What is happening in my circuit?

  1. If I have a relay coil hooked up to the collector port of a transistor and have a solid signal going in the base, can somebody explain what is happening inside the transistor right when the relay turns on? (npn transistor)
     
  2. jcsd
  3. NascentOxygen

    Staff: Mentor

    The transistor is being given base current, so it causes collector current to flow. The circuit is designed so that the base current is greater than the minimum needed to cause this particular collector current, so the transistor saturates (has a very low voltage between E and C) and acts like a good switch. The collector current is the current in the relay.
     
  4. the relay turning on is really just a signal that current is flowing between the collecter and emitter?
     
  5. should be like that as current flow creates a mag field in the relay and turns it on (atleast most of them work that way)

    Just like Nascent said , when a BJT gets a high enough base current the C E junction conducts or is open so current flows from the source you have through the relay then through the transistor to whatever is next there in your circuit.
     
  6. NascentOxygen

    Staff: Mentor

    I recall someone being taken to task ( in a most civil PF way, of course, :smile: ) previously over the OPEN and CLOSED convention for switches. That wasn't you, Crazy, was it? :wink:
     
  7. "Just a signal" isn't the best way to describe it. The current is necessary to turn the relay on.

    If the circuit is working correctly then yes the relay turning on is evidence that current is flowing through the transistor.

    But consider what happens if I replace your transistor with one that is faulty and has a short circuit between the collector and the emitter. The relay would still turn on. Of course it would also stay on even when there was no current going into the base of the transistor.
     
  8. There are a lot of web sites with explanations of how a NPN transistor works. Most are quite complicated and assume you understand how a diode works. Is that the problem? Do you need a simpler explanation?

    Basically if your circuit pushes ?mA into the base of an NPN then upto hFE * ?mA can flow from collector to emitter. hFE is the gain of the transistor and is typically around 60-200 for common transistors. Lets assume the gain of your transistor is 100.

    So if your base current were say 2mA then the transistor will allow upto 2 * 100mA = 200mA to flow between collector and emitter.

    I say "upto" because the actual current in your circuit might be limited by the resistance of the relay coil. If the supply voltage was 12V and the relay coil 1k Ohms then only about 12mA will flow when the transistor is on/conducting.
     
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