Measuring Voltage, Current, and Resistance

Key Points:

  • Voltage is tested using a voltmeter in parallel with the points being tested. An ideal voltmeter has infinite resistance.
  • Current is tested using an ammeter in series with the points being tested. An ideal ammeter has zero resistance.
  • Resistance is tested using an ohmmeter in parallel with the points being tested. Resistance can be tested with a deenergized circuit.
  • These functions are commonly included on most multimeters.

Why it’s Important:

It’s all well and good to understand the concepts and put a circuit together, but it’s also important to be able to measure things like voltage, current, and resistance. Measuring these quantities allows us to test our predictions and to see what’s going on inside a circuit. It can help us to troubleshoot issues when they pop up (they will!) Testing a circuit or component can allow us to verify that it’s working correctly before installing it inside a larger system. The best thing is, testing these quantities is easy with the right tools.

How to Measure Voltage

Recall that voltage is always a potential difference between two points. A device that measures the potential difference is called a voltmeter. A voltmeter will measure the total potential difference between the points being measured. If you want to measure the voltage drop across a single component, be sure to connect the voltmeter to either side of that specific component. If you want to measure the total resistance of a circuit or section of a circuit, a voltmeter connected to either side will do that as well.

Voltmeters can be used in parallel, so it’s not necessary to break a circuit apart in order to use one. Instead, simply connect the two terminals of the voltmeter to the points you are trying to measure. Test leads are flexible wires with either probe tips or ‘alligator clips’ that are used to connect the terminals of the multimeter to the circuit. Probe tips are manually held in place while alligator clips are used when you don’t want to hold the leads. Alligator clips are also safer because they allow you to connect and remove the test leads while the circuit is de-energized so you don’t have to work with a live circuit.

Polarity is important when measuring voltage. Voltmeters are designed to be used with the positive terminal connected to the positive side of the circuit, and the negative terminal connected to the negative side of the circuit. If this orientation is reversed, your measurement will be a negative voltage.

Since voltmeters connect to a circuit in parallel, any current going through the voltmeter will impact the voltage being measured. For this reason, voltmeters are made with as much resistance as possible.

How to Measure Current

Curent is measured using a device called an ammeter. Ammeters measure current in one of two ways:

  1. Most multimeters and traditional ammeters measure current in series with the points being measured. This means that you have to de-energize the circuit, insert the ammeter into the circuit between the measurement points, and then re-energize the circuit.
  2. Clamp-on ammeters have the capability of measuring current running through a clamp. This makes it much easier to take a measurement. In addition, many clamp-on ammeters have other multimeter functionality built in. The trade-off is that clamp-on ammeters typically offer less accuracy.

Ideal ammeters (connected in series) offer zero resistance. Recall Ohm’s Law:

I = \frac{V}{R}

The greater the resistance of the circuit as a whole, the lower the current. If an ammeter offers significant resistance, it significantly influences the measurement of current in the circuit. This is why it’s so important for an ammeter to have as little resistance as possible. That’s what we mean when we say “an ideal ammeter offers zero resistance”.

Recall that in a series circuit, the current is the same at all points of the circuit. In contrast, the current in a parallel circuit depends on the resistance of each branch and voltage applied across that branch. So in a series circuit, an ammeter can be used to measure the current of the entire circuit by placing it in series anywhere in the circuit. In a parallel circuit, two types of measurement are possible:

  1. You can measure the current in any parallel branch by placing the ammeter in series in that branch.
  2. You can measure the total current in the circuit by inserting the ammeter in series at any point where the circuit offers a single path for all of the current in the circuit. This might be between the positive or negative terminal of the power supply and the rest of the circuit, for instance. If there are multiple power supplies, measuring the total current may not be possible.

How to Measure Resistance

Resistance is measured with an ohmmeter. Like voltmeters, ohmmeters are connected in parallel with the points being measured. Unlike voltmeters, the circuit must be de-energized to prevent damage to your ohmmeter/multimeter.

Resistance is a material property that depends on the inherent electrical resistance of the conductors and components in the circuit. It does not depend on the applied voltage or current through the circuit. Therefore, resistance can actually be measured without the circuit energized at all. Ohmmeters are not designed to withstand external current so if you do accidentally use your ohmmeter on a live circuit, it could be damaged.

One thing to keep in mind when using an ohmmeter is that an ohmmeter will read the total equivalent resistance measured between the points on the circuit that are connected to the ohmmeters test leads. So if there are multiple current paths between the measurement points, your reading of resistance could be faulty. In a series circuit, this will not present an issue. However when measuring resistance in parallel circuits, it is best to either completely disconnect the component or section of the circuit to be measured.

Since an ohmmeter or a multimeter can be used to measure the resistance across a single component (like a resistor), it is particularly useful for troubleshooting components.

Using an Ohmmeter to Troubleshoot

Measuring resistance is incredibly useful for troubleshooting, because you can safely determine if your circuit has an unintended open or short circuit.

An open circuit occurs when the current path has been broken. This is a point of infinite (or very high) resistance, where there should be zero or a reasonable amount of resistance. This could mean that you accidentally made the wrong connection. It could also mean that a connection (like a solder joint) is not offering sufficient contact. Or it could mean that a component has critically failed.

A short circuit occurs when there is zero resistance where there should be some resistance. Sometimes this can indicate a serious issue like an electrical arc has occurred. For instance, capacitors should have infinite resistance. If a capacitor is reading zero, or a small number of Ohms, the capacitor has most certainly failed. Make sure however that you aren’t measuring resistance around the capacitor through a different current path in the circuit. As mentioned, an ohmmeter will take any path possible to read the resistance so the resistance across ohmmeter’s test leads has to be the only possible current path for the measurement to be correct.

Congratulations!

You have successfully completed Module 2, and now have a good understanding of some of the most important topics in the fields of electronics and physics.

Module 3 will build on all of these concepts. We’ll work through lots of problems and strengthen our ability to read electrical schematics. By the time you’re done, the topics from Module 2 will be second nature to you.

Let’s continue on to Module 3!