What Is SSR And Its Advantages Over EMR?

Solid state relays (SSR), as the name implies, are stationary in space; whereas electromechanical relays (EMR) have metal contacts that move. The question then becomes, how does a solid-state relay without contacts control a load? The key lies in semiconductor electronics, such as transistors, MOSFET's, thyristors and TRIAC’s. These semiconductor electrons require only a small external voltage or current to conduct a high power circuit. A quick look at the characteristics of an SSR and how it differs from an EMR.

1. Zero-Cross Switching. With the use of SCRs or TRIACs, SSRs can inherently avoid back EMFs and arcing like those generated when EMRs turn off inductive loads. These switching power electronics can remain in the on-state until one’s current crosses the zero point despite the gate signal has gone.
2. Long Lifetime. Solid state relays can be switched a million times or more due to no physical contact wear.
3. Fast Response. From open to close a solid state relay only requires the semiconductor device inside the optocoupler to be excited, an action that takes tens of microseconds. For most SSRs with zero-cross switching, the switching time is added to maximum half an AC power cycle, typically about 17 milliseconds.
4. Wide Range of Applications. Especially where clean operation, fast response or heavy use is required, SSRs are an ideal choice. The SSR's input and output can be either DC or AC, up to 100 amps or more, typical applications like controlling a 240 volts, 10 amps AC load with a 5 volts DC signal.
5. Weaknesses. Although SSRs have many of the above advantages, they still have flaws. More expensive than EMR; requires good heat sinking if the output circuit has too much current, like 50 amps; higher on-state resistance; cannot withstand large transient currents.

Input Circuitry

Understanding how solid state relays work can help you decide whether to adopt them and choose between different specification models. A simple SSR, whose circuit schematic as shown in below, consists of two parts: input circuitry and output circuitry.

An optocoupler is located at their connections, allow both parts of the SSR to use a separate power supply and transfer switching signals easily. The driving voltage of the optocoupler LEDs, depending on the manufacturer, varies from 3 to 24 volts. Protection resistor Rin limits the current passing through the LED. Input terminals series a DC power and a mechanical switch, or pushbutton, relay, contactor, etc. Once the switch closes, the turned-on input circuit lighten the optocoupler LED activating the optocoupler switching device, that makes the output circuit in conduction. Sometimes the mechanical switches can be replaced by power electronics, where the supply voltage should be the SSR input voltage plus the device on-state voltage drop, 1.5 volts eliminated usually.

Solid state relays can also be designed to use AC power as the input source by adding an internal full-bridge rectifier that generates a DC voltage for LEDs emitting light. When the AC power is in the positive cycle, D2 and D3 can conduct, as well as D1 and D4 are blocked; opposite when the AC power is in the negative cycle, D1 and D4 can conduct, as well as D2 and D3 are blocked. The RC circuit then converts the negative-reversed sinusoidal voltage into a smooth, near-DC voltage waveform. The value of R and C are determined by the AC power and the SSR input impedance, often 40kΩ, 10uF or so.

Output Circuitry

In common solid sate relays use transistors or MOSFETs as switching devices for DC output, as well as thyristors or TRIAC’s (because of the higher capacity) for AC output. A thyristor blocks the opposite direction current, but can be still used in an AC circuit with a full-bridge rectifier.

The biggest advantage of solid state relays over electromechanical relays is the zero-crossing detection. Zero current turn-off keeps solid state relays from shutting down the circuit at a peak of the sinusoidal output current, preventing back-EMFs generated by a sudden collapse of an inductive load, which may cause false triggering of semiconductor switches. Zero voltage turn-on as well perform good for the output circuit in terms of devices’ lifetime. The semiconductor switch has just been activated to the on-state, the capacitance in the output circuit behaves as a low impedance. If the sinusoidal voltage is too large at this point, it will cause a serious inrush current which may knock out the power electronics and other devices in the output circuit.

Zero-Cross Switching Waveform

The initial output circuit is disconnected and a full source voltage waveform can be seen at the output terminals of the SSR. No matter when a trigger signal given, the thyristor keep open until the thysistor voltage abates to zero and growth over threshold voltage (less than 15 volts), then the load really gets to work and the output terminal voltage Vout changes to on-state voltage drop about 1-2 volts. Although the trigger signal removed, the thysistor continues conduction and Vout voltage stays close to zero. When the thysistor current abates to zero, the thysistor turns-off the output loop and Vout comes back to the source voltage waveform.

Output Improvements In Practice

For the above output circuits we have directly used an optocoupler thysistor as the switching device for the load circuit. Generally the thysistor T_C rated current is too small, such as 1amp, to match the demand of load current. At this point we have to do some upgrading. Parallel a high capacity thysistor T_L (or other power electronic switch, depending on the power supply and load) to take the main load current. Protection resistor R₁ prevents an excessive current through T_C, its value can be the supply voltage divided by the rated current of T_C. Connect T_C MT1 pole to T_L GATE pole, so that the triggered T_C will trigger the T_L. Resistor R₂ value can be tens of ohms, and it shunts the current generated by the back EMFs when the circuit turns off, preventing mistakenly re-triggering thysistor T_L.