How to calculate output voltage

Written by paul mesler
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How to calculate output voltage
Circuit boards are made up of many resistors. (Maria Toutoudaki/Photodisc/Getty Images)

Ohm's law is an important mathematical formula that electricians and physicists use to determine certain measurements in a given circuit. The formula is V = I x R where V is the voltage, measured in volts, I is the amount of current measured in amps or amperage and R is the resistance, measured in ohms. Resistors impede the electron flow within a circuit and, depending on their material, offer more resistance than others. The voltage in a circuit is nothing more than "a source of electric potential," within that circuit.

Skill level:
Moderately Easy

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Things you need

  • Pencil/Pen
  • Paper
  • Calculator

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    Circuit in Series

  1. 1

    Determine the total amperage in the circuit. If you had a circuit and you found that it carried a total current of 6 amps, you should use this as the amperage in the circuit. Remember that in a circuit the total amperage is everywhere equal.

  2. 2

    Determine the total number of resistance in the circuit. You measure resistance in ohms, which is expressed using the Greek letter omega. If you measure that there is a resistor with 3 ohms of resistance in this circuit and another with 2 ohms of resistance, that means that the circuit has a total resistance of 5 ohms.

  3. 3

    Find voltage output by multiplying the amperage by the total number of resistance in the circuit. In the examples above, we know that the amperage is 6 amps and the total resistance is 5 ohms. Therefore, the voltage output for this circuit is 6 amps x 5 ohms = 30 volts.

    Circuits in Parallel

  1. 1

    Determine the total current in the circuit. Just as it is in a series circuit, the current or amperage is everywhere the same. Using the same example, we'll say that the total amperage is 6 amps.

  2. 2

    Find the total resistance in the circuit. The total resistance in a parallel circuit differs from a series circuit. In the series circuit, we obtain the total resistance by simply adding each individual resistance in the circuit; however, in a parallel circuit, we need to find the total resistance by using the formula:

    1/ 1/R1 + 1/R2 +...+1/Rn. That is, one divided by the sum of the reciprocals of all the resistors in the parallel circuit. Using the same example we will say that the resistors have a resistance of 2 ohms and 3 ohms. Therefore the total resistance in this parallel series is 1/ 1/2 + 1/3 = 1.2 ohms.

  3. 3

    Find the voltage the same way you found the voltage in the series circuit. We know that the total amperage for the circuit is 6 amps and the total resistance is 1.2 ohms. Therefore, the total voltage output for this parallel circuit is 6 amps x 1.2 ohms = 7.2 volts.

Tips and warnings

  • If you're using a calculator to find the total resistance in a parallel circuit, don't forget to put parentheses around the bottom fraction. For example when you calculated the total resistance in a parallel circuit you got 1/ 5/6. In a calculator this is different than 1/ (5/6).

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