We have often used voltage dividers in many circuits. Why are they used? How can we use them?
In this article, we will help you to understand the voltage divider rule. The calculations are also carried out in a simple way. This type of explanation is great for a beginner too. Are you a pro? Because here is all that you need. Sometimes, we forget an important step while making a circuit that can make your project work well.
Are you ready to proceed?
Why must a voltage divider be used?
Imagine you are making a circuit that requires a voltage of 1V but the power supply you have available is 10V. In this situation, you have many options. But the most important step would be to choose a voltage divider.
Also: Learn about Zener voltage regulator
A Zener voltage regulator provides a low voltage out of a higher supply voltage.
This device is quite reliable. It is the most commonly used electronic device. It is also called a reference voltage generator for the amp circuit.
The voltage divider circuit is also called a potential divider. The name is derived from the difference between the electric potential
What is a voltage divider?
Take a look at the simple circuit diagram below. The basic voltage divider circuit consists of two resistors (R1 and R2) that are connected to the power supply (Vs).
The diagram shows a series circuit. Recommended: Learn series circuit works.
In this diagram, the voltage from the power supply is divided between both resistors. It gives the output voltage (Vo). This voltage Vo is the voltage drop across the R2.
This voltage is dependent upon the size of R2 and R1:
- If R2 is much smaller than R1
Vo will be small (very low, near 0V) because most of the voltage drop will be across R1.
- If R2 is equal to R1.
Vo is equal to half of Vs because the voltage is divided equally between R1 and R2.
- If R2 is greater than R1.
Vo is very high near to Vs because most of the voltage is across R2
Voltage divider formula
If we want to understand the output voltage (Vo) in detail, Ohm’s law is very helpful and also provides mathematical knowledge.
Use the formula below to calculate Vo.
Vo = (Vs x R2) / (R1+R2) ……..(#1)
From the above-given principles, we can calculate the truth of the following formula.
First, we use Vs = 10V which makes it easy to calculate. Now test the formula in 3 cases as follows.
- R2 > R1.
Assigned to R2 = 1.5K (1,500 ohms), R1 = 120 ohms.
Vo = (10V x 1,500) / (1,500 + 120)
= 15,000 / 1,620
- R2 = R1
Assigned to R2 = 2.2K (2,200 ohms), R1 = 2.2K.
Vo = (10V x 2,200) / (2,200 + 2,200)
= 22,000 / 4,400
- R2 < R1
Assigned to R2 = 330 ohms, R1 = 4.7K(4,700 ohms)
Vo = (10V x 330) / (330 + 4,700)
= 3,300 / 5,030
Do you have a better understanding now? Look at the circuit diagram below.
The limiting of voltage divider rule
When the current flow through the output is very low, we can use these formulas and approximation rules very well. Only then the Vo value stands true too.
When they are connected to a device with high resistance, such as a voltmeter or input IC the condition is different. For more detailed information, please see the Impedance page. (Coming soon)
Wasted power in a circuit
The voltage dividers draw the current through the 2 resistors so much that there is no load at the Vo terminal. This situation occurs because the current flows through both resistors.
Now, this becomes a circuit that has wasted power.
If the output is connected directly to the base of the transistor, the Vo cannot be higher than 0.7V.
There is a base-emitter in the transistor connection so it behaves like a diode. It can still be used very well. When you go through the examples of other transistor circuits you will have a better understanding.
Voltage Divider: 3 Resistors
Take a look at the example where we add a resistor to the Vo terminal. Does it change Vo? Yes, it will change Vo. But the real question is, how much?
Let’s find out!
Look at the circuit diagram below.
The Vo will drop and the current increases. Here is step by step process to calculate.
- First, calculate the equivalent resistance of R2 and R3 in the parallel combined. Note: R1= 3.9K (3900 ohms), R2=220 ohms, R3=560 ohms.
R = (R2xR3) / (R2+R2)
= (220×560) / (220+560)
= 157.948 ohms
Note: We will compare R = R2 in the formula for Vo (# 1). So, R2 is 157.948 ohms. I hope you understand this idea.
- Find Vo =?
Vo = (Vs x R2) / (R1+R2)
Note: Vs = 12V, R1 = 3.9K, R2 = 157.948 ohms.
Vo = (12 x 157.948) / (3900 + 157.948
= 1,895.376 / 4,057.948
The key principle is to reduce as many resistors as possible to 2 resistors, according to formula # 1.
Using transducer inputs (sensors)
In a normal circuit, an input transducer (sensor) often changes the resistance itself.
For example, the transducer inputs (Sensor) is LDR, Photodiodes, Phototransistors, Microphone, etc. I will explain them later.
We often use them in the voltage divider circuit. After that, when it changes the output voltage also changes.
Next, we have to feed this signal voltage to other parts of the circuit. For example, in the input of an IC or transistor.
The sensor is one of the resistance in the voltage divider circuit. We can connect it to either top (R1) or bottom (R2). When we want the voltage (Vo) to be high, we can work it out in 2 ways.
- Connect the sensor at the bottom (R1). The sensor has high resistance.
Choosing a resistor
Resistor R-value determines the range of the output voltage Vo. For best results, we need a voltage Vo with a wide swing.
- Rmin is the lowest resistance of the sensor.
- Rmax is the highest resistance of the sensor.
And it will be a successful experiment. If R is greater than Rmin, it must be less than Rmax.
A multimeter is used to find the minimum and maximum resistance (Rmin and Rmax) of the sensor so it can be used here. The rough value doesn’t have to be accurate.
Then choose the resistor value: R = square root of (Rmin × Rmax)
or R = √(RminxRmax)
For example, you measure the LDR, Rmin is 100 ohms, and Rmax is 1M.
R = √(RminxRmax)
= 10K (10,000 ohms)
Credit: Voltage Divider Rule with Examples and Applications
Using a variable resistor
Often variable resistors (VR) instead of R are used which are constant. We can adjust Vo more easily.
In this diagram, a variable resistor with LDR in the voltage divider circuit is used.
We can switch between the Variable resistor and the Sensor according to our design requirement.
For example, we can adjust the resistor to set the brightness level high or low that will cause a change in conditions.
We should choose a potentiometer that is greater than the fixed resistor. It will also provide a good adjustment. We should connect the fixed resistor with a Variable resistor in series.
For example, if the suitable constant resistor value is 10K, we can replace it with the R-4.7K series with the VR-10K. This allows the resistance to be adjusted from 4.7K to 14.7K.
How is your understanding now? I hope you understand the principles of the voltage divider better.
You may like these, too.
Sharing is caring!