Answer / maya ramesh

Proximity or vibration measurement circuits use NPN
transistors andNPN transistors are -ve voltage based
devices and have particular advantages and the whole old
lot of proximity measurement circuits are being driven by
NPN transistors.

Answer / rehan riaz

The proximitors are of diffrent types for particular
bentaly nevada the voltage used are negative and that the
circuit use npn transisters which uses negative bias
voltage that is true but that relates to the history in old
times when transistors NpN transisters are cheaper then the
pnp GE then chooses the npn to save the cost and after that
it becomes there design i hope u got the answer in further
then please do share more Thanks

Answer / rajesh

because wrt positive supply other running cables negative
voltages will interfere to the proximitor supply voltage.
to avoid unwanted signal interferance we are using negative
supply.

Answer / anand raj

Proximity sensors are of different types, and the circuitry
can use NPN or PNP switching. I personelly have worked with
NAMUR inductive type proximity sensors where we supply
positive 7.2 to 8 volt to the sensor. When a metallic target
is in its visinity the voltage drops to 4 volts switching
the relay. So dont know if this question is completely correct.

Answer / manish kumar

Depend upon which type of transistor o/p will taken.

Answer / grajesh

Because Proximity will induces the Positive field

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