DMM – Digital Multi Meter is the device which is used to measure different types of electrical quantities like DC Voltage, AC Voltage, DC Current, Resistance. Along with these the latest DMM’s are designed in such a way that they can also be used to test the Diodes, Transistors and many more things.
Now in this post we will discuss about how to measure AC voltage, DC Voltage, DC Current, Resistance, Diode test, Transistor test, Continuity test.
Making ready the DMM :
For a DMM to measure different quantities there will be two test leads. One is of red colour (representing positive) and the other is of black colour (representing negative).
There will be three slots (some may have 4 slots) given in the DMM to place these two test leads.
Place the black test lead in COM slot. The red test lead should be placed in the VΩmA slot if you want to measure AC Voltage, DC Voltage, Resistance, mA & µA DC current. If you want to measure high DC Current then you should place the red test lead in 10A slot.
DC Voltage :
- In DC Voltage measurement, you will have different ranges of voltages to measure like 200mV, 2V, 20V, 200V, 600V etc., (The range readings may vary according to manufacturer)
- According to the range of voltage you need to measure place the rotator switch at that voltage. For example, if you want to measure the voltage in a 12 Volt battery, then place the rotator switch of DMM at 20V.
- Now take the test leads and place the red test lead on the positive terminal of battery and place the black test lead on the negative terminal of the battery.
- Finally, you can then see the voltage reading on the LCD screen of DMM.
Note : While checking if the terminals are interchanged then don’t worry, nothing happens there you will see the reading with the same magnitude but with negative sign only.
- Similarly, you can check different ranges of voltages by placing the rotating switch accordingly.
- But be cautious while measuring high voltages like 200V and above.
AC Voltage :
- In AC Voltage measurement, you will have different ranges of voltages to measure like 200V, 600V etc., (The range readings may vary according to manufacturer)
- Keep the rotator switch in the range you need to measure.
- Place the test leads across the load or source where you want to measure the voltage.
- See the reading on the LCD display.
- While measuring AC Voltages be cautious because it is highly dangerous to deal AC rather than DC.
DC Current :
- In DC Current measurement, you will have different ranges of currents to measure like 200µA, 2mA, 20mA, 200mA, 10A etc., (The range readings may vary according to manufacturer)
- Keep the rotatory switch in the range you need to measure.
- Place the test leads in the series, where you want to measure the current.
- Note the reading from an LCD screen.
- In Resistance measurement, you will have different ranges of resistances to measure like 2MΩ, 200kΩ, 20kΩ, 2kΩ, 200Ω etc., (The range readings may vary according to manufacturer)
- Keep the rotator switch in the range you need to measure.
- Connect the test leads across the resistor for which you want to measure the resistance.
- See the resistor value on the LCD screen
Note : If the resistor for which you want to measure the value is connected in a circuit, then turn off the power supply to the circuit first and if there are any capacitors present in the circuit, discharge them and then measure the resistance value otherwise the power present in the circuit may damage the DMM.
- If you want to check the diode, whether it is working well or not, then keep the rotator switch in (Diode) position.
- Now connect the red lead to anode of the diode and black lead to cathode of the diode.
- If you are checking a Silicon diode then the reading in the LCD display should be between 0.5 to 0.8 Volts (or a reading between 500 to 800). If you are checking a Germanium diode then the reading should be between 0.2 to 0.3 Volts (or a reading between 200 to 300). A shorted diode will give reading between 0 to 0.4 Volts (or a reading between 0 to 400).
- For suppose if you have connected the terminals in the wrong direction then you will see 0V reading in the display that is because a diode allows current in only one direction that too from anode to cathode only. (In some DMMs you will get 1 as reading rather than 0)
- This shows that the diode is working good.
- But even though if you have connected the diode in proper condition and if LCD shows 0V reading then it describes that your diode is not working properly.
- The reading displayed on the LCD screen is the forward voltage drop of the respective diode.
- If you want to check the transistor, whether it is working well or not, the keep the rotator switch in hFE (hFE = small signal forward current gain of the transistor) position.
- First to check the transistor, remove the test leads and place the transistor leads in E C B E slots of either NPN or PNP blocks according to the transistor you are checking.
- For example, if you are checking an NPN transistor having pins in the order E C B then place the transistor in NPN block’s first three slots.
- If you get any value in the display, then it proves that the transistor is working perfectly otherwise not.
- If you don’t know the transistor what you are using then do random check by placing the transistor in different positions of E C B E slots in NPN or PNP block.
- An additional advantage of latest DMM’s is Continuity test.
- By doing a continuity test we can determine whether the circuit is completely closed or not.
- For doing this test place the rotator switch in position.
- Then take the test leads and place them at any two points of the circuit where you want to check the continuity.
- If continuity exists, then the buzzer will make the sound otherwise not.
Some Precautions :
Being enthusiastic is not at all wrong, but being careful is also important.
- Always be careful while working with voltages more than 60V DC and 30V AC. Don’t touch the metal contacts of the test leads while measuring.
- While measuring transistors make sure that the test leads are removed from the DMM.
- No transistor should be placed in the transistor socket while measuring other readings.
- Never perform resistance measurements on live circuits (which have a power supply).
So these are the some of the measuring techniques to measure different quantities using a DMM.
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