I'm searching information about SSR realized with bipolar transistor. Basically I need something like tutorial how they are constructed, how they work, what elements they contain etc... I searched in google but I wasn't very lucky with the results. If anybody give me some source of information I will be very grateful. Note: With bipolar transistor not with MOS transistor.

My background: Familiar with Electrical technique(not sure about the name things like Om law, kirchhoff law etc..) Familiar with building elements in electronics(transistors, capacitors etc..) Familiar with analog circuits learning digital circuits at the moment. Familiar with analysing different schematics.

  • \$\begingroup\$ You're probably not finding much because BJTs are not often used in solid state relays due to drive issues, emitter-base breakdown, etc. Why do you specifically want to use a BJT? \$\endgroup\$
    – John D
    Commented Dec 3, 2014 at 15:53
  • \$\begingroup\$ I don't want, it's academy exercise and I have to prepare somehow, since there aren't any books about electronics in my mother language I was unable to find any information about most of the SSR, that's why I'm searching for information. \$\endgroup\$ Commented Dec 3, 2014 at 15:57
  • \$\begingroup\$ Please take a look at this link: digchip.com/datasheets/parts/datasheet/879/SGC.php \$\endgroup\$
    – R Djorane
    Commented Dec 3, 2014 at 16:02
  • \$\begingroup\$ For some reason the text is in French or Spanish, I don't speak any of those. \$\endgroup\$ Commented Dec 3, 2014 at 16:05
  • \$\begingroup\$ Maybe you should tell us what languages you do speak... There's a slot in your profile for your location - why not fill it in? And give yourself a sensible name too while you're at it. \$\endgroup\$
    – Majenko
    Commented Dec 3, 2014 at 17:19

1 Answer 1


A typical DC SSR has only two output connections, so the current for the transistor base must come from the load current.

So, think of configurations that would do that- for example, a triple darlington with the phototransistor at the 'top', or a hybrid MOSFET/BJT output stage with the MOSFET as a driver.

Edit: Here's a somewhat similar idea with less voltage drop than a 3-transistor Darlington:


The phototransistor causes base current in the PNP transistor which provides base current ot Q3 which switches the load.

Q1 and R2 form a constant current circuit for the LED in the optoisolator allowing wide input range (4-12V by their numbers). R1 and Z1 and D1 protect the input from transient overvoltage or reverse voltage. D2 protects the output from reverse voltage.

By using a Sziklai pair Q2 and Q3 (sometimes called a complimentary Darlington) configuration as the output stage, the voltage drop across Q3 can be used by the phototransistor so the total voltage drop under load is Vce(sat) + Vbe(on).

  • \$\begingroup\$ I was unable to understand this answer. \$\endgroup\$ Commented Dec 3, 2014 at 16:14
  • 1
    \$\begingroup\$ If you want answers you can understand, you need to fill in your profile, telling us something about your background. \$\endgroup\$
    – Dave Tweed
    Commented Dec 3, 2014 at 16:56
  • \$\begingroup\$ @user3868594 Try looking up Darlington transistor and see if it takes you anywhere helpful. \$\endgroup\$ Commented Dec 3, 2014 at 17:15
  • \$\begingroup\$ Dave Tweed, I edited my question and posted with what I'm familiar with, what I was unable to understand is how does this answer part of my question. I remind my question is about the construction and how the relays(with bipolar transistor) work. Spehro Pefgany I'm familiar with Darlington transistor, but I need a guide for SSR relay constructed(not sure if this is the correct word) with bipolar transistor. \$\endgroup\$ Commented Dec 3, 2014 at 19:23

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