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### Topic: Transistors are driving me nuts. Please someone explain them to me. (Read 552 times)previous topic - next topic

#### Pluto1

##### Apr 12, 2015, 03:29 pm
Ok, so I moved from LEDs and displays to motors, and you can't go there without learning transistors. Now, bloody hell. So much things I'm not clear with. Ok, from most basic to not-as-basic:

- Say, if you are following a wire in a circuit and that wire becomes connected to two seperate things at the same time, or, to say it another way, the wire splits in two. I think this is called a node, yes? How does the current go here? Does it choose a way? Does it divide in half and go both ways? Go according to the demands of the separate circuit elements?

- Now, straight to the transistors themselves. I use NPN transistors. How do you connect this up? I understand that once the voltage going through the base leg reaches past a certain threshhold, current is allowed to go through the collector and emitter. But, it's not as straightforward as that at all from what I have been trying. What are the rules concerning where the emmiter should be connected to, the connector? Etc.

- The device connected to the collector also itself needs to be connected to the ground. So, I don't understand how connecting the emmiter to the ground completes the circuit or what the need for the emmiter is.

I'm a newbie, as you can clearly tell, so all of this is still Greek to me. I probably missed some important electronic concepts on the way to learning this, which is in turn probably causing the misunderstandings, so if you can, please go all the way through.

#### weedpharma

#1
##### Apr 12, 2015, 04:06 pm
1. Current splits at the node according to the resistance leading from the node. High R, low current and vice versa.

2.  A basic NPN switch uses a small current in the base emitter Gnd circuit to allow a larger current to flow in the collector emitter Gnd circuit.

The amount of base current depends on the gain (amplification) of the transistor. If the gain is 100 and you need to have 1A flowing, you need to put 10ma into the base.

In a simple switch situation, you just need to make sure there is enough base current to fully turn on the transistor.

3.  The load in a NPN switch situation is connected to the supply and collector. The transistor acts as a switch to Gnd.

NB this is a quick superficial coverage of the workings! For full description you need to study for years.

Weedpharma

#### MarkT

#2
##### Apr 12, 2015, 08:35 pm
At a node the sum of currents into the node equals the sum of current out of the node

Or put another way the signed sum of currents into the node is zero.  See Kirchoff's Laws.
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