What is happening in my circuit?

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Discussion Overview

The discussion revolves around the operation of an NPN transistor in a circuit controlling a relay. Participants explore the relationship between base current, collector current, and the functioning of the relay, addressing both theoretical and practical aspects of transistor behavior in this context.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant asks for an explanation of what occurs inside the transistor when the relay turns on.
  • Another participant explains that base current causes collector current to flow, leading to transistor saturation and acting as a switch for the relay.
  • Some participants note that the relay turning on indicates current flow between the collector and emitter, but one later clarifies that this is not merely a signal but necessary for operation.
  • There is a discussion about the implications of using a faulty transistor, which could still allow the relay to turn on due to a short circuit between collector and emitter.
  • A participant provides a simplified explanation of how base current relates to collector current, introducing the concept of transistor gain (hFE) and its effect on current flow.
  • Another participant mentions that the actual current may be limited by the relay coil's resistance, providing an example with specific values.

Areas of Agreement / Disagreement

Participants generally agree on the basic operation of the transistor and relay interaction, but there are nuances in how they describe the implications of current flow and the effects of potential faults in the transistor. The discussion remains somewhat unresolved regarding the best way to articulate these concepts.

Contextual Notes

Some participants express uncertainty about the complexity of existing explanations for NPN transistors, suggesting that simpler explanations might be needed. There are also references to conventions in describing switch states (open vs. closed) that may affect understanding.

PsychonautQQ
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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)
 
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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.
 
the relay turning on is really just a signal that current is flowing between the collecter and emitter?
 
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.
 
Crazymechanic said:
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 [strike]open[/color][/strike] closed[/color] so current flows from the source you have through the relay then through the transistor to whatever is next there in your circuit.
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:
 
PsychonautQQ said:
the relay turning on is really just a signal that current is flowing between the collecter and emitter?

"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.
 
what is happening inside the transistor

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. Let's 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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