My question regards a component I found in the output circuit of f/U (frequency to voltage) signal converter (Noris Tachometerwerk RV43A 0-3767Hz to 0-10V). See the red box below on the PCB photo.

I tried Googling it, but without any success. Does anyone know what it is?

PCB frontside

PCB backside

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    \$\begingroup\$ It might be a thermistor. You could try measuring its resistance, once when cold and again when hot. \$\endgroup\$ Jul 27 at 10:39
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    \$\begingroup\$ Note reverse side red box needs to be "slid up" one set of pins. \$\endgroup\$
    – rdtsc
    Jul 27 at 11:43

As Spehro explained, it looks like a thermistor touching (i.e. thermally-coupled to) the resistor next to it (R20) (notice the diagonal line on the silkscreen between the two components, also suggesting a coupling between them).

However there is one more piece of evidence not yet mentioned, which adds extra confirmation.

  • Notice the reference designator next to the unidentified component is "H".

  • The product manufacturer (Noris Tachometerwerk) is German, and one German-language term for an NTC thermistor is "(der) Heißleiter".

So the reference designator "H" for that component, definitely fits with it being a thermistor.

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    \$\begingroup\$ +1 Fascinating. Hot ladder? \$\endgroup\$ Jul 27 at 14:30
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    \$\begingroup\$ @SpehroPefhany: Literally "hot conductor." "Leiter" has multiple meanings - mostly to do with "leading." A conductor leads electrons from place to place. A ladder leads you from the floor to some place higher up. "Leiter" can also be the person in charge "Geschäftleitung" is the "leader" of a place of business. \$\endgroup\$
    – JRE
    Jul 27 at 14:43
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    \$\begingroup\$ @SpehroPefhany - Hi, Thanks, you did the main identification (the thermal coupling is interesting) and I already +1 yours :-) I don't claim to know the etymology of the German terms here, but thanks to JRE for that input. I just picked up some useful terminology while working for a German company :-) \$\endgroup\$
    – SamGibson
    Jul 27 at 14:45

I agree that this is probably a thermistor and it appears to be thermally coupled to the 1.78K 1% resistor beside it (which in itself would not normally change much with temperature- perhaps 100ppm/°C for a metal-film resistor).

This old device is pretty low drift (I think max 0.1% for 10°C max overall drift- which is 100ppm/°C) -- they may have tweaked it in ways that are not obvious. For example, typical drift of metal film resistors of a certain value may be much more predictable than the datasheet reveals. If you're trying to repair it I would not touch those parts unless you're convinced they are somehow defective.


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