Transistors in a Circuit

Exercise Eight: NPN Transistors

Background Knowledge:

How to draw a Schematic
Resistors, Push Buttons, Capacitors, NPN Transistors, and LEDs
Current Flow in a Series Circuit and Parallel Circuit
Ohm's Law and Power

What you'll need:

1 - 9 Volt Battery

1 - 9 Volt Battery Harness

1 - Breadboard

1 - 1N4007 Rectifier Diode

1 - Momentary PBNO

1 - 470-Ohm 1/4 Watt THR

1 - 22 K-Ohm 1/4 Watt THR

1 - 10 uF 35V Radial Capacitor

1 - 100 uF 35V Radial Capacitor

1 - 470 uF 35V Radial Capacitor

1 - 3904 NPN Transistor

1 - Red 5mm LED

The NPN Transistor

The NPN transistor is turned on when voltage and current are applied to the base. The NPN transistor acts very much like a water faucet; a little pressure on the handle opens the valve, releasing the water under pressure. When a little voltage and current is on the base of the NPN transistor leads to a very large increase in the flow of current through the NPN transistor from the collector to the emitter. For more information, please check out our page on Transistors. In this circuit, we will be using one of the most basic and common NPN Transistors, the 3904.

The NPN is truly electronic. It acts like a normally open push button but has no moving parts. The transistor is the basic electronic switch. Transistors are commonly packaged in the TO-92 case shown to the right. Notice how the legs correspond to the schematic symbol. Note the arrow inside the schematic symbol. It indicates two things: First, it points in the direction of conventional current, towards ground. Second, it is always on the side of the emitter. It is important to identify the legs of the transistor.

Identifying which leg is which on the TO-92 package is easy to remember. Hold the transistor with the flat face towards you. The legs are in the following order: EBC: Emitter, Base, and Collector. The common expression to remember for the acronym EBC is “Enjoy British Columbia.” EBC: Emitter, Base, and Collector.

There are thousands of different types of transistors. The only way to read them is to read the numbers printed on the face of the package itself. But even with thousands, there are only two basic types of transistors, the NPN transistor, and the PNP transistor. Be sure to always refer to the Datasheet for each transistor you use moving forward. The Datasheet will provide you with key details about the form and function of the transistor.

You should have already used the capacitor to store small amounts of electricity. If not, check out our page one Capacitors in a Circuit. It powered the LED directly, but could only do that for a brief moment. In this experiment, we use the capacitor to power the transistor.

What to Expect:

The LED stays off as you attach your power source. Push and release the push button. The LED will turn on immediately. It will dim and turn off. You will notice how the LED is able to stay on for much longer with the same size capacitor than when the capacitor was directly powering the LED. This is because the transistor base consumes far less power than the LED. This action is faster with small capacitors.

How This Circuit Works:

When you push the button, you allow the charge to flow into the base of the NPN transistor and fill the capacitor. When you let go of the button, the charge from the battery is no longer putting pressure on the base leg keeping the transistor flowing from the collector to the emitter. At this point, the circuit is using the charge held in the capacitor to power the transistor. The transistor provides the path for the current to the LED. Because the base of the transistor uses much less power than the LED, the voltage drains from the capacitor very slowly. The higher value resistor of 22,000 ohms slows the drain from the capacitor significantly. The LED stays on much longer.

Steps

Once you have the circuit built using your breadboard follow the steps below and consider what is happening in the circuit.

Draw the schematic diagram and label the components.

When labeling your components in a circuit each resistor will be R#, so in this circuit R1, R2, R3, and R4. R1 will typically be the resistor closest to the positive node.
Your circuit should also have the nominal values of each component annotated on the schematic diagram.
- With Resistors, you can find this using the Resistor Color Codes.
- With polarized radial capacitors, the nominal value should be written on the casing.
- With transistors, a model number will be written on the casing. A 3904 is an NPN and a 3906 is a PNP. You can also tell an NPN and PNP from the schematic.
Using what we know about current, you should also label the schematic with the anticipated current flow direction.

Press and release the push button.
Consider the path of the current that provides the power to the LED.

*Hint: Something to consider is that the capacitor is not powering the LED. It is only powering the transistor.

Record your data in the table below.

Before you start to record in the table below, how many times longer do you think the 500uF capacitor will stay on compared to the 100uF capacitor? How about a 1mF capacitor compared to the 100uF capacitor?

Questions:

How accurate were your predictions?
Why does it take longer for the capacitor to drain in this circuit than in Exercise Seven: Capacitors and Push Buttons?
What is the purpose of a transistor?
How do you tell which leg of the transistor is the emitter?
Which lead of the transistor is the base?
What two separate things does the arrow inside the transistor symbol indicate?
What is the only way to tell the type of transistor?
After you release the push button, what part provides the power to the base of the transistor?

Exercise Nine: The PNP Transistor:

Background Knowledge:

How to draw a Schematic
Resistors, Push Buttons, Capacitors, NPN Transistors, and LEDs
Current Flow in a Series Circuit and Parallel Circuit
Ohm's Law and Power

What you'll need:

1 - 9 Volt Battery

1 - 9 Volt Battery Harness

1 - Breadboard

1 - 1N4007 Rectifier Diode

1 - Momentary PBNO

1 - 470-Ohm 1/4 Watt THR

1 - 22 K-Ohm 1/4 Watt THR

1 - 100 K-Ohm 1/4 Watt THR

1 - 1 M-Ohm 1/4 Watt THR

1 - 10 uF 35V Radial Capacitor

1 - 100 uF 35V Radial Capacitor

1 - 3906 PNP Transistor

1 - 6-12V Piezo Buzzer

1 - Red 5mm LED

This exercise introduces the PNP transistor, using the 3906 PNP. Above, we introduced the 3904 NPN. They are opposites but evenly matched in their properties. The identity of the legs on the TO92 package stay the same as shown in figure to the right. But look closely at the schematic symbol for the PNP which holds some very important information. The arrow inside the schematic symbol still points in the direction of the conventional current flow, but is on the top side now. This is because it is on the side of the emitter, which means that the PNP emitters and collectors have reversed positions relative to the NPN. The legs on the package are still the same, though. The emitter and collector have traded positions relative to the current flow.

Not only are the emitter and collector positions reversed, but the action is reversed as well. The PNP transistor’s action is the opposite of the NPN. The PNP starts in a closed position allowing current to flow passively. As you increase the voltage to the base, the PNP starts to open, decreasing the flow of current. The circuit that is flowing through the PNP is turned off more and more until the max voltage is on the base and the entire circuit turns off. The valve starts in an open position.

Experiment: PNP Transistor Circuit

The capacitor is powering the transistor. But remember for this PNP transistor, when the capacitor is charged, it is going to put pressure on the base of the transistor that will stop the flow. Note the similarity between this schematic in figure L10-6 and what the schematic was for the NPN transistor. Also, notice that the transistor in the image to the left is physically reversed compared to the NPN transistor in the previous lesson.

What to Expect:

The LED turns on as soon as you attach your power source. Push and release the push button. The LED will turn off immediately. It will slowly turn back on.

How it Works:

When you first attach your power source, the LED turns ON immediately because there is no voltage pressure pushing at the base, so the valve is in the opened position, allowing the current to flow from the emitter to the collector. When you push the button down, the voltage is immediately pushed against the base of the 3906 PNP transistor which closes the valve and blocks the current flow- voltage also fills the capacitor C1. After you release the push button, C1 holds the voltage pressure, and keeps the voltage on the base, keeping the valve closed and the current cut off from flowing through to the LED. As the voltage drains from C1 through R1, the voltage pressure against the base is released. The transistor enters flow control and starts passing current and voltage again slowly. The LED will slowly turn back on to its brightest point.

So, Why the extra resistor (R1)?

Before the push button is closed, both C1 and the base of the 3906 PNP transistor have no voltage. Because there is no voltage pressure on the Q1’s base, the valve is open, and current flows from emitter to collector. When the voltage in the capacitor is high, Q1’s valve stays shut and the path for the current to escape from C1 through the transistor is blocked because the valve on the PNP transistor is closed. R1 is necessary to drain the charge from the capacitor. This allows the 3906 PNP transistor's valve to open again.

In other words, the capacitor is unable to drain and the transistor stays off because the voltage from the capacitor keeps the pressure on the base of the transistor, keeping the valve closed. The capacitor cannot drain through the base of the PNP transistor like it did in the previous 3904 NPN circuit. The extra resistor allows the cap to slowly drain, decreasing the voltage pressure on the base of the PNP transistor, allowing the valve to reopen and let current flow again.

Steps

Once you have the circuit built using your breadboard follow the steps below and consider what is happening in the circuit.

Draw the schematic diagram and label the components.

When labeling your components in a circuit each resistor will be R#, so in this circuit R1, R2, R3, and R4. R1 will typically be the resistor closest to the positive node.
Your circuit should also have the nominal values of each component annotated on the schematic diagram.
- With Resistors, you can find this using the Resistor Color Codes.
- With polarized radial capacitors, the nominal value should be written on the casing.
- With transistors, a model number will be written on the casing. A 3904 is an NPN and a 3906 is a PNP. You can also tell an NPN and PNP from the schematic.
Using what we know about current, you should also label the schematic with the anticipated current flow direction.

Press and release the push button.

Pay attention to what happens and reflect on how it is different from the NPN circuit.

Questions:

In the schematic, Q stands for what component?
The arrow in the transistor represents what action?
The arrow is always on the side of which leg in the schematic?
What would happen if R3 were not in the circuit and the LED was connected directly to the collector of the 3906 transistor?
What would happen if you replaced C1 with the 100 microfarad capacitor?
Why does changing the capacitor affect the circuit this way?
Change C1 back to 10 microfarads. Now, what would happen if you change R1 to 1 megohms?
Think of the capacitor as a sink, holding water. Think of the resistor as the drain pipe. Which statement best explains how changing to a higher resistance has the same effect as changing to a larger capacitor?

The drain is bigger and empties the water faster.
The drain is smaller and empties the water slower.
The volume of water is bigger and takes longer to drain.
The volume of water is smaller and drains faster.

Would you be able to explain how this circuit works in your own words?
Why does this circuit need R1, the 100K-ohm resistor?

Extra Experiment: Replace R3 and the LED with a piezo buzzer. Make sure the buzzer’s positive red wire is getting voltage from the 3906’s collector, and the negative black wire is connected to ground.

Push the PB and release. What happens to the buzzer as the capacitor discharges.

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