and 35Mhohm is because of the wrong polarisation.

[T3sl4co1l]

Try diode test rather than ohm test.

Tim

[TheAmmoniacal]

That would explain a lot..

In diode mode both reads 2.6 V in one direction at room temperature. Neither appears to be faulty then.. Any idea how the temperature sensing works here?

[Kleinstein]

The forward voltage of a diode makes a reasonable temperature sensor: relatively low noise, low power and still low resistance, cheap and linear from some -150 C to +150 C with some -2 mV/K for silicon (more accurate 1.12 V extrapolated to 0 K, so that a single point adjustment can be be sufficient).

The 2.6 V may be several diodes in series for an average temperature - this would be still a little unusual.

[fzabkar]

Could this be what it is?

https://en.wikipedia.org/wiki/Silicon_bandgap_temperature_sensor