Matching a sensor interface with a programmable logic converter or controller hardware that can accept only one type of input—npn or pnp—can be difficult. The last-stage modification circuit described here solves this problem. Designers can use it to convert an npn input to a pnp type or incorporate it in the last stage of the sensor design to generate both npn and pnp outputs.
The npn and pnp transistors used in the circuit should be highcurrent- gain (hfe > 100) devices with minimum 2-A continuouscurrent drive capacity (see the figure). This ensures an output stage that can directly drive a relay with a 22-V switching voltage from a 24-V power supply. The transistors used (a 2N3055 npn and 2N2955 pnp in TO-220 packages) are reasonably good choices. They should be placed on a heatsink for good heat dissipation.
The npn first stage operates from a TTL-level input: a 0.4- to 2.4-V signal with a 20-mA maximum current from the signal. To translate the npn signal into a pnp signal output, the first stage drives a pnp switch to create a high-current source of about 22 V.
The positive temperature-coefficient (PTC) thermistor in the pnp transistor’s current-control circuit prevents short-circuit damage. A short circuit would create excessive current and the resulting heat will cause the thermistor’s resistance to rise sharply. Once cooled, the circuit starts working again, so the thermistor acts like a resettable fuse.
Rafael Videla V. From Chile South America -October 18, 2009
Wow, that's some pretty basic electronics skills you have there. Congratulations on designing your first 24V complementary output!
Kevin Minckler -May 05, 2009
very true.
Anonymous -May 04, 2009
If you didn't buy the sensor with the built-in converter, then for 12-13 bucks, you can use one of our signal inverters. 45323T/NP or 45323T/PN. www.esgllc-usa.com
Mike urenovich -May 04, 2009
Since when did the terms "NPN" and "PNP" become descriptors of signals? This reeks of market-speak when the real meaning is current-sinking or current-sourcing. It's this kind of misuse of engineering terms that promotes confusion among folks trying to decipher what he means. Of course, marketing people often have exactly that as their hidden agenda.
Bill Whitlock -April 23, 2009
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