Section 1.9 Using a multimeter
One thing that's tricky about working with electricity is that voltage and current are usually invisible. And when electricity suddenly becomes visible, it's almost always a bad thing! So tools to measure electrical quantities are essential.
A device which measures voltage is called a "voltmeter", a device which measures current is an "ammeter", and a device which measures resistance is (surprise) an "ohmmeter". However, it's standard to package these instruments together into a single device, which is called a "multimeter", or simply a "meter". These days, multimeters almost always have a digital readout, so they're called "digital multimeters" or DMMs.
Here a picture of a typical handheld digital multimeter, annotated with its main functions:
Subsection 1.9.1 Measuring DC voltage
Remember that voltage is the potential difference between two circuit nodes. So to measure voltage, we'll connect a probe to each node and the meter will tell us the voltage between them.
The multimeter should be set to one of the DCV scales. Each labeled number indicates the maximum value that can be read on that setting.
Most of the scales make sense, but the "2000m" one is a little odd. Why not just put 2V? This is because while the 2000m scale can measure up to 2V, it displays the result in millivolts, just like the 200m scale.
By convention, we'll connect the black lead to the "lower" potential node, and the red lead to the "higher" potential node. The meter will report the voltage of the "higher" node relative to the "lower" node. It's no problem if you get the probes backwards and the black lead ends up on the node with the higher potential; the meter will just display a negative result.
Note that you do not have to modify the circuit in order to measure voltages between various nodes.
Subsection 1.9.2 Measuring DC current
Current is the flow of charge through a wire or device, so a typical DMM must be inserted into the path that the electrons travel through.
By convention, the meter will report a positive current when current flows into the red probe and out the black probe. As with voltage, it's no problem if you get it backwards: if current is flowing in the black probe and out the red, then the meter will report a negative current.
Subsection 1.9.3 Sad truths about the real world
So far, we've pretended that the real-world DMM acts just like the simulated model. Specifically, we have assumed that a voltmeter doesn't allow any current to leak between the two nodes being measured ("infinite impedance"), and therefore doesn't affect the circuit by creating a new path for current to flow. Likewise, we've assumed that an ammeter will allow any amount of current through with no resistance whatsoever ("zero impedance"), meaning that inserting an ammeter into the circuit won't change the behavior of the circuit.
This is actually almost true for voltmeters. It is relatively easy to make a device which measures voltage while allowing almost no current through, so real voltmeters have very high impedance and can be considered ideal for most normal applications.
This is not the case for ammeters. An ammeter has significant resistance (particularly on the lower current settings) and if you're not thoughtful about your measurements you can accidentally change the very thing you're trying to measure!
There's a second real-world effect you need to know: multimeters will be damaged beyond certain limits. Again, voltage isn't the main problem here. Even a cheap DMM can the read the voltage of mains power (120\,V or even 240\,V) without any issue. However, currents of even a few hundred milliamps can spell doom.
How does this happen? One way is simply to try and measure a large unknown current that happens to be too much for the meter. But this is rare — usually you have a decent guess what the current is before you hook up your meter, and (if we've taught you well) you won't try to measure something that you suspect is beyond the meter's limits.
The more common way to damage your meter is a little more subtle, so let's work work through it one step at a time. Suppose you have the circuit from the previous problem, and you were just measuring current using the 200m scale. As before, the multimeter has a series resistance of 2 ohms (i.e., the resistance between the red probe and the black probe is 2 ohms). [ picture ]
Now, suppose you leave your meter in current-measuring mode, but hook it up as if to measure the voltage of the source:
How much current will flow through your multimeter?
How much power will be consumed in your multimeter?
It will display an "over limit" condition
It might display "over limit", but only briefly. Worse things will happen quickly.
An internal fuse will blow
Correct. An internal fuse will blow, breaking the circuit and preventing further damage to the device.
It may shock you or become unsafe to hold
Remember that an over-current situation can occur at quite low voltage, voltages low enough that they will not harm you. Additionally, the DMM case is insulating, so you can safely hold it even when measuring high voltages.
It will get hot or catch fire
Thankfully the designers put in a safety feature to prevent the device from getting so hot that it could catch fire.
What will happen to your multimeter as a result?
Subsection 1.9.4 Measuring resistance
Measuring resistance is easy: set the meter to an approriate scale, and connect one probe to each side of the thing you're measuring the resistance of.
There's only one catch. If you're measuring resistance of a component in a circuit, you'll usually need to disconnect at least one end of the component. Otherwise, you'll be measuring the combined resistance of multiple paths through the circuit.
Also, don't try to measure resistance while the circuit is powered up. At best you'll get nonsensical results, and at worst you could damage your meter or your circuit.
Subsection 1.9.5 Things to ponder
What do you think is inside a multimeter? The brains of the multimeter is a chip which is only able to measure small voltages (roughly +/- 200 millivolts). How do you think it measures large voltages? What about current and resistance? Try drawing a schematic.