Objective
This lab activity demonstrates and defines how electrical charge relates to voltage, current, and resistance. It introduces Ohm’s law and how to use it to understand electricity along with a simple experiment to demonstrate these concepts.
Electricity Basics
When beginning to explore electricity and electrical engineering, it is useful to start by understanding the basic relationships between voltage, current, and resistance. We cannot see with the unaided eye the energy flowing through a wire or the voltage of a battery sitting on a bench. Even lightning in the sky, while visible, is not truly the energy exchange occurring from the clouds to the ground—it’s the heating of the air by the energy passing through it that produces flashes of light. Detecting this electrical energy requires measurement tools such as multimeters, oscilloscopes, and spectrum analyzers to visualize what is happening with the electrical signals in a system. This lab activity will give a basic understanding of voltage, current, and resistance and how the three relate to each other.
Electrical Charge
Electricity is the movement (flow) of electrons. A light bulb, a fan, a radio, a mobile phone, etc., all harness the movement of the electrons to perform some function.
The three basic principles for this activity can be explained using electrons or, more specifically, the charge they create:
- Voltage is the difference in charge (more electrons, fewer electrons) between two points in space.
- Current is the rate at which charge (electrons) flows between two points, usually through some material.
- Resistance is a material’s tendency to resist the flow of charge (electrical current). Materials with very low resistance are called conductors. Materials with very high resistance are called insulators.
A circuit is a closed loop that allows charge to move from one place to another. Components in the circuit allow us to control this charge and use it to do work.
Georg Ohm was a Bavarian scientist who studied electricity. Ohm’s law starts by describing a unit of resistance that is defined by current and voltage.
Voltage
Voltage is defined as the amount of potential energy between two points on a circuit. One point has more charge (electrons) than another. This difference in charge between the two points is called voltage. It is measured in volts, which, technically, is the potential energy difference between two points that will impart one joule of energy per coulomb of charge that passes through.
When describing electrical properties like voltage, current, and resistance, a common analogy is a water tank. In this analogy, charge is analogous to the volume of water, voltage is represented by the water pressure, and current is represented by the water flow. So, for this analogy, remember:
Water = Charge
Pressure = Voltage
Flow = Current
Consider a water tank at a certain height above the ground. At the bottom of this tank there is a hose. The pressure at the end of the hose can represent voltage. The water in the tank represents charge. The more water in the tank, the higher the charge, and the more pressure is measured at the end of the hose.
Think of the water tank as a battery, a place where a certain amount of energy is stored and then released. Draining the tank a certain amount shows that the pressure created at the end of the hose goes down. Think of this as decreasing voltage, like when a flashlight gets dimmer as the batteries run down. There is also a decrease in the amount of water that will flow through the hose. Less pressure means less water is flowing, which is current.
Current
The amount of water flowing through the hose from the tank is like current. The higher the pressure, the higher the flow, and vice versa. With water, the volume of the water flowing through the hose over a certain period of time is measured. With electricity, the amount of charge flowing through the circuit over a period of time is measured. Current is measured in amperes (or amps (A)). An ampere is defined as 6.241 × 1018 electrons (1 coulomb) passing through a point in a circuit per second.
Now consider two tanks the same size with the same amount of water in them, but the hose on one tank is narrower (smaller diameter) than the hose on the other. Measure the same pressure at the end of both hoses because there is the same amount of water pressing down, but when the water begins to flow, the flow rate of the water in the tank with the narrower hose will be less than the flow rate of the water in the tank with the wider hose.
In electrical terms, the current through the narrower hose is less than the current through the wider hose. For the flow to be the same through both hoses, the amount of water (charge) must increase and thus the pressure in the tank with the narrower hose. This increased pressure (voltage) at the end of the narrower hose pushes more water through the tank. This is analogous to an increase in voltage that causes an increase in current.
There is a third factor to be considered here: the diameter of the hose. In this analogy, the diameter of the hose determines the resistance to the flow of water (charge). This means adding another term to the model:
Water = Charge
Pressure = Voltage
Flow = Current
Hose Diameter = Resistance
Resistance
Consider again the two water tanks, one with a small diameter pipe and one with a large diameter pipe.
The same volume of water cannot fit through a narrow pipe as a wider one at the same pressure. This is resistance. The narrow pipe resists the flow of water through it even though the water is at the same pressure as the tank with the wider pipe.
In electrical terms, this is represented by two circuits with equal voltages and different resistances. The circuit with higher resistance will allow less charge to flow, meaning the circuit with higher resistance has less current flowing through it.
This brings us back to Georg Ohm. Ohm defines the unit of resistance of 1 Ohm (Ω) as the resistance between two points in a conductor where the application of 1 volt will cause 1 ampere, or 6.241 × 1018 electrons per second to flow.
Ohm’s Law
Combining the elements of voltage, current, and resistance, Ohm developed the formula:
Where:
V = Voltage in volts
I = Current in amps
R = Resistance in Ω
This is called Ohm’s law. For example, in a circuit with the potential of 1 V, a current of 1 A, and resistance of 1 Ω, using Ohm’s law it becomes:
Going back to the water analogy, this represents the tank with a wide hose. The amount of water in the tank is defined as 1 V and the narrowness (resistance to flow) of the hose is defined as 1 Ω. Using Ohm’s law, this gives a flow (current) of 1 amp.
Now consider the tank with the narrow hose; because the hose is narrower, its resistance to flow is higher. The resistance can be defined as twice as much or 2 Ω. The amount of water in the tank is the same as the other tank so the voltage is the same. Using Ohm’s law, the equation for the tank with the narrow hose is:
But what is the current? Because the resistance is greater, and the voltage is the same, this gives a current value of 0.5 A:
So, the current is lower in the tank with higher resistance as predicted. If we know two of the values for Ohm’s law, we can solve for the third.
Let’s Demonstrate this with an Experiment
For this experiment, let’s light up a light emitting diode (LED). LEDs are somewhat fragile and should have only a certain amount of current flowing through them. Current larger than the maximum allowed can burn them out. In the data sheet for an LED, there will always be a current rating. This is the maximum amount of current that can flow through the particular LED before it is damaged.
Use the 5 V power supply from the ADALM2000 as the voltage source.
Materials
- ADALM2000 Active Learning Module
- Solderless breadboard and jumper wire kit
- One LED (the longer of the two leads is the anode (+) and the shorter lead is the cathode (–))
- One resistor
NOTE: LEDs are known as non-ohmic devices. This means that the equation for the current flowing through the LED itself is not the simple linear relationship V = IR. The LED is a special kind of diode. All diodes have something called an internal voltage drop. However, this experiment is simply trying to protect the LED from conducting too much current, so we can neglect the nonohmic current characteristics of the LED for the moment and choose the resistor value using just Ohm’s law to ensure that the current through the LED will be safely less than 20 mA.
For this example, the V+ output of the ADALM2000 is configured to generate 5 V and a (red) LED with a current rating of 20 mA, or 0.020 A, is used. To be safe, do not drive the LED at its maximum current but rather its suggested current, which is listed on its data sheet as 18 mA, or 0.018 A. Simply connecting the LED directly to the battery, the values for Ohm’s law look like this:
Rearranging for I:
With just wire and no resistor yet:
Dividing by zero results in infinite current! Not actually infinite in practice, but as much current as the +5 V supply of the ADALM2000 can deliver. We certainly do not want that much current flowing through the LED—a resistor needs to be included.
The circuit connections should look like Figure 1.
Use Ohm’s law to determine the resistor value to provide the desired current value:
Rearranging for R:
Plugging in the values 5 V and 0.018 A:
Solving for the resistance:
So, the resistor value needed for R1 is around 277 Ω to keep the current through the LED under the maximum current rating.
This is not a common value for off-the-shelf resistors, so for this experiment use a 470 Ω resistor (yellow, purple, brown), which is the next closest value greater than 277 in the ADALP2000 parts kit. Figure 2 shows what the circuit should look like all put together.
Success! The resistor value is high enough to keep the current through the LED below its maximum rating, but low enough that the current is sufficient to keep the LED nice and bright. Enable the positive power supply to 5 V. If the LED does not light up, ensure that the (+) and (–) ends of the LED are connected correctly.
This LED/current-limiting resistor example is a common occurrence in electronics. Ohm’s law is often needed to change the amount of current flowing through the circuit.
Current Limiting Before or After the LED?
To make things a little more complicated, place the current-limiting resistor on either side of the LED, and it works just the same (Figure 3).
Many readers learning electronics for the first time struggle with the idea that a current-limiting resistor can be inserted on either side of the LED and the circuit will still function as usual. Try swapping the resistor and LED in the circuit. Does the LED still light up with the same brightness for both cases?
Here is yet another water analogy: imagine a water pipe that is a continuous loop with a pump that continuously circulates the water. If placing a valve somewhere in the pipe, with the valve closed, the water in the entire pipe would stop flowing, not just one section. Now imagine opening the valve a little, which restricts the flow of the water. It wouldn’t matter where in the loop the partly open valve is inserted, it will still slow the flow in the entire pipe. The water does not back up behind the valve. The pressure in the section of the pipe between the outlet side of the pump and the valve increases while the pressure in the section of the pipe between the valve and the outlet side of the pump decreases. The pump is analogous to the voltage source increasing the voltage, while the valve is analogous to the resistor decreasing the voltage.
This is an oversimplification, as the current-limiting resistor can only be placed in the circuit in two places; it can be placed on either side of the LED to perform its function.
For a more scientific answer, we turn to Kirchhoff’s voltage law (KVL), which states that the sum of all voltage changes around a closed loop in an electrical circuit is zero. It is because of this law that the current-limiting resistor can be placed on either side of the LED and still have the same effect. For more information and some practice problems using KVL, visit ADI’s Electronics I course.
Measuring the Actual Voltage and Current
The ADALM2000 also has two input channels that can be used as a voltmeter. Connect them as shown in Figure 4 to measure the actual voltages in the circuit. The Channel 1 voltmeter input is connected to measure the 5 V power supply, and the Channel 2 voltmeter is connected to measure the voltage at the positive (+) end of the diode.
Connect the voltmeter inputs as shown in Figure 5.
Start the Scopy voltmeter tool. The interface looks like the one shown in Figure 6.
Click on the green Run button and the circuit voltages will be displayed. The Channel 1 Voltage should display the actual value of the 5 V power supply. The Channel 2 Voltage should display the actual voltage across the LED. In this example for a red LED, the voltage was 1.84 V. The difference between these two voltages, Channel 1 and Channel 2, will be the voltage across the resistor, which, in this example, is 3.12 V.
Use Ohm’s law to calculate the current in the resistor:
Or:
Or:
6.6 mA
Questions
- How does a current-limiting resistor protect an LED, and how can you calculate the appropriate resistor value using Ohm’s law?
- Why can the current-limiting resistor be placed on either side of the LED and still have the same effect?
You can find the answers at the StudentZone blog.
