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Along with the difference between a solid state relay and a transistor, I'm also interested to know if there are transistors that act a lot like the typical relay modules you can find on ebay (where you only need a 5V signal from the microcontroller to toggle the state of the transistor...) A lot of transistor datasheets I've seen so far have wildly different saturation specifications (where the collector/emitter value is highly influenced by how much voltage is applied to the base), but this saturation idea doesn't really apply to a electromechanical relay...

https://www.fairchildsemi.com/datasheets/2N/2N3906.pdf https://www.fairchildsemi.com/datasheets/TI/TIP31A.pdf

Doing a search on Digikey's website for solid state relay gave me a list of components that all have input voltages close to 1.25 V... Am I correct in believing the solid state relay is a lot like what I just described?

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    \$\begingroup\$ "What is the difference between a solid state relay and a transistor?" you ask. "Just about everything is a difference between those" I answer. First of all, the term Solid State Relay (SSR) covers several classes of device with substantially different principles of operation. SSRs for AC are based on SCR or TRIAC (typically), while SSRs for DC are based on MOSFETs (typically). \$\endgroup\$ – Nick Alexeev Apr 13 '15 at 21:17
  • \$\begingroup\$ You can have MOSFET based AC SSRs too. \$\endgroup\$ – Andy aka Apr 13 '15 at 21:18
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One essential feature of a relay, solid state or not, is that the input and output are isolated. In practise this means optical isolation in the SSR (solid state relay) case. In contrast, ye olde phashioned mechanical klunkety-klunk relays are magnetically isolated. One could conceivably make a solid state relay using magnetic isolation in various forms too, but optical isolation makes more sense for the requirements.

So solid state relays are more than just a tranistor, triac, or whatever is used to perform the actual switching. They have an isolated input that then ultimately controls the solid state switch. In practice, this usually means at least a LED and phototransitor in addition to the switching element. That is all packaged together and called a Solid State Relay.

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  • \$\begingroup\$ Thanks for the explanation :-). Am I crazy for thinking I could replace transistors with solid state relays for most switching applications? A relay just makes more intuitive sense to me... I'm super new to electronics (obviously) and the saturation levels of a transistor and how the base current/voltage will effect the collector to emitter current/voltage just seems like it's not worth the bother if I could just use a relay... \$\endgroup\$ – Jacob Edward Apr 14 '15 at 1:12
  • \$\begingroup\$ SSRs are much more expensive than transistors. Cheapest one I could find at digikey was digikey.co.uk/product-detail/en/CPC1017NTR/CLA233TR-ND/655290 for 30c versus transistors at 1.5c. Saturation is not magic and not even a problem in most switching uses; and you still have to read datasheets of the relay to check you're supplying enough hold current. \$\endgroup\$ – pjc50 Apr 14 '15 at 9:37
  • \$\begingroup\$ Thanks :-) I've replied to another comment below with a question on saturation if you could help out \$\endgroup\$ – Jacob Edward Apr 14 '15 at 20:11
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There is another major difference between a transistor and an SSR. SSR is an on/off device, current either flows or it doesn't. A transistor CAN be used as a switch, but in many cases, it operates as an amplifier, ie an extremely small current through the base allows a much larger current through the collector (assuming bipolar transistor). There are actually many different kind of transistors! Bipolar junction, MOSFET, JFET, etc...

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  • \$\begingroup\$ My confusion with transistors are the saturation levels which are dependent on how much current/voltage is applied to the based... so... trying to use this transistor as a switching device fairchildsemi.com/datasheets/2N/2N3906.pdf, what voltage/current would I need to apply to the base for saturation levels to allow me to apply the full 40V from collector to emitter? \$\endgroup\$ – Jacob Edward Apr 14 '15 at 20:09
  • \$\begingroup\$ See figure 1 from this explanation: rason.org/Projects/transwit/transwit.htm By the way, the MAX voltage is 40v, I would not go over 20v or you may easily stress the transistor, and the MAX current for this device is 200ma plan on no more than 100ma! The base current needed depends on the gain of the transistor and the voltage/current you need to source to the load. \$\endgroup\$ – Guest Apr 15 '15 at 3:29
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The short answer is no.

An SSR is typically optoisolated. This means that the input is, for all practical purposes, an LED. It may have a current-limiting resistor included.

The light from the LED then drives the switching element. This may be a MOSFET with an optically active gate, or a triac, or it may be a photosensitive driver circuit which then drives a triac or thyristor.

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