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> Did the protection circuitry of the power supply cause the problem
> I was having?

Hi Sai,

No, your power supply has a very low output impedance, compared
with the sensor. As Bob said, it is too "stiff" for the sensor to overcome
this low impedance and cause a significant dip in the PSU's o/p voltage

What the resistor does is limit the current, or raise the output impedance
of the PSU (as the sensor sees it) high enough for the sensor's drive
circuitry to be able to pull the 'cold' side of the 100R down to 0V.

The supply voltage for the sensor will be fairly stable as it presumably
will have a reservoir capacitor and isolating diode on its Vcc pin that
keeps it powered during the pulse

It's very much like a pull-up on a micro output pin like RA4. You
wouldn't expect RA4 to be able to overpower a battery or PSU

Your overall circuit is probably something like the attached

Joe


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