Measurement

When you are building electronic circuits, it’s often important to be able to verify the the voltage difference between two points in the circuit, or to observe the amount of current flowing through a point in the circuit. In the photo above the multimeter on the left is measuring the amount of current flowing through the circuit in μA (microamperes, or millionths of an ampere), while the multimeter on the right is measuring the voltage. The one on the right I bought in the 1980s, and it is not very good at measuring small amounts of current. The one on the left I just bought for about US$35 from Amazon and it seems to do a pretty good job measuring microamperes. Well, good for a multimeter anyway.

Ideally, we would all have access to, and know how to operate, oscilloscopes for monitoring our electronics creations. Oscilloscopes are really the only accurate tools for measuring the current or voltage in a circuit in realtime. They often capture millions of data samples per second, and they are are able as a result to capture very brief (transient) upward or downward spikes that multimeters like those above simply cannot see. Multimeters tend to sample only a handful of times per second.
Unfortunately oscilloscopes are generally quite expensive, generally US$250 or more, and are complex to operate.

Note though, there are a few new build-it yourself oscilloscope kits available now, like this one for about US$25 (photo above). These little scopes are not as capable as the expensive ones, but they are much better than multimeters for seeing what really is happening in your circuits. Unfortunately they appear to be even more complex to use than those expensive digital oscilloscopes.

On the other hand, inexpensive multimeters are very easy to use.

So let’s take a look at how to use multimeters to get a pretty good idea of what’s going on in our circuits.

Multimeter Basics

The photographs above show this multimeter in the two modes of operation I want to discuss here. On the left is the most standard mode of operation. At the bottom you can see the black probe wire is connected to the “” (ground, or earth) or “COM” (common) terminal. The red probe wire is connected to the generic input terminal. This is how the multimeter probe wires will be connected for everything that the multimeter does, except for measuring current. The other two terminals (labelled “10A” and “μA mA“) are used for current measurement. There will be more on current measurement later.

With the two probe wires connected as shown in the photo on the left, you can measure resistance, capacitance, and voltage (among other things I won’t get into here). To measure the value of a resistor in ohms, turn the selector dial to Ω (ohms) and connect the probe wires to each connector of the resistor. To measure the value of a capacitor in farads, turn the selector dial to the capacitor symbol (the one similar to “˧(-“, just one above and to the right of the Ω position) and connect the probes to the two connectors of the capacitor (taking care to connect the black probe to the “-” terminal of the capacitor if it is a polarized type). Generally the cylindrical capacitors (shaped like a very small can of food) are polarized and are marked to indicate this (e.g., marked with a large stripe on one side to indicate the “-” terminal, or with the “-” lead being shorter than the other one, etc.). So measuring resistance or capacitance is very straightforward:

Voltmeter

To measure voltage, turn the selector dial to one of the two positions marked “V” depending upon whether you are measuring Direct Current (DC) voltage (the one at the bottom left) or measuring Alternating Current (AC) voltage the one above it and to the right. On this multimeter they indicate these with a straight line with dots below for DC, and a sine curve for AC, but often they will explicitly state DC or AC. The black probe should be connected to the ground or “-” or negative, ground, GND, ⏚, etc. terminal for DC voltage measurement, while the red probe connects to “+” or positive, VCC, VIN, 5V, 3V3, etc. terminal. For AC you generally connect black to the neutral wire and red to the hot wire.

Note that when measuring voltage, you connect the voltmeter (e.g., a multimeter in voltage mode) to the positive and negative terminals of the power supply, so it ends up being connected in “parallel” with the rest of your circuit. The diagram below shows a (polarized) DC power source on the right, and the circuit you are running on the left. To check the voltage you connect the voltmeter across the two power lines as shown below. It’s not really a problem if you reverse the voltmeter probes here. If you have them backward it will just show the voltage as a negative number.

Ammeter

Measuring current, or amperage, is a little trickier. First of all, you need to move the red probe wire. It cannot be plugged into the “input” terminal on the multimeter when you are measuring current.
Instead you must select either the “10A” terminal, or the “μA mA” terminal. If you are measuring current in small electronics devices, where you want accurate measurements of small currents in milliamperes (i.e., mA, or thousands of an ampere) or even smaller and more accurate measurements in microamperes (i.e., μA, or millionths of an ampere), then you should use the “μA mA” terminal. For circuits drawing a lot of power you should use the “10A” terminal because it can handle more current. Be careful not to push more than 10A through a “10A” terminal, or any large amount of power through a “μA mA” terminal, or bad things may happen. Here is what happened when I accidentally pushed about 12A (at 12V) through a very small dedicated ammeter in a project:

So be careful. Don’t break your stuff, and don’t start any fires!

Next you need to choose among the amperage settings of the multimeter. The red meter shown in the photographs at the top of the article has 3 settings for this (“10A“, “μA“, and “mA“). Again, based upon what you are trying to measure, you need to select the appropriate one of these. This particular meter displays an error and gives an audible warning if you, for example, select “μA” but then you attempt to run 2A through the meter. Other meters may have many amperage settings to choose from, or they may be completely automatic and determine the range on their own. The main thing that range selection does here, is determine the amount of resistance used in the power “shunt” that runs through the multimeter. More on the shunt in a moment.

The last thing to note when measuring amperage is that you must connect the ammeter (e.g., a multimeter in any of the amperage modes) in “series” with your circuit. That is, the power going to your circuit must run though the internal “shunt” inside the ammeter in order for the amperage to be measured. The diagram below shows a (polarized) DC power source on the right, and the circuit you are running on the left. To check the voltage you cut the wire delivering power to your circuit, and insert the ammeter in series to reconnect the wire through the ammeter shunt. Normally you want the power source to be connected to the red probe, and the “load” (your circuit) to be connected to the black probe wire connected to the “COM” or common terminal on your ammeter. It’s not really a problem if you reverse the ammeter probes here. If you have them backward it will just show the amperage as a negative number.

Note that you may also measure both current and voltage at the same time if you have two multimeters, as shown in the diagram below:

Observer Effect, and Shunts

The Observer Effect (often conflated with the Heisenberg Uncertainty Principle) describes the situation often encountered in physics experiments where the act of observing something actually alters the outcome. Unfortunately whenever we observe our circuits with a voltmeter or ammeter, we are changing the circuit, so we may alter the outcome by observing with these tools.

In the case of a voltmeter, it is wired in parallel, and it has relatively little effect upon the circuit being observed. In the case of the ammeter though, it is wired in series with our circuit, and it introduces a shunt resistor through which all of the current flowing into our circuit travels. This can have a very significant effect upon the circuit being observed, especially if the circuit uses only a very small amount of current. This is one of the reasons that when using the ammeter you may have to explicitly connect to a different terminal on the meter, and/or explicitly select a measurement range — so an appropriate shunt resistor can be selected to minimize the impact on the circuit being observed.

Fortunately the “Observer Effect” of your meter is usually quite precisely quantified by the manufacturer. My multimeter’s manual contains the information below, for example:

(And I am claiming “fair use” for copying this from the AstroAI WH5000A manual).

Notice that the maximum amount of current permitted for each case is also clearly stated here, along with the inline fuse rating. Whichever meter you use, be sure to consult the manual so you don’t end up frying something like I accidentally did with my little dedicated ammeter, above.

I hope this is helpful. If you have any questions or comments, please submit them below.

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