Last edited 30 August 2014 by Tom Igoe
This lab will introduce you to a few basic electronic principles by trying them in action. You’ll learn how to measure voltage, amperage, and resistance using a multimeter. You will also learn about components in series vs. parallel and be introduced to Ohm’s Law in practice. For more information on the theory behind this lab, please check out these notes.
When you’re building electronics, you run into problems all the time. Diagnosing those problems relies not only on a good theoretical knowledge of how circuits work, but also on practical knowledge of how to test them. The most common tool for testing circuits is the multimeter, a device that can measure current, voltage, resistance, and electrical continuity. More expensive multimeters can also measure other electrical properties, but if you can measure these four, you can diagnose most common circuit problems.
To get the most out of this lab, you should be familiar with the following concepts beforehand. If you’re not, review the links below:
Safety Warnings! Check below when measuring Amperage in order to avoid damaging your meter!
|Solderless Breadboard||22-AWG hook-up wire||Voltage regulator||Soldered DC Power Jack||Wire strippers|
|LEDs||Potentiometer||220-ohm resistors (anything from 100 to 1-kilo Ohm will do)||Switch||Multimeter|
by Deqing Sun
Before you get started, you will want to make sure your meter is working. This is a particularly good idea if you’re using a meter that lots of other people use, such as the ones at ITP. Here is how to test that:
The image below, by Erin Finnegan, shows the controls on a typical meter. Different meters have different functions and controls, but this is a fairly typical set:
Continuity is simply whether or not there is a connection between two points. You just used this function to test your meter. Now you’ll use it to test a conductor.
You can use the continuity check to find connections on a switch or if the pushbutton is connected when you press the button. You can also use it to measure whether there’s a break in a wire, or whether a given material conducts electricity. Set your meter as shown here, and try touching the leads together. The meter should beep.
Now try touching the leads to two ends of a wire. Again, the meter should beep. The wire conducts electricity. There is a continuous flow of electricity from one end of the wire to another.
If you touch two points on a switch, what happens when you switch the switch? Beep or no beep? When you close the switch, the meter should beep, indicating that there is continuity between the two leads of the meter.
Put a wire in one hole of a breadboard. Then put another wire in another hole, chosen at random. Measure continuity between the two wires. Did you get what you expected? If the two holes were in the same row (or in the same column, on the side of the board) then you would get continuity and the meter would beep. If they were in different rows, then it would not beep.
Try measuring the continuity across your hand. Do you get anything? Why or why not? You probably don’t because the resistance across your skin is so high that it doesn’t register as a continuous conductor. It can conduct small amounts of current though. You don’t want your body to carry high amounts of current or voltage though, because it can damage or kill you.
For the rest of this lab, you’ll need a breadboard connected to a +5 Volt power supply. You can use an Arduino as your power supply, if it’s connected to a USB power supply or a DC power supply, or you can solder together a DC power jack as shown in the Soldering lab, and use a 9-12V DC power supply and a 7805 voltage regulator. The voltage regulator will take the DC power supply’s Voltage and regulate it down to 5 Volts DC.
|Arduino used to supply 5 volts to the breadboard||DC power supply with 7805 voltage regulator used to supply 5 volts to the breadboard|
From here on out, diagrams will show the DC power supply and voltage regulator version, but feel free to use the Arduino version instead.
Resistance is a material property of any component, like a resistor or a wire. To measure the resistance of a component, you have to remove the component from the circuit. To measure resistance, turn your meter to the setting marked with the Greek letter Omega (Ω):
Ideally, you want the meter set to the approximate range, and slightly higher than, of the component’s resistance. For example, to measure a 10-Kilohm resistance, you’d choose 20K, because 10K and 20K are in the same order of magnitude. For a 50K resistance, you’d have to step up to 200K, and so forth. If you don’t know the component’s resistance, start with the meter set to a high reading, like 2M (2 Mega Ohms). If you get a reading of zero, turn the meter one step lower, and keep doing so until you get a good reading.
Not all components will register resistance. For example, a wire will ideally register 0 Ohms, because you want wires to have as little resistance as possible so they don’t affect the circuit. The longer the wire, the greater the resistance, however. Likewise, switches have ideally zero resistance.
The circuit shown is not complete. The resistor connecting the LED to voltage has been removed to measure its resistance. To measure resistance of a component, you must remove it from the circuit.
If you measure the resistance of a diode (such as the LED shown here), you may see a number flash briefly on the meter, then disappear. This is because diodes ideally have little or no resistance once voltage is flowing through them, but have what’s called a forward bias, which is a minimum voltage needed to get current flowing. Before you reach the forward bias voltage, the diode’s resistance is ideally infinite. After you reach it, the resistance is ideally zero. There are no ideals in electronics, however, which is why you see a resistance value flash briefly as the meter meets the diode’s forward bias.
Try measuring the resistance across your hand. Set the meter really high, perhaps 20 MegaOhms. Do you get anything? You should get a resistance in the 2-20 MegaOhm range. Make your palm sweaty, or lick it, and try again. You should get a lower resistance, perhaps 0.2 MegaOhms or so.
Once a circuit is complete and powered, the first thing you should do is learn to read voltages between different points in the circuit. Wire a 7805 5-Volt voltage regulator on a breadboard as shown in the breadboard lab and connect it to power. The NYU Book store, Radio Shack, and just about every electronics store will carry the voltage regulator, but if you don’t have one, you can use the 5V output from an Arduino as well.
|Made with Fritzing|
Note how the long leg, or anode, of the LED goes to voltage, and the short leg, or cathode, goes to ground.
Voltage is a measure of the difference in electrical energy between two points in a circuit. It’s always measured between two points in a circuit. Measuring the voltage between the two sides of a component like an LED tells you how much voltage that component uses. When you’re measuring voltage between one side of a component and another, for example, it’s called measuring the voltage “across” the component.
When you’re measuring voltage across a component, you’re putting the meter in parallel with the component. In that case, the voltage across both the component and the meter should be the same.
Set your multimeter to measure DC volts. The voltage regulator you’re using can take an input voltage range of about 8 to 15 volts, and it outputs 5 volts, so you know that no voltage you’ll read in this circuit is over 15 volts. If your meter has a variety of ranges for DC volts, choose a range that ‘s closest to, and greater than, 15 volts. For example, many meters have a setting for 20 volts, meaning that they can read up to 20V DC at that setting.
Measure for voltage between the power and ground bus rows on the breadboard. You should have 5 volts, or very close to that.
Did you get a negative voltage? Why would that happen? That means you placed the red lead on the point of lower voltage, and the black lead on the point of higher voltage. In other words, you reversed the polarity.
Connect the board to your power supply and press the switch. It will illuminate the LED. Let go of the switch and it will turn the LED off again. By pressing the switch you are completing a circuit and allowing the resistor and LED to begin consuming electricity. The resistor is very important in this circuit as it protects the LED from being over-powered, which will eventually burn it out. A typical LED operates at a voltage of 2.0-2.6 volts (V) and consumes approximately 20 milliamps (mA). The resistor limits the current to a level that is safer for the LED to consume. The higher the resistor value, the less electricity that will reach the LED. The lower the resistor value (with 0 ohms being no resistor at all), the more electricity that will reach the LED.
Now, while playing with the switch, measure the voltage across the switch as you did in the last step, both in the on position and the off position. Measure the voltage across the LED and the resistor as well. Does the total resistance across all the components add up to the voltage between power and ground on your board? Remember, in any circuit, all of the voltage must be used up. Why? If the voltage across all the components doesn’t add up, that indicates to you that some of the electrical energy is getting converted to light, heat, and other forms of energy. No component is 100% efficient, so there’s always the possibility for some loss.
Measure the voltage across the resistor. Then measure the voltage across each LED. Does the total add up to the voltage from power to ground? If not, where does the missing voltage go? The remaining energy is lost as heat generated from the components.
|Measuring voltage across the resistor||Measuring voltage across the first LED||Measuring voltage across power and ground|
Did you use two different color LEDs and get a different voltage drop across each one? That’s normal. Because different color LEDs are made with different elements, and have slightly different voltage drops. Did you get no reading when you measured? Did you remember to push the button before you took your reading?
Do they light? Why or why not? They most likely will not light up. Each LED needs about 2V to reach its forward bias and turn on. If you have three in series, and a 5-volt supply, each is getting less than the 2V it needs to turn on.
Measure the voltage across each LED. It should be the same across each one.
Now you’re going to read the amperage at various points in the circuit. Move your meter’s red lead to the hole for measuring amperage. On many meters, there are three holes, one marked “Volts/Ohms/Hz”, another marked “mA”, and another marked “10A”. The middle one can be used for measuring amperage when the expected amperage is less than 1A. The latter is for measuring high amperage, up to 10A. If you’re not sure, it’s best to use the hole for 10A. Then set your meter to measure DC amperage.
To measure the amperage through a given component, you need to place your meter in series with the component. When two components are in series, the amperage flowing through both of them is the same. To measure the amperage through any one of the leds in this circuit, you’ll need to disconnect one of its ends from the circuit (disconnect power first!) and use the meter to complete the circuit, like so:
|Correct position for metering amperage||The second LED's anode leg has been moved so that there is no electrical connection with the other LED's anodes: the meter completes the circuit|
You’ll find that the amperage drawn by the LEDs is tiny, on the order of 10 or 20 milliamps at the most. That’s normal for LEDs. Make sure that you check which holes your leads are connected to when you’re using a meter.
Warning: Measuring amperage with the red lead in the voltage hole when you have no idea how big the current is, or measuring voltage with it in the amperage holes is a good way to damage the meter.
In this last step, you’ll generate a changing voltage using a potentiometer. A potentiometer is a resistor that can change its resistance. A potentiometer (or pot) has three connections. The outer leads are the ends of a fixed value resistor. The center lead connects to a wiper which slides along the fixed resistor. The resistance between the center lead and either of the outside leads changes as the pot’s knob is moved.
As you turn the potentiometer from one end to the other, measure the voltage at the center position. The pot is acting as a voltage divider, dividing the 5V into two parts. As the voltage feeding the LED goes up or down, the LED gets brighter or dimmer. The 220-ohm resistor in the circuit protects the LED from overvoltage when the resistance between the pot’s 5V lead and its center lead is 0 ohms.
Now you’ve got a basic understanding of how to use a meter to measure voltage, current, resistance, and electrical continuity. You’ll use these tests all the time.
Next, check out the lab on Switches.