{"id":289,"date":"2014-07-01T18:57:22","date_gmt":"2014-07-01T22:57:22","guid":{"rendered":"https:\/\/itp.nyu.edu\/physicalcomputing\/?page_id=289"},"modified":"2024-08-12T11:58:16","modified_gmt":"2024-08-12T15:58:16","slug":"using-a-transistor-to-control-a-high-current-load","status":"publish","type":"page","link":"https:\/\/itp.nyu.edu\/physcomp\/labs\/motors-and-transistors\/using-a-transistor-to-control-a-high-current-load\/","title":{"rendered":"Lab: Using a Transistor to Control a High Current Load"},"content":{"rendered":"\n

<\/span>Introduction<\/span><\/h2>\n\n\n\n

Transistors are often used as electronic switches, to control loads which require high voltage and current from a lower voltage and current. The most common example you’ll see of this in a physical computing class is to use an output pin of a microcontroller to turn on a motor or other high current device<\/a>. The output pins of a microcontroller can only produce a small amount of current and voltage. But when coupled with a transistor, they can control much more.<\/p>\n\n\n\n

<\/span>What You’ll Need To Know<\/span><\/h2>\n\n\n\n

You should have read the notes on high current loads<\/a> before doing this lab. In order to get the most out of this lab, you should know the basics of electronics<\/a>, as well as how to use a solderless breadboard<\/a>. It would help to do some reading on DC motors as well<\/a>.<\/p>\n\n\n\n

Microcontrollers aren’t the only integrated circuits that produce a low voltage and current on their output pins. There are many components that do this. You’ll see a whole range of so-called logic ICs<\/strong> that can’t produce very much current or voltage, but can produce a small change on their output pins that can be read as a data or control signal. The output voltage from devices is often referred to as a logic<\/strong> or a control voltage<\/strong>, as opposed to the supply<\/strong> or load voltage<\/strong> needed to control the high-current device. You can use transistors from circuits like these. For example, you might put a transistor on the output pin of a 555 timer IC<\/a> (which produces a variable timing pulse), or a shift register IC<\/a> (which allows you to produce multiple control signals in parallel) to control high current loads from those devices.<\/p>\n\n\n\n

<\/span>Things You’ll Need<\/span><\/h2>\n\n\n\n

Figures 1-12 are the parts you\u2019ll need for this exercise.<\/p>\n\n\n\n

\n
\"A<\/a>
Figure 1. A short solderless breadboard.<\/figcaption><\/figure>\n\n\n\n
\"Photo<\/a>
<\/meta>Figure 2. 22AWG solid core hookup wires. <\/figcaption><\/figure>\n\n\n\n
\"5-volt<\/a>
<\/meta>Figure 3. 5-volt voltage regulator, model 7805<\/figcaption><\/figure>\n\n\n\n
\"Photo<\/a>
<\/meta>Figure 4. Resistors. You’ll need 1-kilohm and 10-kilohm for this<\/figcaption><\/figure>\n\n\n\n
\"Diodes.<\/a>
<\/meta>Figure 5. Diodes. Shown here are 1N400x power diodes.<\/figcaption><\/figure>\n\n\n\n
\"Photo<\/a>
<\/meta>Figure 6. DC motor<\/figcaption><\/figure>\n\n\n\n
\"TIP120<\/a>
<\/meta>Figure 7. TIP120 Transistor. You can also use a MOSFET, see below<\/figcaption><\/figure>\n\n\n\n
\"Photo<\/a>
<\/meta>Figure 8. Pushbuttons<\/figcaption><\/figure>\n\n\n\n
\"Photo<\/a>
<\/meta>Figure 9. Potentiometer<\/figcaption><\/figure>\n\n\n\n
\"Photo<\/a>
<\/meta>Figure 10. Force Sensing Resistor (FSR) or other variable resistor<\/figcaption><\/figure>\n\n\n\n
\"DC<\/a>
<\/meta>Figure 11. DC Power Supply<\/figcaption><\/figure>\n\n\n\n
\"Small<\/a>
<\/meta>Figure 12. Small Incandescent lamp bulb or LED bulb and socket<\/figcaption><\/figure>\n<\/figure>\n\n\n\n

<\/span>Set Up the Breadboard<\/span><\/h2>\n\n\n\n

Connect a 7805 5V voltage regulator to your board<\/a>, and power it from a 9-12V DC power supply<\/a>. Connect the ground rows on the sides together. Don’t connect the two red rows on the side of the breadboard to each other, though. Wire the breadboard so that the right side of the board receives the 5V output from the regulator, but the left side gets 9-12V directly from your DC power supply. The 5V line is the 5-volt bus<\/strong> or logic supply<\/strong> and the 9-12V line is the high-voltage bus<\/strong> or load supply<\/strong>. The two ground lines are ground<\/strong>. Figure 13 shows the schematic drawing and Figure 14 shows the breadboard view of the circuit explained here.<\/p>\n\n\n

\n
\"Schematic<\/a>
Figure 14. Schematic drawing of a DC power jack connected to a 7805 5-volt voltage regulator.<\/figcaption><\/figure><\/div>\n\n
\n
\"At<\/a>
Figure 14. DC voltage jack and 7805 voltage regulator on a breadboard. The regulator is supplying 5V and ground holes are supplying voltage to the rest of breadboard.<\/figcaption><\/figure><\/div>\n\n\n
\n\n\n\n

<\/span>Add a Motor and Transistor<\/span><\/h2>\n\n\n\n

The transistor allows you to control a circuit that’s carrying higher current and voltage from the a lower voltage and current. It acts as an electronic switch. The one you’re using for this lab is an NPN-type transistor called a TIP120. The datasheet for it can be found here<\/a>. It’s designed for switching high-current loads. It has three connections, the base<\/strong>, the collector<\/strong>, and the emitter<\/strong> as shown in Figure 15 and Figure 16. Attach high-current load (i.e. the motor or light) to its power source, and then to the collector of the transistor. Then connect the emitter of the transistor to ground. Then to control the motor, you apply voltage to the transistor’s base. When there’s at least a 0.7V difference between the base and the emitter, the transistor will “turn on” — in other words, it’ll allow voltage and current to flow from the collector to the emitter. When there’s no voltage difference between the base and the emitter, the transistor turns off, or stops the flow of electricity from collector to emitter.<\/p>\n\n\n

\n
\"The<\/a>
Figure 15. The schematic symbol of an NPN transistor. B is the base, C is the collector, and E is the emitter.<\/figcaption><\/figure><\/div>\n\n
\n
\"Pinout<\/a>
Figure 16. Pinout drawing of a TIP-120 transistor. From left to right the legs are labelled 1. base, 2. collector, 3. emitter.<\/figcaption><\/figure><\/div>\n\n\n
\n\n\n\n

<\/span>Using a MOSFET instead of a TIP120<\/span><\/h3>\n\n\n
\n
\"Figure<\/a>
Figure 17. FQP30N06L MOSFET transistor pin diagram and schematic symbol<\/figcaption><\/figure><\/div>\n\n\n

You can also use an N-channel MOSFET transistor<\/a> for this. The diagram and schematic symbols are shown above in Figure 17. The IRF520<\/a> and the FQP30N06L<\/a> MOSFETs are similar in function, and have the same pin configuration as the TIP120, and perform similarly. They can handle more amperage and voltage, but are more sensitive to static electricity damage.<\/p>\n\n\n\n

Connect a 1-kilohm resistor from the transistor’s base to another row of the breadboard. This resistor will limit the current to the base.<\/p>\n\n\n\n

You also need to add a diode in parallel with the collector and emitter of the transistor, pointing away from ground as shown in Figure 18 and Figure 19. The diode to protects the transistor from back voltage generated when the motor shuts off, or if the motor is turned in the reverse direction. This is called a snubber diode<\/strong>, or protection diode. <\/strong>Related topics: Transistors, Relays, and Controlling High-Current Loads<\/a><\/p>\n\n\n

\n
\"Schematic<\/a>
Figure 18. Schematic drawing of a transistor controlling a DC motor.<\/figcaption><\/figure><\/div>\n\n
\n
\"Breadboard<\/a>
Figure 19. Breadboard view of a transistor controlling a DC motor.<\/figcaption><\/figure><\/div>\n\n\n
\n\n\n\n

Be sure to add the diode to your circuit correctly. The silver band on the diode denotes the cathode which is the tip of the arrow in the schematic, as shown in Figure 20:<\/p>\n\n\n

\n
\"Schematic<\/a>
Figure 20. Schematic representation and physical representation of a diode.<\/figcaption><\/figure><\/div>\n\n\n

This circuit assumes you’re using a 12V motor. If your motor requires a different voltage, make sure to use a power supply that’s appropriate. The TIP120 transistor can handle up to 30V across the collector and emitter, so make sure you’re not exceeding that. Connect the ground of the motor’s supply to the ground of your microcontroller circuit, though, or the circuit won’t work properly.<\/p>\n\n\n\n

<\/span>Add a Switch to Control the Transistor<\/span><\/h2>\n\n\n\n

To turn on the transistor, you need a voltage difference between the base and the emitter of at least 0.7V. Since the emitter is attached to ground, that means any voltage over 0.7V applied to the base will turn the transistor on.<\/p>\n\n\n\n