In this video, we will learn about Transistors, Potentiometers, and basic concept behind Simple DC motor Speed Controllers.

Hi everybody, these are components we need to setup simple motor Speed controller. This is potentiometer, this one is transistor, and these are simple resistors.
First of all I will jump into potentiometer and test it with multi meter. A potentiometer maybe be like this or this or this, no matter all of them are basically the same but a little difference. All of them got three pins and a resistance value. Resistance between side pins is equal to potentiometer’s resistance. Let’s name resistance between side pins as R and resistance between left side pin and middle pin as R1 and resistance between middle pin and right land side pin as R2. Then we got this equation in all types of potentiometers, R = R1 + R2. Exact value of R1 and R2 is related to handle position. If the handle turned in CW, R1 will grow and R2 will shrink and vice versa. This growing and shrinking behavior of R1 and R2 causes another characteristic for potentiometers which is used in DC motor Speed Controllers. Let’s explain it with a simple experiment. Connect 5 and zero volts to potentiometer’s side pins and then measure middle pin voltage.
You see what happens when I turn handle left and right? Voltage of middle pin changes. But don’t be happy too soon, this voltage can’t even run an LED, let alone DC motors. So what we can do to make this variable voltage usable for DC motor speed controller? That’s right, next component will help up in this journey. Yes, that is transistor.
Transistor is a 3pin component which often used as a digital switch, but it is not all of the story, because transistors can amplify voltage or current too. These amazing components got three pins named collector, emitter and base which collector and emitter are for switching voltage or current and base pin used for control. In this case, we got variable voltage from middle pin potentiometer but the current sucks and we have to amplify current on this variable voltage. Then we need a current amplifier. Fortunately transistors can do it simply.
There is a secret about BJT transistors, they use to hold 0.7 volts between their base and emitter pins and if you change base pins voltage, the transistor arrange everything to keep this voltage difference and it is exact thing that we need, one of these arrangement is current on collector to emitter
This is transistor C945 a simple NPN transistor and its pins are in this order (E C B) and I found it in datasheet. If you have to use another transistor, you must refer to its datasheet.
Think of this simple circuit, variable voltage is produced by 10kohm potentiometer and applied to base pin of transistor through this 560ohm resistor, this resistor is used here for current restriction in case the potentiometer turn left to end, and if you don’t put this resistor here, actually base pin will directly connect to supply voltage and draw a lots of current through potentiometer and burn it.
So this transistor will do its best to keep 0.7 voltage difference between base and emitter pins by allowing or blocking voltage from its collector pin to emitter. The transistor will amplify current on its collector pin to keep voltage on its emitter pin constant, no matter the load on output draw how much current. That is it.

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