An interposing relay (sometimes called an interface relay or isolation relay) is essentially a relay used to interface two circuits that have different electrical characteristics (e.g. different voltages, current levels, or common references). It “interposes” itself between a control circuit (e.g. PLC, sensor output) and a load or another circuit that may not be directly compatible.
Key roles include:
- Electrical isolation between circuits
- Voltage and current level translation (amplification)
- Protecting sensitive control electronics from spikes or faults in the load circuit
- Allowing use of a relay as a “buffer” so that damage happens on the relay rather than on expensive control electronics
Article Content
Why / When Use an Interposing Relay
Here are common motivations and use cases:
| Reason | Description |
|---|---|
| Voltage mismatch | If your control circuit (say 24 VDC) cannot directly drive a load circuit (say 120 VAC), you use an interposing relay whose coil works at 24 VDC and whose contacts switch 120 VAC. |
| Current mismatch / load handling | The control output may not be able to source/sink the current demanded by the load; the interposing relay can handle the heavier load. |
| Protection / isolation | To protect the control device (PLC, sensor, etc.) from voltage spikes, transients, faults, or EMI generated in the load circuit. The interposing relay acts as a buffer. |
| Panel segregation / safety / wiring clarity | For practical layout: isolating sections so that AC/DC circuits are separated, or to make maintenance safer. |
| Improving switching performance / contact life | In some cases, a small relay (interposing) switching a bigger device can improve speed or reduce stress compared to directly driving the load. |
| Modularity & easy replacement | If the relay gets damaged, it is often easier to replace the interposing relay module than repairing the control card. |
How It Works — Basic Principle
The interposing relay has two “sides”:
- Control (input) side: a coil that is energized by the control circuit (low voltage, low current)
- Load (output / contact) side: contacts (normally open, normally closed, etc.) which switch the load circuit (which may be higher voltage or current)
When the coil is energized, the contacts actuate, thereby making or breaking the load circuit. Because the control and load circuits are separate (only magnetically coupled via the relay), you get galvanic isolation.
In circuits with inductive loads, you often include a flyback (commutating) diode across the coil to safely dissipate the stored magnetic energy when the coil is de-energized. Otherwise, the collapsing magnetic field generates a high voltage “kickback” which can damage the driving electronics.
Also, the contacts are rated for the voltage/current of the load side, while the coil is rated for the control side’s conditions. So design requires matching coil vs contact ratings appropriately.
Design Considerations & Selection
When choosing or designing an interposing relay, you need to check:
| Parameter | What to consider |
|---|---|
| Coil voltage / current | Must match the control circuit (e.g. 5V, 12V, 24V DC, or some AC) |
| Contact configuration & rating | Number of poles, normally open / normally closed, and rated voltage/current must exceed what the load requires |
| Dielectric / isolation | The insulation must handle the differences (voltage, surges) between control and load circuits |
| Speed, bounce, switching time | For some applications timing matters |
| Contact protection / suppression | For inductive loads, you may need RC snubbers, diodes, MOVs, etc. |
| Mechanical lifespan / cycles | Ensure it can handle the expected number of switching cycles |
| Package & mounting | DIN-rail, socketed, PCB, plug-in, etc. |
| Environmental / mechanical constraints | Temperature, vibration, enclosure ratings, etc. |
For example, Rockwell’s relay guide includes interposing relays as a category, stressing their use in interfacing mismatched voltages, reducing PLC load, and increasing life.
ABB also produces interposing relay modules with dual relays, multiple contacts, etc.
Example Applications & Circuits
Example 1: PLC Output to AC Load
Suppose you have a PLC that outputs 24 VDC but you need to switch a 230 VAC motor or device. You place an interposing relay whose coil is 24 VDC (powered by the PLC), and whose contacts switch 230 VAC to your motor. This way, when the PLC output goes high, the relay coil energizes, closing its contacts, and powering the motor. The PLC never sees the high voltage side.
Example 2: Input Isolation / Signal Interfacing
If a sensor outputs 24 VDC, but your control input expects 120 VAC, you can use an interposing relay to “translate” the signal. The sensor energizes the relay coil (24 VDC), which closes a contact that feeds 120 VAC into the control input.
Example 3: Improving Contactor Performance
In one scenario, a PLC directly drives a large contactor. The dynamics (inductive kick, suppression, slowing of opening) can degrade response. By inserting an interposing relay between PLC and the contactor, the interposing relay handles the suppression/clamping, allowing the contactor to open more cleanly and reduce stress.
Differences vs Other Types of Relays
- An interposing relay is not typically used for protection (like overcurrent) or complex logic — its role is more of interface/isolation than decision making.
- It overlaps conceptually with “interface relay” or “buffer relay” or “isolation relay.” Some high-voltage systems use “interface relays” that provide galvanic isolation over large voltage differences (e.g., in power systems).
- Solid-state relays and optoisolators sometimes serve similar purposes, but relays have the advantage of true galvanic isolation and handling of larger currents/voltages.
Conclusion
An interposing relay is a crucial component for safely connecting control and power circuits with different voltage or current levels. By providing electrical isolation and protection, it prevents damage to sensitive electronics, ensures reliable operation, and simplifies maintenance. Whether used in PLC systems, industrial automation, or electrical panels, interposing relays enhance both safety and performance, making them an essential part of modern control design.