General Assist Technical Reference
Posistor Check
PTC thermistor operation and diagnostic guidance
A posistor is a positive temperature coefficient (PTC) resistor used to protect electronic circuits and components during power-up and abnormal DC circuit conditions.
What does the posistor do?
During initial power-up, the PTC bypasses the main power relay and allows a low current to flow to the circuit boards.
| 1 | Initial power-up: The PTC permits low current flow to the circuit boards while the main power relay is bypassed. |
| 2 | Fault condition: If a DC circuit fault causes excessive current during power-up or operation, the ceramic PTC heats rapidly. |
| 3 | Protection: As the PTC temperature increases, it may go open circuit and interrupt the current path. |
| 4 | Recovery: Once the cause of the fault is rectified, the PTC cools and returns towards ambient temperature. |
| 5 | Normal operation: If no circuit abnormality is detected during start-up and operation, the main relay closes and supplies line voltage to the circuit boards. |
Circuit reference
| Item | Reference shown |
|---|---|
| PTC thermistor | ZPROYCE400A300 |
| Connection | W25 (B) to W26 (B) |
| Lead specification shown | UL3271, AWG20, orange |
Diagnostic approach
If the posistor is found open circuit or becomes very hot during start-up, treat this as an indication that the PTC may be reacting to an abnormal current condition. The source material indicates that the underlying DC circuit fault should be identified and rectified before normal operation is expected to resume.
| 1 | Isolate power before inspecting the circuit or component. |
| 2 | Inspect the PTC and associated wiring for visible signs of overheating, damage or poor connection. |
| 3 | If the PTC has operated, investigate the DC circuit for the fault condition that caused excessive current. |
| 4 | After the fault is corrected, allow the PTC to cool back towards ambient temperature before reassessing operation. |
| 5 | On normal start-up, confirm the main relay closes and supplies line voltage to the circuit boards. |
Safety: This procedure relates to powered electronic circuitry. Testing and diagnosis should only be carried out by appropriately qualified service personnel using suitable electrical safety procedures.
General Assist technical reference — Posistor / PTC thermistor check.