Look inside an SSR and we see classic circuitry which includes galvanic isolation, Darlington output, and surge suppression. This engineering brief examines an SSR at the component level. It then addresses fundamental questions about heat dissipation and sinking vs sourcing connection to the load. It shows that a SSR electrically flexible. However, the real-world wiring constraints can favor one circuit configuration over the other.
Key Takeaways
- The small SO6 packaged photodarlington (Figure 1) has little margin for heat dissipation.
- Low input voltage is an understated hazard for an SSR. If the input voltage is not high enough the transistor will not fully turn on.
- Broadly speaking, an SSD designed for galvanic isolation is neither sinking nor sourcing. However, the preferred configuration is often determined at the real-world hardware implementation level.
- The sinking vs sourcing distinction becomes a installation-level distinction. It is not necessarily a label attached to the SSR itself.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Select It → Relays
Difficulty: 
Technician — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 16 Apr 2026
Figure 1: PCB for a WAGO SSR showcasing the TLP127 Photo−Transistor.
Anatomy of a DIN-rail mounted SSR
As a case study, we examine the WAGO 859-794 as shown in Figure 1. This is a DIN-rail-mounted SSR designed to operate within a 24 VDC industrial environment. The internal design is centered on the Toshiba TLP127 Darlington phototransistor. Please refer to the related datasheet for TLP187(TPL,E), which is a newer member of the Toshiba optocoupler family.
A simplified schematic for the WAGO SSR is included as Figure 2. The Darlington pair TLP127 phototransistor is shown in the center.
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To the left is the input circuitry. This includes surge protection from 600 W (peak) STMicroelectronics TVS, a current limiting resistor, indicator diode, capacitor filter, and the photodiode located inside the TLP127.
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To the right is the output circuitry. A second TVS, with a higher voltage rating, is included. A steering diode provides polarity protection for the transistor. Finally, the Darlington pair inside the TLP127 includes an integral flyback suppression diode.
Tech Tip: Observe that the Darlington transistor within the TLP127 has very little ability to dissipate heat. Consequently, it is important to operate the transistor in a fully on (saturated) mode to keep V_{CE} low and minimize waste heat.
The input voltage and input resistor determine the transistor’s conduction. Together, they work to provide optimal current in the input diode, thereby fully turning on the photodarlington. In this application, the series resistance is selected for the 24 VDC nominal input voltage.
Input voltage is an understated hazard for an SSR. If the input voltage is not high enough the transistor will not fully turn on. A transistor operating in its linear range will run hot compared to a saturated transistor leading to reduced transistor life. That is why SSRs are available with different input voltages.
Figure 2: Simplified schematic for the WAGO 859-794 SSR.
Is a DC solid-state relay considered sinking or sourcing?
For an SSR, the distinction between sinking and sourcing is really a question about polarity.
Assuming a conventional current flow (positive to negative), the schematic in Figure 2 demonstrates:
- Input Circuit: Current flows into A1 and exits on A2.
- Output Circuit: Current flows into A and exits on -2.
Note that this says nothing about the placement of the input switch or the output load. In fact, there are four different ways to utilize the WAGO SSR:
- Switch input on A1, load connected to A
- Switch input on A1, load connected to -2
- Switch input on A2, load connected to A
- Switch input on A2, load connected to -2
The SSR is neither intrinsically sinking nor sourcing.
For clarity, the remaining portion of this article assumes an A1 input with terminal A2 connecte to the 24 VDC return. This allows us to focus on the sinking (Figure 3) and sourcing (Figure 4) output configurations.
Sinking Output Configuration
Figure 3 presents a wire diagram showing the SSR connected in a sinking output configuration. By definition, a sinking configuration has one side of the load permanently attached to the voltage rail (24 VDC) and the SSR sinks the load current to the return (ground) rail.
As a reality check, ensure that the flow of current is correct for both the input and output circuits. The input circuit is valid as current flows into A1 and exits A2. The output circuit is also valid as current enters A and exits on the -2 terminal.
Tech Tip: The term “ground” is a source of confusion in 24 VDC industrial control panels. Many designers choose to float the 24 VDC supply. What we loosely consider “ground” is better described as “the return for the 24 VDC supply” or simply “return.” We see this in Figures 3 and 4 where the right-hand rail is labeled “return.”
At the same time, we recognize that earth safety ground (PE) is present in the control panel. All exposed metal surfaces are bonded to ground.
Figure 3: SSR configured in a sinking configuration with one end of the load attached to 24 VDC.
Sourcing Output Configuration
Figure 4 presents the wire diagram for the SSR configured for sourcing. In this case, one end of the load is permanently attached to the return rail while the current is supplied by the SSR.
Figure 4: SSR configured in a sourcing configuration with one end of the load attached to the return rail.
When does an SSR installation favor sinking or sourcing?
The socket design suggests a preferred way to connect the SSR. In this particular example, the sinking configuration is strongly favored.
Background
We have established that the WAGO SSR is neither sinking nor sourcing. However, that is only part of the answer, as we need to integrate the SSR into a larger system. This requires us to think in terms of wiring, adherence to common technician expectations (ease of maintenance), and use of accessories to efficiently connect multiple side-by-side SSRs. WAGO calls this commoning. For example, we could connect the -2 terminal for multiple SSRs using a WAGO 859-404 push-in-jumper.
Support
That brings us to the fundamental distinction, not all terminals allow jumpers. For the WAGO 859-794 only the -2 and the A2 terminals allow jumpers. Discrete wires are required for all other connections. We see the jumper connections in Figure 6 as cup-like symbols.
Knowing that the -2 output terminals may be bussed together casts this representative SSR cleanly into the sinking category. Back to Figure 3, we see that a sinking configuration ties the -2 terminal to return rail. If we add commoning jumpers, we can instantly form a node with all the SSRs. A single wire may then be used to connect the entire assembly to return. Consequently, the SSR and accessories push the designer to use a bank of sinking SSRs.
We see a similar approach on the input side where all of the A2 terminals may be connected to a common return.
This preference for the sinking configuration also holds for a free-standing SSR. In this case, we recognize the efficiency of installing a short jumper between the A2 input terminal and one of the -2 output terminals. This is an efficient use of connecting wire as only three long wires are required to integrate the SSR into the larger system. As a counterexample, we know something is off in Figure 4, as one of the -2 connections is unused. The implication is that one additional long wire is required when compared to Figure 3.
Limits of the Sinking vs Sourcing discussion
Throughout this discussion we have framed the SSR within a single voltage domain. This is clear as both input and output circuits are returned to the common right-hand side rail.
This is limiting as we have abandoned one of the greatest advantages of the SSR, namely galvanic isolation. But then again, there is nothing that says we need to connect the jumper between A2 to -2 as shown in Figure 3. The input domain and the output voltage domain can be different.
This distinction does not change the practical conclusion that socket plus jumper are biased toward the sinking configuration.
Figure 5: Push-in connectors for “commoning” the WAGO SSR.
Figure 6: Close up image of the WAGO 859-794 showing the jumper connection (cup like symbols).
Parting Thoughts
A binary classification of an SSR as either sinking or sourcing is too simple. While the SSR may be used in either configuration, it is instructive to look deeper into how the SSR will be used in the circuit. Sometimes small implementation details like commoning via jumpers can crystallize the OEM’s intended use for the SSR.
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.