Using relays are 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 single pin is very limited – varies around 20mA. 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 higher voltage supply.
And there you have several options for switching loads. One and most oldest method is using mechanical relays. They are still very popular way of switching power electronics. One biggest disadvantage of using mechanical relay is that it has moving parts with all rising problems. These are:
- slow switching time;
- relatively high control current;
- may produce sparks on switching;
- sensitive to environment factors like vibration, humidity;
To overcome those problems there is more modern solution – a Solid State Relay (SSR). Instead of switching loads mechanically, SSR does this with help of electronics.
If we look at SSR-25DA internal circuit we can see that control circuit is isolated from 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 zero crossing detector. This means, that relay will be turned on only when AC voltage crosses zero line. This ensures that switching is always performed at safest moment to minimize RFI noises in line. Relay also turns off when AC voltage crosses zero line. This is due SCR properties.
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As you can see in this particular relay a snubber circuit is already included. It prevents relay from unintended triggering due to voltage spikes on 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 transformer.
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 you electronics is able to light a LED, you can trigger relay. SSR-25DA relay already include current limiting resistor. Additionally it also houses an indicator LED. Other SSR relays need external LED current limiting resistor and they may not have 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 you would need transistor switch for mechanical relay.
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 heatsink to 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), case is capable to dissipate heat. But if ambient temperature is high enough, then you need to use heatsink. How to calculate safe margin?
Lets 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 junction temperature lower than maximum125ºC. Lets say we want junction temperature to be about 80ºC. From 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 maximum junction temperature. You can operate without heatsink if load is switched in periods, but for constant driving it would be better to use minimal heatsink to lower junction temperature and extend operating life.