Divide the HVAC System Into Protection Zones

The service entrance and disconnect see lightning-induced and utility-borne surges. The compressor and contactor generate local inductive transients. The 24 VAC transformer, rectified DC rails and thermostat wiring each have different normal voltage peaks and source impedances. Treating them as one node usually leads to either nuisance conduction or weak protection.

Create a boundary at the AC input, another at the low-voltage supply, and separate boundaries at every external control or communication connector. This makes it possible to use a high-energy MOV where power enters and a lower-voltage TVS near the controller without asking either device to perform the other job.

Select the Mains MOV From Continuous Voltage and Heat

For 120 VAC equipment, the YINT 7D241K offers a compact MOV option with a 150 VAC maximum continuous rating for moderate-energy locations. Larger installations or harsher surge environments can move to 10 mm or 20 mm discs. The YINT 20D471K is a high-energy candidate for 230 VAC-class inputs because it is rated for 300 VAC continuous operation.

The enclosure temperature, repetitive switching duty and temporary overvoltage determine the real margin. Install the MOV after appropriate overcurrent protection, away from hot refrigerant lines and power resistors, and include a thermal safety strategy when the product standard requires it.

Protect 24 V Controls Without Confusing AC and DC Ratings

A 24 VAC transformer can produce a peak above 33 V before tolerance and regulation are added, so a 24 VDC TVS or 33 V PTC cannot be placed blindly across the raw AC winding. Measure the actual waveform and choose a bidirectional clamp whose stand-off remains above the highest normal peak.

After rectification and regulation, YINT SMBJ or SMCJ TVS families can protect the controller DC rail. Resettable fuses from the YINT SMD2920 family can limit fault current in thermostat, accessory or communication power feeds when their voltage and temperature-derated hold current fit the circuit.

Layout Around the Contactor and Compressor Current

Keep the AC surge loop at the line-entry area and separate it from the thermostat ground and sensor reference. Snubbers or suppression across contactor coils should be located at the coil, while the controller TVS belongs beside the connector or rail it protects. Long shared ground traces can turn a clamped motor transient into a controller reset.

For outdoor units, use short connections to the equipment bonding point and maintain creepage around mains devices. Route external thermostat and communication cables away from compressor and fan motor conductors where practical.

Test the Conditions That Cause Field Returns

Perform surge immunity testing at the specified mains phase angles, then cycle the compressor, blower, reversing valve and contactor while monitoring the control rails. Check thermostat hot-plugging, brownout recovery and communication errors. Re-run functional tests at low and high temperature because MOV leakage, PTC trip behavior and power-supply margin all change with temperature.

Measure the voltage directly at the microcontroller and relay-driver rails, not only at the transformer output. A short disturbance can reset firmware without leaving visible damage. Run the complete thermostat cable length and representative accessories during the test, since wiring inductance and third-party loads often explain the difference between a laboratory pass and a field failure.

YINT Models by HVAC Circuit Location

HVAC assemblies combine mains power, inductive loads, low-voltage controls and external wiring. Assign a device to each electrical domain instead of placing one MOV across the entire product.

FAQ

1.Is one HVAC surge protector enough for the whole system?

A service-entry protector reduces incoming energy, but local MOV, TVS and interface protection are still valuable inside sensitive equipment.

2.Can a 24 V TVS be used directly on 24 VAC?

Not automatically. The AC peak, transformer regulation and tolerance may exceed the TVS stand-off voltage. Select from the measured maximum waveform.

3.Why does a controller still reset when the MOV survives?

The surge path may share inductance with the control ground, or the residual voltage may still exceed the controller limit. Improve placement, grounding and secondary clamping.

4.Which YINT parts fit HVAC designs?

YINT 7D, 10D and 20D MOVs address mains transients, SMBJ and SMCJ TVS families protect regulated rails, and SMD2920 PPTCs support resettable branch protection.

Summary

Build a YINT Protection BOM for the Complete HVAC Assembly

YINT enables HVAC designers to coordinate mains surge absorption, low-voltage clamping and resettable overcurrent protection from one product portfolio. The result is a quieter control board, fewer nuisance resets and a stronger reliability message for residential and commercial equipment. To discuss a configuration for your project, you are welcome to contact YINT Electronics at global@yint.com.cn for model-selection and sample support.