Multimeter

Digital Multimeter is one of the most essential tools an electronics hobbyist should own. While developing electronics circuits, we come across lots of different parameters which needs to be checked, for example, voltage, current, resistance, continuity etc… there are separate instruments available for each of these parameters, and multimeter is the one instrument having facility for doing all above measurement. There are analog and digital multimeters available. But digital are becoming more common these days and hence we use digital multimeters, which is also referred as DMM. Now and onwards, we use simply multimeter for referring to a DMM. It’s basically a measuring instrument which can measure.

  • Voltage (AC/DC)
  • Resistance
  • Continuity (diode test)
  • Current (AC/DC)

So this single instrument serves all the major tasks required in electronics measurement. If we don’t have multimeter, there are very few chances to troubleshoot a circuit. Multimeter enables us to measure parameters and debug lots of problems in electronics circuit. Let’s see how to use one of the most helpful of all instruments, multimeter!!!

Look at below image for understanding three major parts of multimeter.

parts of dmm

A multimeter consists of three Major Parts

  • Display
  • Selection Knob
  • Probes

The display usually has four digits and the ability to display a negative sign. A few multimeters have backlight displays for better viewing in low light situations.

The selection knob allows the user to set the multimeter to read different things such as current, voltage (V) and resistance (Ω). Have a look in above image

As shown in above figure, multimeter is having a central rotary switch for selecting various knob positions, each position corresponds to a particular parameter, the wheel can be easily rotated to select one of our required measurement knobs.

If we want to measure voltage, or say measure resistance, then we have to connect multimeter with the testing point in our circuit. This job is done by special wires which connect multimeter to components or test points of our circuit. These wires are called as probes. Two probes are plugged into two of the ports on the front of the unit. Have a look at below figure.

DMM insert RED

 

DMM insert Black

As you can see above, COM stands for common and is almost always connected to Ground or ‘- Negative’ of a circuit. The COM probe is generally black colored but there is no difference between the red probe and black probe other than color.

Apart from COM, there are two more ports on multimeter, One is labeled as mAVΩ. mAVΩ is the port that the red probe is conventionally plugged in to. This port allows the measurement of small current (up to 200mA), all the voltage (V), and resistance (Ω). The probes have a specific type connector on the end that gets easily fixed into the multimeter. Any probe with this kind of connector will work with this meter. This allows for different types of probes to be used.

Measuring Voltage

To start, let’s measure voltage on a 9V battery Plug the black probe into COM and the red probe into mAVΩ.

DMM SetVolt

Set the multimeter to “20V” in the DCV range. Almost all electronics use direct current, not alternating current. Connect the black probe to the battery’s ground or ‘-’ and the red probe to power or ‘+’. Press the probes with a little pressure against the positive and negative terminals of the 9v battery. If you’ve got a fresh battery, you should see around 8.9V on the display (this battery is used,so its voltage is slightly less than 9V).

DMM VoltMeasure

If accidentally, you connect the probes in reverse direction, means red probe to negative and black to positive, the multi meter shows the negative reading, no harms!!!

DMM NegVolt

 

If you’re measuring DC voltage in any circuit, you must know a tentative range of voltage that you’re measuring to set the knob. If the measured voltage is more than the specified range, the meter shows “1” means out of range

Overload

What happens if you select a voltage setting that is too low for the voltage you’re trying to measure? Nothing bad. The meter will simply display a 1. This is the meter trying to tell you that it is overloaded or out-of-range. Whatever you’re trying to read is too much for that particular setting. Try changing the multimeter knob to a next highest setting. Look Below Image

DMM outofrange

 

Measuring Resistance

Normally resistors have color codes on them. If you know how to read color codes, its OK! Understanding color codes is really easy and you can read more about it here. However, if you’re having a multimeter at hand, its much easier to measure resistance value and measure it accurately. Let’s see how to do this

Pick out a random resistor and set the multimeter knob to the 20kΩ setting. Then hold the resistor between the metallic ends of the probe and keep it pressed for few seconds

The meter will either read 000 or 1 or actual resistance value

If multimeter reads 0, it means you’ve selected very high range for measurement and hence value cannot be read properly. So decrease the multimeter know to some lower resistance setting

If display reads 1, it means you’re measuring voltage out of the selected range, and now you should increase the range knowb

If the resistor value is in the range, then the accurate value should be displayed, as seen in below shows 998Ohm

See below pictures for example.

DMM measure res

Remember that resistors have a minimum of 5% tolerance. This means that the color codes may indicate 1,000 Ohms (1kΩ), but because of differences in the manufacturing process a 1kΩ resistor could be as low as 950Ω or as high as 1.2kΩ. please note that this is completely OK, and neither multimeter or resistor is damaged J

Continuity

While developing circuits, we often need to check that if one particular point is correctly going to the other point or not. Means if we’re doubtful that there is something loose in the connections, and then we need to check the continuity of connection

DMMcontinuity

Continuity testing is actually testing the resistance between two points. If there is very low resistance (less than a few Ωs), the two points are said to be connected electrically, and a tone is generated for audio feedback. If there is more than a few Ωs of resistance, than the circuit is assumed open or not-connected, and no tone is generated. This test helps insure that connections are made correctly between two points. This test also helps us detect if two points are connected that should not be.

Set the multimeter to ‘Continuity’ mode. As shown below. It may vary among various multimeters, but look for a diode symbol with picture like sound coming from a speaker. This is continuity knob for most meters.

Now touch the probes together. The multimeter generates a tone (Note: if battery of multimeter is low, the tone should be very low). This shows that a very small amount of current is allowed to flow without resistance between probes.

On a breadboard that is not powered, use the probes to poke at two separate ground pins. You should hear a tone indicating that they are connected. Continuity is a great way to test if two pins are touching. The audio feedback of multimeter ensures that you can look at circuit under test and hear the tone for confirmation