Do SSR require current limiting in some fashion?

• Do SSR require current limiting in some fashion?
  Posted by ryanmills on 10 Jul, 2019 17:29
• I'm pretty sure the answer is no. But I just made some real basic stupid mistakes and before I move on I wanted to give some EE's a chance to use the RTFM emoji. 
  
  Do solid state relays with a MOSFET (specifically PhotoMOS AQY211EHAX) output require any current limiting on the output? My first setup was with an LED and obviously it needed a current limiting resistor. However i'm also going to drive an incandescent bulb and a small DC motor. I assume its just going to be like driving anything else regular MOSFET?
• #1 Reply
  Posted by MagicSmoker on 10 Jul, 2019 18:10
• Unless the datasheet says it is fully protected against shorts, overcurrent, overtemperature, etc., then assume current limiting is up to you.
  
  EDIT - I checked that datasheet for you and it is not protected.
• #2 Reply
  Posted by ejeffrey on 10 Jul, 2019 18:14
• Nothing special.  If you are running off of mains or any other circuit capable of delivering massive current into a fault you need a fuse.  Solid state relays tend to fail short circuit rather than open -- if a failed short is hazardous you need some ability to do a safety cutout -- for instance if you are controlling a heating element you might want a thermal fuse.
  
  They are MOSFETs, so they are more susceptible to transient overvoltage compared to mechanical relays or TRIACs. When interrupting an inductive load you will want snubbers, just as with any other MOSFET.
• #3 Reply
  Posted by ryanmills on 10 Jul, 2019 21:50
• Thank you, just feeling like an idiot today and wanted to check.
• #4 Reply
  Posted by Zero999 on 11 Jul, 2019 08:08
• Quote from: ryanmills on 10 Jul, 2019 17:29

  I'm pretty sure the answer is no. But I just made some real basic stupid mistakes and before I move on I wanted to give some EE's a chance to use the RTFM emoji. 
  
  Do solid state relays with a MOSFET (specifically PhotoMOS AQY211EHAX) output require any current limiting on the output? My first setup was with an LED and obviously it needed a current limiting resistor. However i'm also going to drive an incandescent bulb and a small DC motor. I assume its just going to be like driving anything else regular MOSFET?
  

  Make sure the relay can pass enough current to drive the load and it will be fine. The load resistance will limit the current to a safe level.
  
  If you're asking about protecting the SSR from faults, then the only thing which can offer any kind of protection is an electronic overcurrent protection circuit. A fuse, circuit or PTC resistor will be far too slow to protect a solid state relay from damage, due to the load drawing too much current.
  
  Finally, the input of the AQY211EHAX and many solid state relays is just an LED, which of course needs a current limiting resistor.