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INDUSTRIAL AUTOMATION

8-Relay Interface Rack for Water Treatment

A worked industrial automation example, rendered live. Open it in the AI editor and adapt it to your own case.

UPDATED 2026-09-13
TYPECircuit
USE-CASERelay Interface Pnp Npn
EXAMPLE8-Relay Interface Rack for Water Treatment
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CASE ANALYSIS

Scenario

A water treatment PLC uses NPN sinking transistor outputs to energize relay coils, and each relay's NO contact sources +24V PNP to a pump starter input.

Key decisions

  • Use a dedicated 24V_ctrl rail for relay coils and a separate 24V_pnp rail for PNP contact outputs to preserve isolation.
  • Add flyback diodes across each relay coil to protect the NPN PLC output transistors.
  • Assign a 10k load resistor on each pump starter input so the PNP output net has a defined return path.

When to reuse this

Reuse this pattern when interfacing sinking NPN PLC outputs to sourcing PNP field devices through relay isolation in industrial panels.

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