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Electromechanical vs. Solid-State Relays

Solid-state relays (SSRs) and electromechanical relays (EMRs) provide the same basic functionality, but they also exhibit significant differences with regard to both implementation and performance. Overall I still appreciate the simplicity and versatility of electromechanical relays, but there’s no doubt that these devices have some serious disadvantages.

A schematic symbol for an electromechanical relay.

A schematic symbol for an electromechanical relay.

One issue is that they have rather long switching times; nowadays it’s pretty much inevitable that an electronic device will be considered slow if its operation is based on physical movement. EMRs are also bulky—again, it’s safe to assume that any mechanical system will be considered a bit too large in comparison with a functionally comparable semiconductor-based system.

A transistor can never be as straightforward as a piece of conductive material. Thus, solid-state relays may require a bit more attention during the circuit-design stage, but this is a small price to pay if your application can’t tolerate the limitations of electromechanical switching. If you’d like to learn more about these two types of relays, take a look at AAC’s article on the basics of SSRs.

SSRs and Optical Control

Electromechanical relays don’t just provide electrically controlled switching. Their structure also creates galvanic isolation, which is quite convenient—many applications that require electrically controlled switching also benefit from an implementation that prevents any type of current flow between the circuit that controls the relay and the circuit connected to the relay’s output contacts.

When isolation is required, we can’t simply replace an EMR with, for example, a BJT used as a switch. Likewise, if manufacturers of solid-state relays want to offer a replacement for EMRs, they need to incorporate isolation, and they accomplish this by using light: the EMR’s coil is replaced by a light-emitting diode, and the switching device is designed to be photosensitive. For example:

Example of photosensitive switching device

Top view of terminal arrangement and internal connections of G3VM-21MT. Image used courtesy of OMRON

As you can see, the G3VM-21MT uses field-effect transistors as electronic switches, but it is also possible to use bipolar junction transistors or TRIACs. Note that each switch requires two MOSFETs; if there were only one, an output voltage that forward-biases the FET’s body diode would cause current to flow regardless of the state of the control signal.

Low-Leakage Solid-State Switching

The G3VM-21MT is an SPST, normally open solid-state relay that comes in a surface-mount package and is suitable for low-power (and maybe medium-power, depending on how you define “medium”) applications. The output device can sustain 200 mA of steady-state current or 600 mA of low-duty-cycle pulse current, and the recommended maximum load voltage is 16 V.

OMRON emphasizes the extremely low leakage current in the off state: only 1 pA with 20 V across the output contacts. Be careful with derating; maximum output current decreases significantly in high-temperature environments.

Continuous load current vs. ambient temperature

Continuous load current vs. ambient temperature. Image used courtesy of OMRON

It’s important to keep in mind that an SSR emphasizes relay functionality rather than state-of-the-art switching performance. The G3VM-21MT’s on-state resistance and switching delays are much higher than what we would expect from a newly released power MOSFET.

G3VM-21MT’s on-state resistance and switching delays

G3VM-21MT’s on-state resistance and switching delays. Image used courtesy of OMRON

Actuating the Relay

The key to controlling an SSR such as the G3VM-21MT is to remember that we’re driving an LED, not some sort of typical high-impedance input pin. We need to limit the forward current (just like when we’re working with a discrete LED), keep the LED’s forward voltage in mind, and design our control circuitry in accordance with the specifications for turn-on and turn-off current.

In the case of the “main control” LED in the G3VM-21MT, the absolute maximum forward current is 30 mA and the typical voltage drop is 2.54 V. The LED will produce enough light to trigger the relay when the forward current is 3 mA (or less, since 3 mA is identified as a “maximum” spec), and release occurs when the current drops to 0.1 mA (this one is a “minimum” spec).

Do you have any thoughts on the advantages and disadvantages of electromechanical relays and solid-state relays? Feel free to leave a comment and let us know.  

Source: All about circuit News

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