How to Use a Multimeter
Reading current is one of the trickiest and most insightful readings in our world of embedded electronics. It’s tricky because you have to measure current in series. Where voltage is measure by poking at VCC and GND (in parallel), to measure current you have to physically interrupt the flow of current and put the meter in line. Said another way, pull out the VCC wire going to the breadboard and then probe from the power pin on the power supply to the VCC rail on the breadboard.
Now you can connect the multimeter in series to the white wires.
Let’s say you want to measure an ATmega328 with a blinking LED. The current consumption for many breadboard projects is under 200mA. Make sure the red probe is plugged into the 200mA fused setting. On our favorite multimeter the 200mA hole is the same port/hole as voltage and resistance reading (the port is labeled mAVΩ). This means you can keep the red probe in the same port to measure current, voltage, or resistance. Just remember: you have to measure current in series, not parallel.
Measuring current works the same as voltage and resistance - you have to get the correct range, so set the multimeter to 200mA. In the image above, we’ve pulled the wire out of the VCC pin on the bread board power supply and added another wire to the VCC rail. This way we can more easily connect the red probe to the wire coming out of the power supply, and the black probe to the wire sticking out of the bread board. This effectively “breaks” power to the breadboard. We then insert the multimeter inline so that it can measure the current as it “flows” through to the multimeter into the bread board.
Now connect the red probe to the VCC pin on the power supply, and the black pin to the VCC rail on the bread board. Realize that the multimeter is like a piece of wire - you’ve now completed the circuit and the breadboard is now energized (and will start running). This is important because as time goes on the microcontroller, or sensor, or device being measured may change its power consumption (such as turning on an LED can resulting in a 20mA increase for a second, then decrease for a second, etc). On the multimeter display you should see the instantaneous current reading. All multimeters take readings over time and then give you the average so expect the reading to fluctuate. In general, cheaper meters will average more harshly and respond more slowly, so take each reading with a grain of salt. In your head, take an average range such as 7 to 8mA under normal 5V conditions (not 7.48mA). To give you a rough idea of power consumption: 8mA (0.008A) on a 5V system is incredibly small (0.040W). A light bulb will easily consume 40W, or 1,000 times more energy.
For the pictures above we cheated and used alligator clips. When measuring current, it’s often good to watch what your system does over time, a few seconds or minutes. While you might want to stand there and hold the probes to the system, I’m lazy. So these alligator clip probes come in handy. Note that almost all multimeters have the same sized jacks (they’re called “banana plugs”) so if you’re in a pinch, you can steal your friend’s probes.
Similarly, when measuring current the color of the probes does not matter. What happens if we switch probes? Nothing bad happens! It simply causes the current reading to become negative:
Current is still flowing through the system, you’ve just changed your perspective and now the meter reads negative.
Remember: When you’re done using the meter, always return the meter to read voltage (return the probes to the voltage port, set the meter to read the DC voltage range if necessary). It’s common to grab a meter and begin to quickly measure the voltage between two pins. If you have left your meter in ‘current’ mode, you won’t see the voltage on the display. Instead you’ll see ‘0.000’ indicating that there is no current between VCC and GND. Within that split second you will have connected VCC to GND through your meter and the 200mA fuse will blow = not good. So before you put the meter down for the night, remember to leave your meter in a friendly state.
Measuring current can be tricky the first couple of times. Don’t worry if you blow the fuse - we’ve done it dozens of times! Keep reading, we’ll show you how to replace the fuse in a later chapter.