Considering Solid-State Relays (SSR) for your projects

Using relays are a common way of switching high-power loads with electronics. If you take any microcontroller or any other digital IC, you will see that their output current on a single pin is minimal – varies around 20mA. The same situation is with voltage. Digital pin output voltage is limited to IS supply voltage like 3.3v or 5V. Usually, we need to switch loads that draw significantly higher currents and are powered from a higher voltage supply.

And there, you have several options for switching loads. One and oldest method is using mechanical relays. They are still a trendy way of switching power electronics. One most significant disadvantages of using a mechanical relay is that it has moving parts with all rising problems. These are:

• slow switching time;
• relatively high control current;
• noisy;
• may produce sparks on switching;
• sensitive to environmental factors like vibration, humidity;

There is a more modern solution to overcome those problems – a Solid State Relay (SSR), also known as a single-phase power controller. Instead of switching loads mechanically, SSR does this with the help of electronics.

If we look at SSR-25DA internal circuit, we can see that the control circuit is isolated from the load by using optics. Usually, inside is an IR LED which turns on phototransistor controlling SCR (Silicon-Controller Rectifier). So the working horse of solid-state relay is SCR or TRIAC. Relay is triggered through a zero-crossing detector. This means that relay will be turned on only when the AC voltage crosses zero lines. This ensures that switching is always performed at the safest moment to minimize RFI noises in line. Relay also turns off when the AC voltage crosses zero lines. This is due to SCR properties.

As you can see in this particular relay, a snubber circuit is already included. It prevents the relay from unintended triggering due to voltage spikes on the AC line. Spikes usually appear from inductive loads. RC snubber circuit reduces those spikes to ensure safe and reliable operation of SSR. Other solid-state relays don’t have snubber circuit, so you need to include it externally – especially when switching inductive loads like motors, solenoids, or transformers like in https://myskyhawk.com/parts/ listings.

If we look at SSR-25DA solid state relay datasheet, we can see that it can switch 25A loads. Switching voltage is from 24V up to 380VAC with blocking 600VAC repetitive voltage. So it is safe to use on 240VAC line. What about triggering current and voltage. SSRs are great because triggering current is equal to LED current. So if your electronics can light a LED, you can trigger the relay. SSR-25DA relay already includes a current limiting resistor.Additionally, it also houses an indicator LED. Other SSR relays need an external LED current limiting resistor, which may not have an indicator LED. Our relay can be turned on from 3V to 32V DC signal and requires 7.5mA. 7.5mA is practically safe sourcing current for any microcontroller like AVR (Arduino), PIC, and even Raspberry Pi. No other external components are required, like the transistor switch for the mechanical relay.

The last thing to discuss on SSR is power dissipation. Like any other electronics, they generate excessive heat. If you need to drive high current loads, it is necessary to ensure overheat protection. Best way to do this is to attach the heatsink to the backplate. Ambient temperature, load current and switching speed are the main factors that may cause overheating. At low currents (up to 5A for this SSR), the case is capable of dissipating heat. But if the ambient temperature is high enough, then you need to use a heatsink. How to calculate the safe margin?

Let us say we want to operate SSR at ambient temperature 40ºC. And drive 5A current. We know that SSR voltage drop is about 1.6V. Then we need to dissipate about 1.6V·5A=8W power. For semiconductors, we need to keep the junction temperature lower than maximum125ºC. Suppose we want junction temperature to be about 80ºC. From a similar datasheet, we can find that thermal resistance junction to ambient Ra = 10ºC/W. So junction temperature in our case:

Tj = 40ºC + 8W ·10ºC/W = 120ºC.

This is close to the maximum junction temperature. You can operate without heatsink if the load is switched in periods. Still, for constant driving, it would be better to use minimal heatsink to lower junction temperature and extend operating life.