A non-isolated SSR solves a not-so-simple “simple” power problem

Providing power via contact-closure circuits historically required a new third wire, but perhaps no longer.

Say the words “solid-state relay” (SSR) and most engineers also naturally think of two unspoken adjectives: “optical” and “isolation” (although the galvanic isolation can also be implemented using magnetic, capacitive, RF, or other techniques).

But that doesn’t have to be the case, as an SSR can also be non-isolated as well as non-optical. An example is a small IC introduced by Littelfuse, Inc. in late 2025: the CPC1601M, a 60 V, 2 A normally open (1-Form-A) solid-state latching relay targeting critical integration and power challenges in thermostat, HVAC, and building automation wiring (Figure 1).

Figure 1 The block diagram of the Littelfuse CPC1601M solid-state latching relay shows its basic input and output connections as well as internal function blocks. (Image source: Littlefuse)

What’s the problem here that needs solving? It’s largely a legacy issue and one that sounds simple enough – but it’s not.

Consider the classic two-wire thermostat still in use in millions of homes. It’s simple, reliable, and easy to troubleshoot. These thermostats provide a “dry” contact closure to call for heat when the sensed temperature drops below their setpoint. The heating system provides 24 VAC to this contact-closure loop via an AC-line transformer; when the circuit is closed, the 24 V energizes the coil of an electromechanical relay that turns on the 120 V/240V heating system.

Note: Dry contacts have a power source going through them that is independent of the power in the circuit they are controlling (often done by a relay). In a “wet” circuit, the controlling switch or element is directly handling the full load current and voltage. A standard wall light switch is a wet circuit, as that switch handles the 120 VAC that goes to the light bulb. The terms “wet” and “dry” are holdovers from the pre-electronics days of electricity when wet electrochemical cells were used as higher-voltage batteries.

But there’s the problem with this elegantly simple two-wire dry scheme: when the homeowner wants to upgrade from the unpowered contact-closure unit to a better thermostat with digital readout or a smart Wi-Fi-enabled thermostat, that thermostat needs a power source. However, there is no power source in the open loop that thermostat controls: when the loop is open, there is no current flow.

In many such “upgrade” situations, the unpleasant solution is to run a new third wire for needed power, designated as the “common” or “C” wire. (Note that this “common” in unrelated to what electronic circuit designers called circuit common, as the HVAC industry has its own terms and designations.

Of course, running that third wire can be difficult, especially in a house with multiple floors and rooms. It often involves cutting openings in the walls to snake the wire around obstacles such as framing, fire stops, wiring conduits, and plumbing.

Littelfuse maintains that CPC1601M is the first PCB-mounted solid-state relay of its kind that combines load-powered operation with a latching architecture in a 3 × 3 mm DFN IC package. It can harvest operating power directly from the load or draw less than 1 μA from the system supply, thus enabling zero-power operation while dramatically extending battery life or eliminating the need for batteries altogether.

Solving missing-power challenge, CPC1601M relay can obtain operating power from the open-circuit load or system power supply. When power is supplied by the load, the relay opens periodically to obtain power via the open-circuit load voltage. In most applications this very short interruption is transparent to the load (Figure 2).

Figure 2 In basic load-powered mode, relay K1 is controlled by turning the CPC1601M relay on and off. (Image source: Littlefuse)

Its use is not limited to thermostats; it is also a viable solution to upgrading other contact-closure designs such as fire-control panels, security systems, and building automation subsystems.

What about the lack of galvanic isolation? That’s easy: it’s not needed here. The electromechanical relay that activates the heating system provides the needed isolation between the thermostat control loop and the 120/240 VAC heating system power (relays easily provide thousands of volts of isolation).

If you do need galvanic isolation – a requirement in dual-transformer HVAC systems where the transformer returns are separate and isolated from each other – it can be implemented with the addition of a few capacitors providing capacitive coupling of a PWM signal (Figure 3).

Figure 3 In the galvanically isolated configuration, the system microcontroller generates several multiple cycles of a PWM signal that is capacitively coupled by isolation capacitor C1. This PWM signal is filtered by R2 and C2 thus creating a DC signal that is used to trigger the SET input of the CPC1601M. (Image source: Littlefuse)

Additionally, the CPC1601M provides a power output pin that can supply external circuits with a maximum of 10 mW of power. The CPC1601M can sense whether it is powered by the load or by the system power supply automatically by monitoring the HV_CC input pin. The load-powered mode of operation applies to an AC source, such as a 24 V_AC transformer secondary voltage.

If all this seems confusing – and even simple circuits can be, depending on context – Littelfuse offers the LEB-0024 Evaluation Board with full documentation (Figure 4). The kit includes input and load-circuit terminal blocks along with switches for mode selection and manual relay operation.

Figure 4 The LEB-0024 Evaluation Board makes it easy to “play around” with the CPC1601M to better understand its functions in each application. (Image source: Littlefuse)

Have you ever had to deal with an upgrade issue where a conceptually simple requirement such as “just add another wire” had a ripple effect with respect to design-in, installation, bill of materials, or retrofit issues? How did you resolve these challenges?

References

Worth HVAC Training, “What are ‘Dry Contacts“
Control by Web, “Understanding Dry Contacts and How to Monitor Them”
Electrical4U, “Dry Contacts: What is It?”
High Performance HVAC, “How to Wire a Thermostat | HVAC Control”
High Performance HVAC, “Running New Thermostat Wire | HVAC Heating and Cooling”
High Performance HVAC, “Programmable Thermostat Wiring Diagrams | HVAC Control”

—Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors and signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing, and he also developed significant mechanical-engineering insight while designing control electronics for large materials-testing systems.

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