Science Forums 2

[engineerdude]

To expand a bit on what Turtle said above,

Component S1 is called a solid state relay. It is similar to a transistor, but it is an on-or-off device only. B1 is a 9v battery, SW1 is a push button switch, and R3 is a resistor that will limit the current that flows from the battery B1. If you didn't have the resistor R3 you'd probably get more current than S1 could handle.

If we close switch SW1, a current will flow in the loop, including through the relay S1. These flowing electrons "switches on" the relay, and allows current to flow from 3 to 1 in the relay S1. This current that affects the state of the solid state relay is called a "bias current". When the bias current stops, the relay will shut down.

Why have the relay at all you may ask - doesn't the switch SW1 do the same thing as this circuit in the red box? The answer is that you don't want humans to come close to dangerous voltages - it looks like the main part of the circuit runs at 200VDC, which is way dangerous. With the circuit like this the switch SW1, which a person would push to make things happen, never sees more than 9v, so if there was a short or something nobody would get zapped too harshly.