How to design ultra-low-power smart thermostats without a C-wire

By Barry Brents, Littelfuse, Inc.

Load-powered latching relay architecture enables battery-free HVAC control designs while reducing system complexity, size, and power consumption.

Designing around power constraints, not features

Despite advances in connectivity and embedded processing, one limitation continues to constrain smart thermostat design: power delivery.

Many HVAC systems in the field still rely on two-wire connections originally intended for mechanical thermostats used as series switches in a control line. These systems provide no dedicated power path, forcing engineers to either depend on batteries or require installation of a common wire when including a smart thermostat in a system—both of which introduce tradeoffs in cost, maintenance, or usability.

The challenge is not simply reducing power consumption. It is designing a control system that can operate reliably with almost no available energy, while remaining compatible with legacy infrastructure and supporting modern functionality.

In smart thermostat design, the limiting factor is rarely processing capability or connectivity—it is power availability. Many residential and light commercial HVAC systems still rely on two-wire connections originally intended for simple electromechanical thermostats. These installations lack a dedicated common wire, making it difficult to supply continuous power to modern electronics.

For engineers working in home and building automation, this constraint reshapes the entire system architecture. Every component—from the relay to the microcontroller—must be evaluated not just for performance, but for how it contributes to the overall energy budget. In many cases, the relay selection becomes the defining factor, since traditional switching approaches can consume more power than the rest of the system combined.

A more scalable approach is to rethink how energy is sourced and consumed at the system level.

Why relay selection defines the power budget

Relays remain central to HVAC control because they switch inductive loads such as compressor and fan contactors. However, traditional electromechanical relays impose a constant energy penalty. Their coils require continuous current to remain energized, often consuming tens of milliamps. In a system with limited available energy, that requirement alone can exceed the power budget.

Beyond power consumption, mechanical relays introduce audible noise, occupy more PCB space, and are subject to wear over time. These limitations make them poorly suited for modern, low-power, maintenance-free control systems.

Moving to latching solid-state relay architectures

A latching solid-state relay changes the system-level power model. Instead of requiring continuous current, it uses a short control pulse to switch states and then maintains that state without additional power draw.

Devices such as the CPC1601M solid-state latching relay integrate switching elements, control logic, and power management into a compact semiconductor package. From a design perspective, this consolidation can reduce component count and board space, and may improve reliability by eliminating mechanical wear mechanisms.

At the electrical level, the relay supports standard HVAC switching requirements, including load currents up to 2 A and blocking voltages compatible with 24 VAC systems. The low on-resistance of typically 308 mΩ minimizes conduction losses, allowing efficient switching of inductive loads.

Control simplicity and system integration

From the standpoint of embedded system design, the relay behaves like a digital component. Control is handled through SET, RESET, and TOGGLE inputs, which are compatible with standard TTL or CMOS logic levels. This allows direct interfacing with microcontrollers without additional driver circuitry.

The absence of hold current fundamentally changes system power consumption. With a standby current below 1 µA, the relay effectively removes switching as a major contributor to overall energy usage. This is especially important in systems where every microampere impacts operational lifetime or feasibility.

Load-powered operation: a system-level shift

A key feature is the ability to operate in a load-powered mode, where the control system derives energy from the HVAC circuit itself.

When the relay is open, the full transformer voltage appears across its terminals. This voltage is rectified and used to charge an external filter capacitor, which then powers both the relay and associated electronics. Once sufficient energy is stored, the relay closes and allows current to flow through the load.

As the capacitor discharges, the relay briefly opens to recharge, then closes again. This cycle repeats continuously.

From an energy perspective, this approach effectively converts the HVAC control loop into a low-power energy-harvesting system, where switching and power delivery are tightly coupled.

Why intermittent operation does not affect HVAC loads

At first glance, periodic relay opening may appear problematic. However, HVAC loads such as contactor coils respond slowly due to mechanical inertia. Short interruptions—typically well under one second—do not affect their operation.

From the system perspective, the load experiences continuous power, while the control circuit opportunistically harvests energy during brief off cycles. This enables a stable operating condition without requiring a dedicated power source.

The external filter capacitor determines the timing of this cycle. Larger capacitance extends the interval between recharge events and reduces voltage ripple but trades off increased size. Selecting the appropriate value is therefore a key design decision.

With a stable source of harvested energy available, the system no longer depends on external power wiring.

Eliminating the C-wire without compromise

The ability to harvest energy from the load circuit directly addresses one of the most persistent challenges in thermostat design: the need for a C-wire.

In conventional smart thermostats, a third wire is required to provide continuous power. Installing this wire often involves running new cable through walls, increasing cost and complexity.

With a load-powered architecture, the thermostat operates using only the existing two wires. This preserves compatibility with legacy installations while enabling modern functionality. For product designers, it also simplifies system architecture by eliminating batteries and associated charging circuits.

Practical design considerations

While the architecture simplifies many aspects of system design, several considerations remain important.

The size and selection of the filter capacitor directly influence system behavior, including recharge frequency and voltage stability. Engineers must balance electrical performance with physical constraints, particularly in compact thermostat designs.

Thermal performance must also be evaluated. Although low on-resistance reduces power dissipation, switching inductive loads still generates heat that must be managed within the system’s operating temperature range.

Zero-cross switching further improves system robustness by minimizing inrush current and electromagnetic interference. By switching at or near zero current, the relay reduces electrical stress on both it and the load.

Finally, the ability to operate in either load-powered or system-powered mode adds flexibility. A single design can support multiple product variants, simplifying development and inventory management.

Extending the architecture beyond thermostats

Although thermostat design provides a clear use case, the same principles apply across a wide range of building automation systems.

In fire alarm and safety systems, ultra-low standby current extends backup battery life. Security systems benefit from improved reliability and silent operation. Distributed building controls can reduce wiring requirements by harvesting power locally, simplifying installation in large facilities.

Industrial control systems also benefit from compact, vibration-resistant switching solutions, while utility metering applications take advantage of low power consumption for demand-response control.

Across these applications, the common requirement is efficient, reliable switching in environments where power and space are constrained.

Redefining power in control systems

This approach also aligns with broader trends in building electrification and IoT deployment, where distributed control nodes must operate autonomously while minimizing installation complexity. Designs that reduce wiring dependencies and eliminate maintenance requirements are increasingly favored in both residential and commercial environments.

Ultra-low-power thermostat design is no longer just about reducing consumption. It requires rethinking how energy is sourced and managed within the system.

Latching solid-state relays with load-powered operation provide a practical solution by eliminating continuous current draw and enabling energy harvesting directly from the load circuit. This approach allows engineers to design battery-free, two-wire-compatible systems without sacrificing performance.

As building automation systems continue to evolve, these architectures offer a scalable path toward smaller, more efficient, and maintenance-free control solutions.