Notes

Transistors, Relays, and Controlling High-Current Loads

Electromechanic Relay:

• a device that controls a switch using magnetism(electromagnet)
• mechanical >>slow
• doesn't have polarity issues

Draw by Excalidraw

Solid-state Relay:

• a series of transistors and diodes...no moving parts
• NOT mechanical
• can control AC/alternative current(2 pins for AC; 2 pins for DC )

Transistor:

• used as a switch/amplifier

• fast

• can't control AC, because it has polarity

• 2 major classes: bipolar transistors and field-effect transistors(FETs)

  They all have three connections: the base, the collector, and the emitter; on FET transistors: the gate, the drain and the source.

  • bipolar transistors

    • NPN transistor(e.g. TIP120 Darlington transistor)

      apply a positive voltage to the base >> turns on the collector-emitter connection >> allows current to flow from collector to emitter

      

    • PNP transistor

      apply a positive voltage to the base >> turns off the collector-emitter connection >> allows current to flow from collector to emitter

      

  • MOSFET

    • N-channel MOSFET(the equivalent of NPN transistor, e.g. IRF520)

      

    • P-channel MOSFET(the equivalent of NPN transistor)

Optoisolator:

• a hybrid between transistor and relay
• the transistor inside uses light(LED on the other side of itself)