I was looking at This model train web page about using solid state relays. He gives a few examples of what he calls “home made” solid state relays – I would call them discrete SSRs because they are made from separate components. There are a few things that I would like to add.
He pointed out that there are advantages and disadvantages to using SSRs. One thing he forgot to mention is that SSRs are clickless, you won’t hear any clicks or ticks when the ‘contacts’ close. One disadvantage is that they are generally only a single set of ‘contacts’. If you want to switch multiple ‘contacts’ you have to add more SSRs (but see note at end).
He did say that they are much faster than a mechanical relay, but the speed may be limited by the speed of the optoisolator, which may not be as fast as the transistors used. Another point is that the optoisolator does not have to be one you’ve purchased. You can also make your own using a CdS photocell and LED, or two LEDs, with one LED being used as a sensor. The LED will act as a very small solar cell and generate a small voltage at a small current that can be amplified by a transistor or two. You can put two LEDs facing each other in a piece of black heat shrink tubing to hold them in place. You may need to put black sealant on the ends to keep external light out.
One thing that I don’t agree with is he uses the 2N3904 or 2N3906 for currents up to 200 milliamps. When the current gets to more than 100 milliamps, the 2N3904 or 2N3906 has been pushed to its limit and should be replaced by a transistor that can handle the load without strain. Choices might be the PN2222A (NPN), PN2907A (PNP), 2N4401 (NPN), 2N4403 (PNP), BC337 (NPN), BC327 (PNP). These transistors should handle a few hundred milliamps; for currents up to 1 amp the BD139 (NPN) or BD140 (PNP) could be used. Remember that these transistors are for low voltages; if switching power line voltages is needed, there are high voltage transistors available.
One has to remember that an electromechanical device connected to the SSR, such as a solenoid or motor, might generate a high back EMF or inductive kick. So it is important that the SSR have the protective diode across its output. Also the SSR is much faster than a regular relay, so they are more sensitive to external transient interference and it may be necessary to add filtering to prevent interference.
He does not mention anything about using a SSR with AC. The SSR can be connected to the plus and minus leads of a full wave bridge rectifier, and the AC connected to the AC leads. The bridge rectifier will drop a volt and a half, due to the forward voltage drops of the diodes, so this has to be taken into account. If the AC is low voltage, a bridge using Schottky diodes will reduce the voltage drop to half that of a regular bridge.
A lot of equipment now use MOSFETs to switch high current and/or high voltages. These can be used as SSRs. But at high power, it is probably better to buy a commercially made SSR to minimize the amount of design decisions and possible failures associated with wrong design decisions. Also, there may be SSRs made to be used with AC. These may use a TRIAC that is optically isolated from its control leads.
Note: generally, a common reason for using an electromechanical relay or SSR is for isolating the power circuit from the controller circuit. The relay can switch much higher currents and voltages than the controller itself could handle. But relays, especially power control relays, often take quite a bit of power to operate, and once they are energized, they continue to take power. SSRs can switch high power with less wasted power than regular relays, but there are relays that take no power after they have been energized. The latching relay may use a ratchet, magnets or other mechanical means to hold the contacts in one position or the other, so there is no contact spring that has to be overcome. Once energized, the energizing power can be removed and it will stay in whatever position it is in.