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Testing Ohms With a Voltmeter

The ohm is the unit of electrical resistivity used in electronics and engineering. Ohms are most commonly tested using an ohmmeter or multimeter, instruments that are specifically designed to measure resistance. You can use a voltmeter to find the resistance in ohms of an electronic circuit or component, but you will also need to use a second tool called an ammeter.

Ohm̵7;s Law
- Ohm̵7;s law is an equation that describes the relationship between current, voltage, and resistance in an electrical circuit. The equation is written as I = V/R, meaning that I, or current in amperes, is equal to voltage, or V, divided by resistance, or R. Because these three quantities are related, if you can measure the voltage and current in a circuit, you will also be able to find the resistance in ohms. Simply substitute the voltage and amperage for the variables in the equation, then solve for R.
Voltmeter Design and Usage
- Voltmeters, like other electronic measurement tools, are intended to interact as little as possible with the circuitry they measure. Although it is impossible to design a test instrument that has no effect on a circuit, voltmeters typically have only a negligible effect, because they are designed with very large resistors. This high level of resistance prevents the voltmeter from drawing current away from the circuit. Voltmeters are always connected in parallel with the circuit or component they are measuring.
Using a Voltmeter with an Ammeter
- Because a voltmeter measures only voltage, you will also need to use an ammeter in order to test the ohms of the circuit or component. Ammeters measure the current in a circuit, and use amperes, or ̶0;amps,̶1; as their unit of measurement. Although voltmeters are always connected in parallel with a component, ammeters should be connected in series. Using a variable output power supply, test the voltage drop and current values for several power levels. You can plot these measurements to find the component̵7;s resistance.
Multimeters
- Multimeters are devices that can be set to measure several different variables, including ohms, volts and amperes. If you use a multimeter as your voltmeter, finding the value of a component in ohms is much easier. Simply set the multimeter to measure ohms instead of volts, then connect the leads in parallel with the component to obtain your reading. The result is the value of the component̵7;s resistance in ohms.

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