Refrigerator Compressor Wiring Diagram: C, S, R Terminals, PTC Start Relay and Overload Protector
This is a free printable refrigerator compressor wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A refrigerator compressor wiring diagram maps the mains supply through the overload protector and PTC start relay to the Common, Start, and Run terminals of the hermetic compressor motor.
The compressor in a domestic refrigerator is a hermetically sealed single-phase induction motor driving a refrigerant pump. The motor windings are inaccessible once sealed but their terminals are brought out through the compressor body on three pins labelled Common (C), Start (S), and Run (R).
The Run winding (between C and R) has a lower resistance — it is a heavier-gauge winding designed for continuous operation. The Start winding (between C and S) has a higher resistance — it provides additional phase shift at startup to give the motor sufficient starting torque. Once the motor reaches operating speed, the start winding must be disconnected, because leaving a high-resistance winding energised indefinitely causes overheating.
Two protective and starting devices are fitted externally to the compressor terminals.
The overload protector (also called a Klixon, after a common trade name) is a thermal bimetal disc device that breaks the Common terminal connection if the motor draws excessive current or if the compressor body temperature rises above a safe limit. The overload is mounted in direct thermal contact with the compressor body. When its bimetal disc reaches the trip temperature, it snaps open, disconnecting the motor until it cools and resets automatically.
The PTC (positive temperature coefficient) start relay replaces the older electromagnetic current relay in most modern refrigerators. A PTC relay is a thermistor — a resistor whose resistance increases sharply (by several orders of magnitude) when its temperature rises above a transition temperature. The PTC device is connected between the Run terminal (R) and the Start terminal (S) in series with the start winding. At the moment of startup, the PTC is cold and its resistance is low, allowing full current through the start winding to provide starting torque. Within 0.5–1 second of energisation, the PTC self-heats to its transition temperature and its resistance rises dramatically, effectively disconnecting the start winding from the circuit. The motor then runs on the run winding alone.
Standard wiring sequence: mains live connects to the overload protector pin. The overload output connects to the Common (C) terminal. The Run (R) terminal connects to the mains neutral. The PTC relay connects between R and S, with its body plug mounted on the compressor terminal pins.
A refrigerator compressor wiring diagram shows the hermetic compressor motor connected via a start relay and overload protector to the mains supply, with the thermostat (cold control) switching the circuit on and off to maintain cabinet temperature. Understanding this wiring is essential for diagnosing common faults such as a compressor that hums but does not start (failed start relay), one that trips on overload (seized motor or dirty condenser), or a unit that runs continuously (faulty thermostat). You can draw a refrigerator compressor circuit diagram free in the online diagram editor without any software to install.
How to wire refrigerator compressor wiring diagram
- Identify the compressor terminals Remove the compressor terminal cover (the plastic housing on the compressor body containing the pins). Identify or measure the C, S, and R terminals as described above using a multimeter in resistance mode. Photograph the terminal positions and existing wiring before disconnecting anything.
- Check the overload protector Disconnect the overload protector from the Common terminal. At room temperature, measure resistance across the overload protector terminals with a multimeter set to continuity. It should read near-zero (closed). If it reads open circuit at room temperature, the overload has failed closed-trip and must be replaced.
- Check the PTC start relay Remove the PTC relay from the compressor terminals. Shake it — no rattle should be heard. Measure resistance across the PTC relay terminals at room temperature: expect a low resistance (typically 5–30 Ω). If open circuit when cold, or if it rattles, replace the relay. Do not attempt to test the PTC when hot — it will read open circuit normally at elevated temperature.
- Measure compressor winding resistance With the PTC relay and overload protector removed, measure resistance between C–R (run winding), C–S (start winding), and S–R (both windings in series). Typical values vary by compressor rating — smaller domestic compressors might show C–R ≈ 10–20 Ω and C–S ≈ 20–40 Ω. An open circuit on any winding indicates a failed motor. No repair is possible for an open motor winding in a hermetic compressor — the compressor must be replaced.
- Reconnect the circuit correctly Connect the overload protector to the Common (C) terminal. Connect the mains live supply wire to the overload protector's external terminal. Plug the PTC relay body onto the compressor terminal block — the relay bridges the R and S terminals. Connect the mains neutral to the Run (R) terminal. Confirm the earth conductor is connected to the compressor mounting stud or chassis earth point.
- Power up and verify operation Restore mains power. The compressor should start within a few seconds. Listen for the sound of the motor starting and running. The compressor should not cycle repeatedly on and off within a short time — repeated short cycling indicates an overload trip, which may be caused by a low refrigerant charge, excessive head pressure, or a marginal motor.
Specifications
| Mains supply voltage (North America) | 120 V, 60 Hz |
|---|---|
| Mains supply voltage (UK / EU / AU) | 230 V, 50 Hz |
| Terminal identification — Common (C) | Shared connection for run and start windings; mains supply through overload |
| Terminal identification — Run (R) | Run winding terminal; connected to mains neutral |
| Terminal identification — Start (S) | Start winding terminal; connected to R via PTC relay at startup |
| PTC relay cold resistance (typical range) | 5–30 Ω (device dependent) |
| Run winding resistance (C–R), small domestic compressor | 10–20 Ω (device dependent) |
| Start winding resistance (C–S), small domestic compressor | 20–40 Ω (device dependent) |
Safety warnings
- Refrigerator compressor wiring is connected directly to mains supply voltages (120 V or 230 V). Always disconnect the refrigerator from the mains socket before removing the terminal cover or making any measurements or connections at the compressor. Verify dead with a non-contact voltage tester.
- Hermetic refrigerant systems are pressurised with refrigerant gas. Do not pierce, cut, or attempt to open the compressor body or refrigerant lines. Release of refrigerant is environmentally harmful and may be illegal without appropriate certification. All refrigerant handling must be performed by a certified refrigeration technician.
- The compressor body, terminal cover components, and the refrigerant lines may be very hot if the compressor has been running. Allow adequate cooling time before handling any components.
- A failed compressor that cycles on and off repeatedly (short cycling) may indicate a refrigerant undercharge. Running a compressor significantly undercharged with refrigerant can cause the motor to overheat due to insufficient cooling from the refrigerant vapour passing through the motor.
Tools needed
- Digital multimeter (resistance and continuity)
- Non-contact voltage tester
- Flat-blade screwdriver (for terminal cover removal)
- Needle-nose pliers (for connector handling)
- Clamp ammeter (to measure running current for diagnosis)
Common mistakes
- Shaking and hearing a rattle in a PTC relay but reinstalling it anyway — a cracked PTC element will fail to develop sufficient resistance during startup, leaving the start winding energised and causing it to overheat.
- Measuring PTC relay resistance when the relay is still warm from a recent run cycle, observing open circuit (which is normal when hot) and incorrectly concluding the relay has failed.
- Connecting the overload protector to the Start (S) terminal instead of Common (C), leaving the run winding unprotected.
- Omitting the earth (protective ground) connection to the compressor chassis, leaving the metal compressor body unearthed and creating a shock hazard.
- Replacing the PTC relay with a relay of significantly different cold resistance from the original, causing the start winding to be either under-energised (insufficient starting torque) or over-energised (excessive current and heat) at startup.
Troubleshooting
- Compressor hums briefly then goes silent, cycling on and off every few minutes
- Cause: The compressor motor is failing to start because the PTC relay has failed (open circuit when cold, preventing start winding energisation) or the overload protector is tripping due to excessive starting current, indicating a hard-start condition. Fix: Test the PTC relay cold resistance (should be low, typically 5–30 Ω) and listen for rattling (indicates cracked element). Replace if failed. Test the overload protector for continuity at room temperature. If both are intact, the problem may be a seized compressor or low refrigerant charge requiring a refrigeration technician.
- Compressor runs continuously but refrigerator does not cool
- Cause: The compressor is mechanically running but not pumping refrigerant — typically caused by refrigerant undercharge (leak in the refrigerant system) or a failed compressor pump mechanism. Fix: This requires a certified refrigeration technician. Do not attempt to add refrigerant without the correct equipment and certification. The technician will check the refrigerant charge, locate any leaks, and determine whether the compressor pump is functional.
- Compressor does not start at all — no hum
- Cause: Open circuit in the mains supply wiring, a failed overload protector in the open position, a failed compressor motor winding (open circuit), or a failed thermostat or defrost timer that is not calling for compressor operation. Fix: Verify mains voltage is reaching the compressor terminal (use a multimeter at the supply wiring with mains connected — perform live electrical testing with extreme caution). Measure overload protector continuity. Measure winding resistances at C, S, R terminals. If all supply and wiring checks are correct and windings are open circuit, the compressor motor has failed and the compressor must be replaced.
Frequently asked questions
What do the C, S, and R terminals on a refrigerator compressor represent?
C is Common — the shared terminal for both windings, connected to the mains supply through the overload protector. S is Start — the terminal of the high-resistance start winding, connected to the mains supply through the PTC start relay (which temporarily allows current to energise the start winding at startup). R is Run — the terminal of the low-resistance main run winding, connected to the neutral return of the mains supply.
How do I identify the C, S, and R terminals if they are not labelled?
Measure the resistance between all three pairs of terminals with a multimeter. The highest resistance reading is between S and R (Start winding + Run winding in series). The lowest reading is between C and R (Run winding alone). The intermediate reading is between C and S (Start winding alone). The terminal common to both the lowest and intermediate readings is C.
What is a PTC start relay and how does it work?
A PTC (positive temperature coefficient) relay is a ceramic thermistor with a low resistance at room temperature and an extremely high resistance above a transition temperature. At startup it is cold, so it passes current through the start winding to boost starting torque. Within about half a second of energisation it self-heats past its transition point and its resistance increases by several orders of magnitude, effectively opening the start winding circuit. The motor then runs on the main (run) winding only.
How do I test a PTC start relay?
Shake the PTC relay next to your ear — a rattling sound indicates the internal ceramic element has cracked and the relay must be replaced. Measure cold resistance with a multimeter set to resistance: a PTC relay should read a low resistance at room temperature (typically 5–30 Ω depending on the motor). If it reads open circuit when cold, it has failed and must be replaced.
What is a Klixon overload protector?
Klixon is a common trade name for a bimetal disc thermal overload protector used in compressor applications. The bimetal disc is sensitive to both compressor body temperature and the current flowing through it. If either excess current or excess temperature is detected, the disc snaps open, breaking the circuit. When the compressor cools, the disc snaps back closed and the circuit resets automatically.
What does a wiring diagram for a refrigerator compressor include?
A wiring diagram for a refrigerator compressor shows mains supply feeding through the thermostat (cold control), then through the overload protector clipped to the compressor body, to the start relay and finally to the compressor motor's run and start windings. The start relay — either a current-sensing relay or a PTC (positive temperature coefficient) type — connects briefly in series with the start winding to boost starting torque, then drops out once the motor reaches operating speed. The overload protector is a bimetallic thermal switch that opens the circuit if the motor draws excessive current or overheats, protecting the compressor from damage.
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