Hello Everyone. I have gotten control of my dc motor using the following guide:
But what I have a question about is the circuit that was used on the bottom of the page:
What I want to know is what exactly is going on in this circuit, I have never really used a transistor before so I don't understand what is happening in circuit. Although I understand how the basics of a transistor. The things I want to know is how do i know what the value of the resistor should be before the transistor, and how exactly is this overall circuit working. Like are we varying the voltage with PWM so that we can vary the current across the collector and emitter? And is that current that goes across the collector and emitter the current my motor receives? Also shouldn't we be trying to vary the voltage across my dc motor in order to change the speed instead of the current?
Firstly motors are inductive, so although the voltage is a PWM rectangular wave, the current doesn't have to
be - it may be fairly constant in fact. All motor controllers rely on the current-smoothing effect of the
motor's inductance in fact. When the transistor is off current can flow through the diode and stay
constant (on small timescales). With fast enough PWM this is what happens (although the current
decays until the transistor is next on)
The transistor is being used as a switch, not an amplifier. We aren't controlling the current, we are closing
and opening the switch.
As a switch a transistor is not operating linearly at all. So long as the load doesn't take more current than
the device can handle, and not more than 20 times the base current, the transistor will behave like a closed
switch (on) - the load determines the current that flows.