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Name: Ellis
Status: student
Grade: 9-12
Location: IL
Country: USA
Date: Fall 2012


Question:
Is there a big difference between PNP transistors and NPN transistors? I want to control a motor with some transistors and I need to know if I really need both. Could I just use PNP transistors?



Replies:
Hi Ellis,

The "difference" between NPN and PNP transistors is simply that PNP transistors are opposite in polarity as compared to NPN types. Technically, it is said that NPN and PNP transistors are "complimentary".

You asked if there was a big difference between NPN and PNP transistors. In fact, it is common for transistor manufacturers to make so-called "complimentary pairs" .....that is, a pair of transistors (one NPN and one PNP) that perform identically, except that one is the opposite polarity from the other. These complimentary pairs are very commonly used, for example, in audio amplifiers where one NPN and one PNP transistor are connected together and form the output power stage of the amplifier. Another common use of these complimentary NPN-PNP pairs, is in some types of motor controls.

It is important to understand that the choice of either NPN or PNP is dictated by the designer (that is, YOU!) according to the needs of the circuit being designed. You do not decide before starting the design, to use (say) only PNP transistors! Instead, a skilled designer will use whatever type of transistors (ether NPN, PNP, or both) will best achieve the simplest and best design of the circuit in question.

My suggestion to you is that you get a book from your local library on basic transistor theory. This should help you better understand the differences between NPN and PNP transistors, and where each is best suited for use.

Regards, Bob Wilson


Hi, Ellis

NPN and PNP transistors are sort of mirror opposite of each other. In an NPN transistor, positive current flows into the base and collector and out the emitter. In a PNP transistor, positive current flows into the emitter and out of the base and collector.

Referring to current as negative or positive is merely a matter of convention. Electrical engineers are accustomed to discussing positive current flow, even though electrons are considered to be negative.

Before I went to college and learned circuit design, I was hoping to find some sort of "cookbook" method for doing circuit design. I was not able to find this. Once I learned to design circuits I understood why. Designing with transistors is like designing with lumber or screws (but more complicated). Rather than there being one way to use a transistor, there are a huge number of things you can do. Most of them will not work but many will. I think your only two options are to copy a design which works, or to learn electrical engineering.

When you say "control a motor":

1) Do you want to just turn it on and off, vary speed, reverse direction?

2) There are MANY different types of motors; AC induction, AC synchronous (single phase or three phase for all AC) DC brush, DC brushless......on and on. Power ranges from 0.1 watt to 100,000 watts, voltages from 1-2 to many thousands. I can guess that you have some sort of small DC motor.

3) Will you use a switch or knob control or optical sensor or perhaps output from a computer? The last will be most difficult.

You see there are so many possibilities and your question tells me that you have a long way to go.

Please consider that purchasing a control circuit might make more sense unless you want to first study electrical engineering.

If you want to try again, please describe as completely as possible: Your motor, your power source, and what you want to accomplish with your control.

I wish you a long and rewarding career in electrical engineering as I have had.

Bob Zwicker


Ellis,

An NPN transistor and a PNP transistor are essentially oppositely-polarized. If you look at a circuit as having two power terminals­one positive and one negative­you can look at the two “flavors” of transistors as working with each terminal in the same fashion. In a simple sense, an NPN transistor is good at pulling a voltage to the negative terminal while the PNP is good at pulling a voltage to the positive terminal in the same way. They are completely symmetric in this case. In some circuits, it is desirable to drive current in two opposite directions “pushing” the current in one direction and “pulling” it in another. In these circuits it is easiest to use a completely symmetric circuit with NPN and PNP transistors. This having been said, however, it is also common to create motor drive circuits with NPN transistors only, just working around the simplicity of a symmetric circuit. The reason to use one “flavor” is that if you are designing an integrated circuit with many transistors in one package, the cost of mixing the two types of transistors is not worth simply working around a design using purely NPN transistors. This explanation would best be shown with a schematic of different circuits and how to design each case. Now, in your case, I would have to know the type of motor you are trying to control. A simple direct current (DC) motor is the simplest. All you need is to drive a variable current through the winding to get the motor to spin at different speeds. An NPN transistor with a variable current control will work fine here. In this case, you are driving current in one direction only and a single type of transistor is fine. If you want to reverse the motor, the current needs to flow in the opposite direction, and using both types of transistors will be easiest. One suggestion I would have is to do some digging on the web for “motor control circuits” and look at how others have designed specific motor drivers.

Kyle Bunch, PhD, PE



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