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Voltage Divider Calculations with and without Load



  • Introduction

    A voltage divider provides a simple means to step down the voltage in a circuit. For example, the voltage divider below with R1 = 500 [ohm] and R2 = 358 [ohm] can step down a 12 Volt source down to approximately 5 Volts. However, it should be noted that this 5 V output is only obtained without load on the voltage divider. Or, stated in another way, 5 Volts is obtained when the load has infinite impedance. With a resistive load on the circuit, the actual voltage seen at the output will be less than 5 Volts. In the example below, if you select the second row in the table, a 100 [ohm] load is added to the circuit. With this load, the output voltage of the voltage divider drops down to 1.6 Volts. If smaller resisters are chosen for R1 and R2, the voltage drop for a low impedance load will be reduced with the drawback of more power lost due to increase current passing through R2, as discussed next.


    Another drawback of voltage divider circuits is that power is lost to the current that is always passing through resistor 2. This current is sometimes referred to as quiescent current, which is the name given to the current consumed by a circuit while it is idle. The end of this sheet contains the calculation of the current and the electrical power dissipated by the voltage divider.


    Inputs

    The input voltage and the two resistances of the voltage divider circuit are entered below. The table below allows the configurations with and without load to be compared. Use the radio buttons at the left side of the table to select the configuration. The output voltage equation for the loaded case was derived using the equation solving capabilities of EngineeringPaper.xyz using this sheet.









  • Without Load
    With Load


  • Output Voltage





  • Quiescent Current and Dissipated Power Through Resistor 2