Compressor Starting Relays and Capacitors) PTC Thermistor Starting Relays

 

Characteristics and Operation

• Solid State Device: No moving parts, highly reliable.
• Positive Temperature Coefficient: Resistance increases with temperature.
• Heat Generation: Current flow through the device generates heat, increasing resistance.
• Wiring: Wired in series with the start winding.
  • Resistance increase stops current flow to the start winding, removing it from the circuit.

Construction and Application

• Ceramic Disk: Acts as a switch due to its resistance variation.
• Horsepower Range: Fits a wide range (e.g., 1/20 to 1/2 horsepower).
• Current Flow:
  • Initial: Full current through the cold PTC relay, allowing startup.
  • Heating: Resistance increases, stopping current flow to the start winding.

Cutaway View and Physical Wiring

• Ceramic Disk: Held by wire springs, acting as conductors.
• Parallel Wiring: Typically wired parallel to the run capacitor, in series with the start winding.
• Full Voltage and Current: Passed at startup.
• Low Initial Resistance: Below 100 ohms when cold.

Sequence of Operations

• Startup: Full voltage and current to the start winding.
• Heating: Resistance increases within a second, stopping current flow to the start winding.
• Running: Disk remains hot due to bleed current, keeping start winding out of the circuit.
• Cooling: Disk cools down in 3-5 minutes after compressor stops, ready for next start.
• Modern Relays: Faster cool down, within seconds.

Diagnosis

• Turn Off Power: Allow to cool for 3-5 minutes.
• Measure Resistance: Disconnect and measure resistance across the PTC relay.
  • Cold Resistance: Typically below 100 ohms.
  • Faulty Relay: Resistance higher than specified, often fails to open (infinite resistance).

Troubleshooting

• Cold Resistance Value: Stamped on the device or in model number.
• Measurement Tolerance: Should be within 10% of stamped value.
• Failure Mode: Typically fails to open, not shorted.