# Using a DMM

A multimeter is an indispensable tool when analyzing electronic circuits. Let's discover how to use it!

## Digital Multimeter

In the last tutorial we took a look at the fundamentals and learned what electric current and voltage are. Now time to measure them. For this purpose we will use a digital multimeter (DMM).

A DMM allows as to measure different physical quantities relevant to electronics, like voltage and current. Almost all multimeters allow measuring resistance too and some have additional capabilities to measure inductance or capacitance. Today we will focus on voltage, current and resistance. ## A Simple Circuit

For our measurement we will use the circuit shown below. It uses a 9V battery as power source to power an LED protected by a 470 Ω resistor. In electronics, we usually use a different representation form for circuits. We use circuit diagrams with schematic symbols. Here is what our circuit looks like as a schematic: If you are already familiar with that, this should be no problem, but for those who are new to electronics I created a list with some basic schematic symbols. In future tutorials we will continue to add more symbols to this list.

Note, that there are two possible representations for resistors. I will use the IEC version which is used here in Europe. In America an alternative version specified by the ANSI standard is used. ## Measuring Voltage

As discussed in the last tutorial voltage is always measured between two points. One of these points is the reference point. The voltage reading will be relative to it.

The black reference probe should be always connected to the COM connector. For measuring voltage the red probe needs to be connected, to the connector labelled with a V for volts. I think it is actually pretty self-explanatory.

Next we need to select the measuring range. For that we select the next voltage above the one we expect to see. Some multimeters automatically select the correct measurement range. If you don't know what voltage you have to expect, start with the highest voltage and continue by step-by-step narrowing down the range. If you have guessed the voltage incorrectly this is usually not a problem, especially for low voltages. Most DMMs won't be damaged and you can adjust the measurement range at any time if you see that the value is out of range. The maximum voltage acceptable in each range is usually given in the manual of your DMM.

Ready to measure? To measure the voltage of our battery, which should be roughly around 9 V, we have to connect the voltmeter in parallel to it. We can do this by directly probing the battery terminals or by adding some loose wires to the breadboard. If we want to measure the voltage drop over a component in our circuit, e.g. the resistor we would connect the test leads to both pins of our resistor. Or to put it in other words, we measure the voltage by connecting the voltmeter in parallel to the resistor.

A question that might arise, is whether it matters which way around one connects the probes. Well, as told the voltage is always measured relatively to a point. If you swap around the probes you just change the point of reference. This causes a sign change in the reading. The meter now tells you that the negative terminal of the battery has a potential that is 9 V below the one of the positive terminal, by displaying - 9 V. The correct way around (red to + and black to -), it tells you that the positive terminal of your battery has a potential of 9 V above the negative terminal. When interpreting the measurement value, you have to take into account the reference point you used too. Using the probes the wrong way around does not decrease the measurement quality.

## Measuring Current

When measuring the current, we need to connect the meter in series to the other components. If you understood what current is, that will make perfect sense to you. As current is the flow rate of electrons, one has to route these electrons through the meter to allow a measurement. When measuring voltage with the multimeter there is not really a current flowing through the meter. We need to change something. To route the current through the meter we need to remove the red test lead from the voltage measuring connector and put it into the appropriate current connector for current measurement. As this enables current to flow through the meter you can easily short circuit your circuit with the test leads. This happens for instance, if you accidentally try to measure voltage without reconnecting the test leads back to the voltage measurement input. Be careful!

There is a maximum for current the multimeter can handle. A lot of DMM have a connector for small currents in the range of a few hundred milliamperes and one for measuring bigger currents up to multiple amperes. Most meters use a fuse to protect the input. In case you exceed the maximum rating you will blow this fuse. You have to open your DMM and replace the fuse, then. The maximum rating is usually printed next to the connector. If you don't know what current to expect then start with the input rated for higher currents to check whether it is safe to use the more precise input for lower currents. Better safe than sorry.

## Measuring Resistance

Last, but not least, let's measure resistance. We will cover resistance in the next tutorial. A DMM does not measure it directly. It uses ohms law to calculate it. I will explain to you how this works in a later tutorial. To keep it short, you need to know the current and the voltage to calculate the resistance of a component. The DMM does this by supplying a very small fixed current to the component. It then measures the voltage drop above the component. To allow this the probes have to be connected the same way, as for measuring voltage. There is however an additional requirement: the circuit need to be powered off. If the circuit is powered on this will interfere with the fixed current supplied for measurement and you get a totally wrong result. 