Video summary

How PNP Transistor Works as a Switch?

Main summary

Key takeaways

Technology

Summary of Technological Concepts (PNP Transistor as a Switch)

PNP vs. NPN behavior (sourcing vs. sinking)

  • A PNP transistor is described as a “sourcing” transistor.
  • In the U.S. instrumentation world, many sensors/proximity/inductive/capacitive devices are PNP because their associated inputs are typically NPN/sinking on PLCs and related control electronics.
  • Sourcing and sinking must be complementary:
    • If you have a sourcing sensor, you need a sinking input.
    • If you have a sinking sensor, you need a sourcing input.

How to turn on a PNP transistor (base/emitter requirement)

  • To turn on a PNP transistor, you must apply a negative voltage to the base relative to the emitter.
  • It requires a voltage drop across the base-emitter junction to activate the transistor.
  • In the industrial context (U.S. typical), “common” is framed as 0V, and using that reference enables the transistor.

Switching action (the “invisible wire” concept)

  • When the PNP is active, it behaves like a switch that effectively connects the collector to the emitter/load side, described as an “invisible wire.”
  • Because the transistor is sourcing, it provides positive voltage to the load when turned on.

Output voltage logic (example with 24V)

The summary describes typical output behavior as follows:

  • Active / ON:
    • The load side (collector side) is shown as being at ~24V.
  • Inactive / OFF:
    • The collector side is floating (not forced to 0V).

It also notes a complementary case, where the behavior is essentially reversed (active/floating vs. inactive/24V), depending on how the complementary output is wired.

Why “floating” matters (common mistake)

  • The key emphasis is that when the PNP is inactive, it does not drive the collector to 0V—it floats.
  • Therefore:
    • If you need a true 0V output, the speaker suggests using an NPN sensor/transistor.
    • If you need 24V sourcing, use PNP sensors (described as common U.S. practice).

Real-world wiring/relay explanation (conceptual)

  • The speaker uses a relay-style explanation:
    • Turning on the transistor “pulls in” the relay by applying the correct base potential.
    • Removing the base drive (or applying it in the opposite direction, per the explanation) opens the switch and the load turns off.

Practical note about schematics (base pin may not be drawn)

  • For many proximity/inductive sensors, simplified wiring diagrams may omit internal details.
  • In such diagrams:
    • The base may not be explicitly drawn, because internal electronics generate the base drive circuitry.
  • The practical takeaway remains:
    • A PNP sensor active state effectively connects the load/collector to the positive rail (often 24V).

Tutorial/guide intent

  • Framed as a “how it works” guide specifically for using PNP transistors as switches.
  • It also mentions that transistors can be used as amplifiers, but that topic is intended to be covered separately.

Main Speakers / Sources

  • Single speaker/creator: The instructor speaking through the walkthrough (no other named sources mentioned).

Original video