Voltage divider: device, principle of operation, purpose

Often, when designing an electronic circuit, it becomes necessary to obtain a point with a certain signal level. For example, create a reference point or bias voltage, power a low-power consumer by lowering its level, and limit the current. It is in such cases that you need to use a voltage divider. We will tell you what it is and how to calculate it in this article.

Definition

A voltage divider is a device or device that lowers the output voltage level relative to the input voltage, in proportion to the transmission coefficient (it will always be below zero). It got this name because it represents two or more series-connected sections of the chain.

They are linear and non-linear. In this case, the first are active or reactance, in which the transfer coefficient is determined by the ratio from Ohm's law. Pronounced nonlinear dividers include parametric voltage stabilizers. Let's see how this device works and why it is needed.

Types and principle of action

It should be noted right away that the principle of operation of the voltage divider is generally the same, but depends on the elements of which it consists. There are three main types of linear circuits:

• resistive;
• capacitive;
• inductive.

The most common divider on resistors, due to its simplicity and ease of calculation. Using his example, we will consider the basic information about this device.

Any voltage divider has Uinput and Uoutput if it consists of two resistors, if there are three resistors, then there will be two output voltages, and so on. Any number of division steps can be made.

Uinput is equal to supply voltage, Uoutput depends on the ratio of resistors in the divider arms. If we consider a circuit with two resistors, then the upper, or as it is also called, damping shoulder will be R1. The lower or exit shoulder will be R2.

Suppose we have a power supply of 10V, the resistance R1 is 85 ohms, and the resistance R2 is 15 ohms. It is necessary to calculate Uoutput.

Then:

U = I * R

Since they are connected in series, then:

U1 = I * R1

U2 = I * R2

Then if you add the expressions:

U1 + U2 = I (R1 + R2)

If we express the current from here, we get:

Substituting the previous expression, we have the following formula:

Let's count for our example:

The voltage divider can also be made on reactances:

• on capacitors (capacitive);
• on inductors (inductive).

Then the calculations will be similar, but the resistances are calculated using the formulas below.

For capacitors:

For inductance:

A feature and difference between these types of dividers is that a resistive divider can be used in alternating circuits and in circuits direct current, and capacitive and inductive only in alternating current circuits, because only then will their reactive resistance.

Interesting! In some cases, a capacitive divider will work in DC circuits, a good example is the use of such a solution in the input circuit of computer power supplies.

The use of reactance is due to the fact that during their operation they do not generate such an amount of heat as when using active resistances (resistors) in structures

Examples of use in a schema

There are many circuits where voltage dividers are used. Therefore, we will give several examples at once.

Let's say we are designing an amplifier stage, on a transistor, which works in class A. Based on its principle of operation, we need to set such a bias voltage (U1) on the base of the transistor, so that its operating point is on a linear segment of the I - V characteristic, while the current through the transistor is not excessive. Let's say we need to provide a base current of 0.1 mA with U1 of 0.6 Volts.

Then we need to calculate the resistance in the divider's arms, and this is the reverse calculation relative to what we have given above. First of all, find the current through the divider. So that the load current does not greatly affect the voltage on its shoulders, we set the current through the divider by an order of magnitude higher than the load current in our case, 1 mA. Let the power supply be 12 Volts.

Then the total resistance of the divider is equal to:

Rd = Upower / I = 12 / 0.001 = 12000 Ohm

R2 / R = U2 / U

Or:

R2 / (R1 + R2) = U2 / Upower

10/20=3/6

20*3/6=60/6/10

R2 = (R1 + R2) * U1 / Upower = 12000 * 0.6 / 12 = 600

R1 = 12000-600 = 11400

Let's check the calculations:

U2 = U * R2 / (R1 + R2) = 12 * 600/12000 = 7200/12000 = 0.6 Volts.

The corresponding upper shoulder will extinguish

U2 = U * R2 / (R1 + R2) = 12 * 11400/12000 = 136800/12000 = 11.4 Volts.

But this is not the whole calculation. For a complete calculation of the divider, it is necessary to determine the power of the resistors so that they do not burn out. At a current of 1 mA, power will be released on R1:

P1 = 11.4 * 0.001 = 0.0114 Watt

And on R2:

P2 = 0.6 * 0.001 = 0.000006 Watt

Here it is negligible, but imagine how much power the resistors would need if the divider current was 100 mA or 1 A?

For the first case:

P1 = 11.4 * 0.1 = 1.14 Watt

P2 = 0.6 * 0.1 = 0.06 Watt

For the second case:

P1 = 11.4 * 1 = 11.4 Watts

P2 = 0.6 * 1 = 0.6 Watt

That is already considerable numbers for electronics, including for use in amplifiers. This is not effective, therefore, impulse circuits are currently used, although linear ones continue used either in amateur constructions or in specific equipment with special requirements.

The second example is a divider for forming Uref for an adjustable zener diode TL431. They are used in most inexpensive power supplies and chargers for mobile phones. You can see the connection diagram and calculation formulas below. With the help of two resistors, a point with a Uref of 2.5 volts is created here.

Another example is connecting all kinds of sensors to microcontrollers. Let's consider several schemes for connecting sensors to the analog input of the popular AVR microcontroller, using the Arduino family of boards as an example.

Measuring instruments have different measuring ranges. This function is also realized using a group of resistors.

But the scope of application of voltage dividers does not end there. This is how the extra volts are extinguished when the current is limited through the LED, the voltage is also distributed across the bulbs in the garland, and you can also power a low-power load.

Nonlinear dividers

We mentioned that a parametric stabilizer belongs to nonlinear dividers. In its simplest form, it consists of a resistor and a zener diode. For a zener diode, the schematic symbol looks like a conventional semiconductor diode. The only difference is the presence of an additional feature on the cathode.

The calculation is based on the U stabilization of the zener diode. Then if we have a 3.3 volt zener diode, and the U supply is 10 volts, then the stabilization current is taken from the datasheet to the zener diode. For example, let it be equal to 20 mA (0.02 A), and the load current is 10 mA (0.01 A).

Then:

R = 12-3.3 / 0.02 + 0.01 = 8.7 / 0.03 = 290 Ohm

Let's figure out how such a stabilizer works. The zener diode is included in the circuit in reverse connection, that is, if Uoutput is lower than Ustabilization, the current does not flow through it. When Upower rises to Ustabilization, an avalanche or tunnel breakdown of the PN junction occurs and a current begins to flow through it, which is called the stabilization current. It is limited by resistor R1, which dampens the difference between Uinput and Ustabilization. When the maximum stabilization current is exceeded, a thermal breakdown occurs and the Zener diode burns out.

By the way, sometimes you can implement a stabilizer on diodes. The stabilization voltage will then be equal to the forward drop of the diodes or the sum of the drops of the diode circuit. Set the current suitable for the rating of the diodes and for the needs of your circuit. However, this solution is rarely used. But such a diode-based device is better called a limiter, not a stabilizer. And a variant of the same circuit for AC circuits. This will limit the amplitude of the AC signal to a forward drop of 0.7V.

So we figured out what a voltage divider is and what it is for. There are even more examples where any of the variants of the considered circuits is applied, even a potentiometer in essence is a divider with smooth adjustment of the transmission coefficient, and is often used in tandem with a constant resistor. In any case, the principle of operation, selection and calculation of elements remains unchanged.

Finally, we recommend watching the video, which takes a closer look at how this element works and what it consists of:

Related materials:

• Ways to lower the voltage
• What is active, reactive and apparent power
• How does a voltage relay work?