Does Transistor NPN only amplify Positive charge?

Question

I know my question might be a bit "stupid", as I am now a student.

I created a circuit like this ... But it does not work. (I mean the DC Motor does not turns.)

^{simulate this circuit – Schematic created using CircuitLab}

(The DC Motor I can't find a real part, so I just simply made a box like that, sorry.)

Then, I searched the internet for help, and I found out the circuit shall be like this ... (I don't know weather this is correct or not.)

^{simulate this circuit}

What does it mean and why should "put" the Transistor at Positive side but not at Negative side? Does Transistors only amplify Positive charge? Thank you.

Answer 1

Think of a BJT as a valve. By default, the valve is off. No current can flow through our valve. In order to get our valve to turn, we need to make current flow through the base of the BJT. This lets current flow through the transistor, from the collector to emitter. The more current we apply to the base, the more open our valve becomes.

In your first diagram, current can reach the base the BJT, which turns it on. I have drawn the current flow with red arrows.

Current can flow through the base and the collector of the BJT, which turns it on.

Now, in your second picture, the current cannot reach the base of the BJT because the BJT is turned off.

In addition, the BJT is in there backwards. The current needs to flow from the collector to the emitter. The way it is wired, the current tries to flow from the emitter to the collector. The current can't reach the base to turn the BJT on because the BJT is off. There's a chicken-and-egg problem with the second schematic.

You can put the BJT on either side of the DC motor, but it has to be wired correctly in order to work.

To fix the second image, run the line from the motor to the collector. Connect the collector and the base together, and connect the emitter to the negative on the power supply.

Answer 2

You can put the transistor in either the positive side or the negative side.
There are technical reasons for choosing one over the other, but for the theoretical circuits you are discussing, they are the same.

Low side switching:

^{simulate this circuit – Schematic created using CircuitLab}

High side switching:

^{simulate this circuit}

Note that in both cases the base is connected to the positive side of the power supply. Also note that the arrow of the transistor (the emitter) always goes towards the low side. In one of your diagrams, you have the emitter pointed towards the positive side through the motor.

In order for an NPN transistor to switch on, you must place a voltage on its base that is at least 0.7Volts higher than the voltage on the emitter.

In the schematic you provided in which the motor didn't turn, the base was at a lower voltage than the emitter, so no current could flow.

Additional notes:

1. You should always have a resistor between the base and the current source to limit the current through the base. With no base resistor in high side switching, you are depending on the load (the motor) to limit the current through the base. With low side switching, there is no limit. So, low side will destroy the transistor immediately without a base resistor and high side will destroy it somewhat later.
2. You need to use diodes to protect your transistors from the spikes generated by the motors. The motors store energy in the coils. When you turn off the power to the motor, the stored energy is released and can create very high voltages. The diodes short that impulse and protect the transistor from damage to to over voltage and over current.