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ELECTRICAL ENGINEERING

2-Relay Interface PNP-NPN Motor Start-Stop

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UPDATED 2026-09-13
TYPECircuit
USE-CASERelay Interface Pnp Npn
EXAMPLE2-Relay Interface PNP-NPN Motor Start-Stop
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CASE ANALYSIS

Scenario

A 24Vdc motor start/stop interface uses a PNP transistor and START pushbutton to energize two relay coils; K1 seals the START command while K2 drives an NPN transistor that switches the contactor coil.

Key decisions

  • K1 seal contact is wired across the START pushbutton to latch the run command until STOP opens the coil path
  • K2 NO contact supplies Q2 base current through R3, keeping the NPN contactor driver isolated from the relay coil circuit
  • D1 across the K3 contactor coil clamps inductive kickback and protects Q2

When to reuse this

Use this wiring when converting a 24Vdc PNP start/stop input into an NPN low-side output for a motor contactor in a control panel.

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