Thursday, April 16, 2015

Divide that voltage!


Ah yes, the voltage divider -- the foundation for many circuits to come in this learning series.  This circuit performs just that, it divides the input voltage down by a specific ratio that is determined from passive components.  In the simplest case, the voltage divider can be represented with two resistors connected in series, as illustrated in Figure 1.

Figure 1. Voltage Divider Circuit

 To find the voltage, Vx, KVL is performed on the voltage divider circuit.


Now that we have a better understanding of what a voltage divider is, let's now think of some applications where we can use voltage divider circuits.  You may think "Well, by adjusting the R2 resistor I can decrease the output voltage.  If I can do this, can I use this circuit in volume control, like controlling the volume of a speaker?", and you'd be spot on.  You can definitely use a voltage divider for volume control, and in fact some of the earlier radios simply used a potentiometer in place of a resistor to effectively reduce volume with the turn of a knob.  This is illustrated in Figure 2.

Figure 2. Volume attenuation with divider circuit

Sensor Applications

Voltage dividers can also be used in sensor applications, more specifically sensors that change resistance in respect to their environment.  One such sensor that comes to mind is a simple moisture sensor.  Many moisture sensors output a different resistance value with respect to the moisture of their environment.  But how do we determine this resistance value, especially in autonomous applications? In applications such as these, a microcontroller is used to source an input voltage to the resistor divider, and is also responsible for measuring the output voltage of the resistor divider.   The moisture sensor is connected as the second resistor in the voltage divider network.  The image below displays the concept and equations in determining the resistance of the moisture sensor.

Figure 3. Sensor application with divider circuit

Digital-to-Analog Converters

The next application demonstrates how a voltage divider can be used to create a simple digital –to-analog converter.  The function of a digital-to-analog converter, or DAC, is to convert digital data to analog voltage.  The simplest architecture of a DAC can be shown as a simple voltage divider, where a reference voltage is connected to a string of equally sized resistors. The nodes of the resistor string display a voltage that is related to the equivalent resistor-divider impedance of the inspected node.  An example of a simple DAC is provided below.

Figure 4. 2-bit DAC example

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