Latching SSRs Simplify Thermostat, HVAC, Security, and Alarm-Panel Contact Switching Designs

Common applications such as thermostats, HVAC systems, fire alarm panels, security systems, building automation, and industrial controls require a simple signal to control the flow of AC or DC power in a supervised adjacent circuit. While electromechanical relays (EMRs) have traditionally supported these applications, many designs increasingly require smaller form factors, higher long-term reliability, greater configurability and functionality, and lower overall noise. Solid-state relays (SSRs) in small IC packages address these needs.

This article explores the challenges of switching power using relays across a range of two and three-wire applications. It then introduces a latching SSR from Littelfuse and shows how it can be used to address these challenges.

Start with a problem that appears simple

Experienced designers know that it is often the basic problem that is among the most difficult to resolve with respect to the technical solution, bill of materials (BOM), printed circuit board (pc board) space, cost, and user experience. A good example is adapting the installed wiring used for the classic two-wire arrangement in homes and other settings to trigger a heating system. This is known as “call for heat” in the HVAC industry.

Historically, systems such as thermostat-controlled heating have been quite simple in design and implementation. A thermostat, such as the classic T-86 (Figure 1), simply closes a switch (metallic or mercury-wetted) when the sensed temperature falls below the setpoint. As a testament to its longevity, tens of millions have been sold since being introduced in 1953, and many are still in use.

Figure 1: Shown is a classic two-wire T-86 thermostat. (Image source: Cooper-Hewitt Museum)

This contact closure, called a “dry” contact, allows 24 V_AC that is stepped down from the AC line to energize the coil of an EMR, which then activates the boiler or other heat source. The thermostat is totally passive, and neither requires nor supplies any power. The relay also provides galvanic isolation between the 24 V_AC thermostat control loop and the AC line that powers the heating system. It is simple, reliable, and easy to troubleshoot.

This long-standing arrangement changed with the arrival of thermostats featuring a digital setpoint setting and temperature readout (Figure 2, left). These were soon followed by smart thermostats with user-controlled day and time settings and then by Internet of Things (IoT) units that added connectivity and greater sophistication (Figure 2, right). The transition from passive to active thermostats introduced a new, unforeseen requirement: a power source. Since the old-style passive thermostat has only two wires, there is no easy way to supply the required power.

Figure 2: The classic switch closure loop cannot supply power to a basic digital thermostat (left) or a connected IoT version (right), raising questions about how to power those loads. (Image sources: PRO1iaq, Ecobee)

This power problem is not unique to legacy thermostats and HVAC systems; it also appears in security systems, building automation, industrial controls, metering applications, and anywhere there is a simple switch closure indicating “activation.”

There are two power-delivery solutions to this dilemma, each with drawbacks. One is to use a replaceable battery in the thermostat, which is inconvenient for both residential and industrial settings. The other is to run a new, third wire to deliver 24 V_AC power to the thermostat. This wire is called the “common” (C) wire.

In many real-world settings, especially in homes, adding a new wire run from the thermostat to the heating system is challenging, involving running and snaking wires, cutting holes in walls, and installing fire stops in wall cavities.

SSR solves the battery and C wire dilemma

Fortunately, a solution is available. The CPC1601M (Figure 3) is an SSR with features designed to address the limitations of the two-wire system.

Figure 3: Shown is the CPC1601M non-isolated, 1-Form-A solid-state latching relay powered by the load. (Image source: Littelfuse)

The CPC1601M is a non-isolated, 1-Form-A solid-state latching relay with low operating current integrated into a miniature 3 mm × 3 mm DFN package with eight contacts. The IC includes a SET input that turns the relay ON; a RESET pin that, when pulsed, turns the relay OFF; and a TOGGLE input that alternately turns the relay ON and OFF.

An important innovative feature is that the CPC1601M relay IC has two power modes and, by monitoring its HV_cc input pin, can obtain its needed operating power either from the open-circuit load or from the system power supply.

The load-powered mode of operation applies to an AC source, such as a transformer with a 24 V_AC secondary-side voltage. When the load supplies power, the relay draws no power from the system supply, thereby extending battery life. The relay periodically opens, allowing it to “harvest” power from the open-circuit load voltage. In most applications, this brief interruption does not affect system operation. No auxiliary power supply is needed in load-powered mode, so a thermostat C lead is not required.

In a typical HVAC system, a thermostat drives a contactor relay (K1). The contactor is usually a high-current EMR that controls the HVAC load. Relay K1 is controlled by turning the CPC1601M relay on and off.

When the CPC1601M is in OFF mode, the full open-circuit voltage from transformer T1 appears across the load’s output pins (RLY1 and RLY2). This AC voltage is rectified by the internal DMOS body diodes (D1 and D2) and the external diodes (D3 and D4), forming a full-wave rectifier. The rectified output is then passed to the filter capacitor (C_FILT), which serves as a reservoir capacitor when operating in load-powered mode.

The CPC1601M adds another power-related feature: it provides a voltage output to power the associated microcontroller unit (MCU) and external circuitry. Furthermore, if this output voltage is within the voltage rail range of the user’s chosen MCU, a separate low-dropout regulator (LDO) may not be required. To protect the switch output against reverse transients when switching an inductive load, a real situation in these applications, a transient voltage suppressor (TVS) diode is placed across RLY1 and RLY2.

In system-power mode of operation (Figure 4), power for the CPC1601M is derived from the power supply and not the load. In a typical thermostat application, the power source is a battery. The extremely low power consumption of the CPC1601M makes it an appropriate choice for applications where extending battery life is critical.

Figure 4: The CPC1601M can also be configured to operate from the system power source. (Image source: Littelfuse)

In this arrangement, the V_CCIN/P_OUT pin of the CPC1601M is connected to the system battery while the HV_CC pin is left open. Here, the CPC1601M acts as a simple latching relay that can be controlled by using SET and RESET, or in the TOGGLE mode.

What about isolation?

Although the basic CPC1601M circuits shown so far do not include galvanic isolation, galvanic isolation is sometimes required to ensure proper system operation, such as in dual-transformer HVAC systems where the transformer returns are separate and isolated from each other. There are many ways to implement isolation, each with its own tradeoffs.

It is easy and cost effective to implement isolation with the CPC1601M using simple capacitive coupling of a pulse width modulation (PWM) signal (Figure 5). The system MCU generates multiple cycles of a PWM signal, which is capacitively coupled through an isolation capacitor (C1). This PWM signal, typically at 200 kilohertz (kHz) with a 50% duty-cycle square wave, is filtered by R2 and C2. This generates a DC signal that triggers the SET input of the CPC1601M.

Figure 5: Galvanic isolation can be implemented by adding a capacitor and a few passive components to the CPC1601M circuit. (Image source: Littelfuse)

Look at key electrical specifications

While it is important to provide efficient functionality, a viable device must also deliver the voltage, current, and other ratings and attributes required by the system. To this end, the CPC1601M features:

• A supply input power voltage of 3 V to 5.5V
• Less than 1 µA of system-powered standby current
• A low typical “on” resistance of 308 milliohms (mΩ)
• TTL/CMOS compatible logic inputs
• Bidirectional, load-connected RLY1 and RLY2 contacts that can be used for 60 V_peak AC or DC operation
• RLY1 and RLY2 contacts that support a continuous load capability of 2 A, AC or DC
• A load-harvesting power pin for powering external circuitry up to 10 mW
• Turn-on time after SET or TOGGLE pulse is applied of 1 µs (maximum); the complementary turn-off time after RESET or TOGGLE pulse is also 1 µs (maximum)
• Reduced electromagnetic interference (EMI) due to switching at zero current in load-powered mode
• Silent operation, as there is no EMR clicking

Conclusion

Updating dry-contact switch closure arrangements, such as those used in traditional passive-thermostat control loops, to now provide power to active thermostats via a local battery or third wire is simple in concept but challenging in practice. An SSR such as the Littelfuse CPC1601M addresses the issues and provides other useful features that enhance system performance and consistency.