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## ASK AN EDUCATOR! – “How can I visualize what transistors and capacitors do?”

Is there a good visual way to see what capacitors & transistors do? While I’ve read the descriptions of what they do, I’m more of a visual person who would like an “ah ha” moment… Thanks!

Prior to teaching our freshman to program microcontrollers, we introduce them to basic breadboard electronics and try to emulate the functions of the microcontroller. It just so happens three of the simple circuits, the LED flip-flop, LED dimmer and transistor amplifier, help to demo the use of a transistor and capacitor. The advantage of using LED circuits, is that it gives you visual feedback. There are also a million other simple circuits that demo transistors and capacitors using things like buzzers, photo-resistors, etc…..I just happen to like these.

The first circuit, an LED dimmer, uses the capacitors ability to store electrical energy (like a battery) and discharge it through the circuit when the primary power is removed. I liked this site, because the author has the circuit’s output connected to a ‘scope and gives a nice visual to the falling voltage, although I don’t condone their breadboard tidiness.

The second circuit, the LED flip-flop, demonstrates the use of transistors to switch on and off a pair of LEDs. This action is controlled by the two base resistors and capacitors in the circuit (RC circuit). When you increase the value of the capacitors the switching frequency slows down and vice versa.

The third circuit, a transistor amplifier, shows a nice demonstration of using a transistor to amplify a high resistance input in order to light an LED. I liked the general description of how a transistor functions as well as using a darlington pair to produce a high-gain amplifier.

These are just two examples that help to visualize the components use. If you are interested in diving deeper, looking at things like filters, you might want to get a scope (i happen to like my DSO nano as it’s cheap and easy to use).

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“Ask an Educator” questions are answered by Adam Kemp, a high school teacher who has been teaching courses in Energy Systems, Systems Engineering, Robotics and Prototyping since 2005.

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1. To explain a capacitor, I’ll tell you three lies that are increasingly accurate:

First lie: A capacitor is like a balloon. As you pump air into it, the pressure inside it rises. If you let air out of it, the pressure inside it falls. That gives you a rough idea of what happens, but doesn’t give you a way to explain things like ‘negative voltages’.

Second lie: A capacitor is like a sealed balloon in a pressure tank. At atmospheric pressure, the balloon fills half the volume of the tank, and the rest of the tank is full of water. As you pump water into the tank, it compresses the balloon and the tank pressure rises. As you pump water out of the tank, it expands the balloon and the tank pressure. ‘Negative voltage’ is what happens when the tank pressure falls below atmospheric pressure. If you open the valve of a tank that’s below atmospheric pressure, it will suck water into the tank.

That one’s better, but doesn’t account for relative voltages.

Third lie: A capacitor is like a pressure tank with a sealed balloon inside, and a knob that lets you set the reference pressure for the tank. The tank’s internal pressure gets added to the reference pressure, whatever that may be. Let’s say you start with the reference pressure at 25psi and pump water in until the tank pressure reaches 30psi. Then you raise the reference pressure to 50psi. That will put the tank pressure at 55psi. If you let water out of the tank so the tank pressure falls to 45psi, then lower the reference pressure to 25psi again, the tank pressure will be 20psi.

There are still some weaknesses in that model, but by the time you can explain them you’ll understand capacitors fairly well.

To explain transistors, I’ll tell you another three lies:

First lie: a transistor is a hovercraft that follows your finger. If you move your finger up, the hovercraft rises with it. If you move your finger down, the hovercraft sinks with it. The neat thing is that the hovercraft doesn’t care how much weight it’s carrying. If you put an ounce on the platform, the fan will push hard enough to make the platform rise and fall with your finger. If you put a hundred pounds on the platform, the fan will blow harder, but it will still keep the platform on the same level as your finger.

In this case, the platform level is voltage. The weight on the platform is the load impedance, and the fan pressure is the current through the transistor. The position of your finger is the base voltage. (all of this is based on an NPN bipolar junction transistor BTW).

Second lie: The hovercraft doesn’t just magically track your finger.. there’s a string that you pull. The string is 6-1/2" long, and the tension in the string is what causes the fan to push harder. There are a couple of important implications from this: first, the platform will always be 6-1/2" below the point where you’re holding the string. If you raise your hand as far as it will go, the platform will still come to rest 6-1/2" below that. Second, your hand has to be at least 6-1/2" off the ground for the fan to start blowing at all.

Third lie: The string isn’t just 6-1/2" long. It’s connected to a linkage that increases the fan pressure exponentially as you pull the string. For every extra 1/4" of string you pull out, the fan pushes ten times as hard. So.. if you have to pull out 6-1/2" of string to lift a 1oz load, you’ll have to pull out about 8-3/8" of string to lift a 100 pound load.

Those should give you an operational model for how BJTs behave in a circuit, and the same basic model works for mosfets (the length of the string will be different, but the overall behavior is the same). If you want to understand the electrodyamics inside a BJT or a mosfet,
that’s a whole series of other progressively-less-wrong lies.

2. Ugh.. forgot to mention some of the conditions for the transistor series of lies:

That model works for an NPN transistor with a resistor at the emitter. The voltage across the resistor is what causes the platform to rise. If you take away the emitter resistor, the platform can’t rise off the ground.. the fan can still move a lot of air, and will still try to blow ten times as hard for every 1/4″ of string you pull out, but the platform will never rise off the ground. The important implication here is that removing the emitter resistor makes it A Bad Idea to try and lift your hand more than about 8-9″ off the floor.

A collector resistor is like a ball on a spring that’s connected to the celing, and is pulled toward the fan’s air intake. The harder the fan blows, the lower the ball falls, and the closer it comes to the input port. If the spring is elastic enough, you can pull the ball close enough to the input port that it starts messing with the fan’s output.

Those models are still weak, but they should give you enough intuition to help you come up with a better model on your own.

3. Zak Zebrowski

Awesome, this has helped a lot. Thanks all!
Zak

4. There is a great explanation of an early transistor on engineerguy.com

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