What Is a Solid State Relay?
A solid state relay is a four-terminal active device consisting of an Input Circuit (Control Circuit), a Drive Circuit and an Output Circuit. This type of SSR is used for controlling AC power loads with DC control signals.
The PNPN structure of the SSR acts like two BJT transistors with common Drain & Source terminals for switching. A diode D1 protects against damage due to reverse connection of the input voltage and a 56 ohm resistor shunts any di/dt currents allowing the SSR to switch off completely.
No Moving Parts or Contacts
The lack of mechanical moving parts or switching contacts in a solid state relay allows it to operate faster and last much longer than electromechanical relays. This also means that it is less likely to be impacted by shock and vibrations.
Solid-state relays (SSR) use semiconductor devices to switch input power to an output circuit. This is in contrast to an EMR which uses a coil, magnetic field and springs to operate and control its armature and contact switches.
When a DC power supply voltage is applied to the control terminals of an SSR the device will activate and switch on. This action is not triggered by mechanical contact, switches or other relay contacts but rather by a light-sensitive photo transistor, a triac or FET and on a separate circuit an LED. When the LED is lit up it sends a signal to the triac or FET to switch on and the output circuit is powered.
This simple operation does not cause any mechanical strain on the relay and it can therefore be cycled millions of times without a deterioration in performance. gate driver ic This long lifetime makes SSRs ideal for repeated applications or where a switch is expected to be in constant use.
High Reliability
Solid state relays have carved out a niche in high-reliability applications where longevity is paramount. They may be more expensive than their electromagnetic counterparts but they can often pay for themselves in the long run due to their higher reliability. The main reason for their better reliability lies in the fact that they have no moving parts, thus eliminating a major source of failure.
Solid-state relays can be used to switch AC or DC input control signals to power supplies connected to their output terminals by using semiconductor switching devices like SCRs, TRIACs and transistors. These devices offer complete electrical isolation between their input and output circuits by using an opto-coupler to detect the actuating pulse and switch on/off the power supply accordingly.
A typical SSR is also equipped with a varistor absorption circuit and an RC snubber to protect it from voltage spikes, current surges and transient overvoltage. This is especially useful in applications that are subject to vibration and EMI.
Because of their superior performance, solid-state relays can be used in a wide range of applications such as military equipment, chemical and industrial automation control devices, explosion-proof motors, electric vehicles, etc. They are also suitable for any device that requires analog divider optical isolation, low-power consumption, high reliability and small size. However, it is important to note that their life depends on the load type and the switching frequency. Inductive loads, momentary shorts and thermal cycling can all significantly reduce their lifespan.
Low Power Dissipation
Solid state relays use a combination of power semiconductor devices to perform their input-to-output isolation and switching functions. They are often based on SCRs or triacs, with an opto-coupler to activate them. These are all built into one package.
The result is a relay that doesn’t have coils and magnetic fields, or springs and mechanical contacts – which means less wear and tear. But this also means that there is a little bit more voltage drop across the output device when it’s on. This drop generates some heat, and that heat dissipates into the case of the relay.
For many applications, this is enough – but some designers want to reduce the amount of heat dissipated even further. They can do that by choosing an SSR with a non-zero crossing control method – the output device is switched ON at any time after the application of the control signal, rather than waiting for the next zero-crossing point in the AC sine-wave.
SSRs with this type of random switching are commonly used in lamp dimming applications where the load needs to be energised for only a small portion of the cycle. However, it should be noted that any SSR that is operated at its full rated current will develop considerable heat – and this heat must be dissipated effectively (via cooling by free flowing air or forced air flow around the SSR) to avoid exceeding the maximum allowable operating temperature of the device.
Minimal Heat Dissipation
All SSRs generate heat and must be mounted on a suitable size heat sink. This is especially important for panel mount SSRs since they must be able to operate at maximum load current without generating too much heat, or their internal transistors will begin to self-destruct under these conditions.
The heat generated is dissipated by a metal plate or heat sink. For best results a layer of silicone thermal transfer grease should be applied to the surface of the heat sink before mounting the SSR, and the heat sink should be firmly attached to the metal base of the relay. This improves the heat transfer efficiency and enables SSRs to handle higher load currents.
Unlike electromechanical relays, SSRs switch “ON” and “OFF” much faster than the mechanical armature of the relay can move and they also require significantly less power to do this, and they don’t generate electrical noise or contacts bounce (electronic or mechanical). This makes them ideal for harsh environments where shock and vibration may occur.
Note that when SSRs are used to drive HID or fluorescent lamps they must be properly sized based on the inrush current characteristics of those loads, as well as their own rated current specifications. Failure to do so can cause overheating, premature failure of the SSR and/or damage the lamp ballast. This is why all SSRs sold by Power-io must be mounted on a proper sized heat sink, and it is important to check the internal temperature of an SSR with a thermocouple, or a similar device, in order to ensure that the relay will not reach its maximum safe operating temperature under load.