Looking at a digital multimeter diagram will greatly help you to understand why it is called a multimeter. Just as its name states, multimeter means a meter with a multi function, multi measurement, and multi purpose. Of course all of them need to be in the scope of electrical and electronics.
Below are the uses of digital multimeter, measure various electrical variables such as:
- Voltage (AC or DC),
- Current (AC or DC),
- Resistance,
- Transistor gain,
- Continuity,
- Diode,
- Frequency,
- Capacitance,
- Inductance, and
- Temperature.
Some expensive and advanced multimeters are able to measure power as well. Keep in mind not every multimeter has the same measurement choices. The most common measurements to find in every multimeter are voltage, current, continuity, diode, and resistance.
There are two types of multimeter:
- Analog multimeter
- Digital multimeter
Digital multimeters are widely used nowadays so we will focus on digital multimeters instead. But it is not a bad thing to at least learn what an analog multimeter is and what makes it different and lose popularity.
What is an Analog Multimeter
Just as an analog voltmeter and analog ammeter, this multimeter has a printed background with several numbers and symbols to indicate the measurements and their values.
The values will be displayed by an arrow, moved by magnetic force generated by a coil. The arrow will move when there is:
- Current flows through the coil, or
- Internal electrical power to measure resistance, or
- Electrical pressure.
Even though its popularity is less than the digital one, it still has some advantages. Its size is relatively small and you can observe the changes in current and voltages real-time even if it is only by small movement.
What makes it lose popularity is you need a quick and precise mathematical calculation in your head before getting the measurement values. Analog multimeter uses scales in its probe terminal and thus you need to do a quick calculation. And we already know that time is short in the practical field and of course we don’t like math very much. As long as we get the value, we are good. This is where a digital multimeter kicks in.
What is a Digital Multimeter
A digital multimeter uses an LCD for displaying the measurement value. This display helps us a lot because it allows us to read the value immediately without doing any calculation.
For its measurement capability, a digital multimeter doesn’t differ much from an analog multimeter. The basic measurement such as voltage, current, and resistance will be the same and they have their own scale we can choose with a rotary switch.
Nowadays, a common digital multimeter also has additional measurements such as diode, continuity, capacitance, inductance, and temperature along with their scales.
A more advanced digital multimeter is capable of measuring power and even has auto-ranging measurement. But in our opinion, having an auto-ranging measurement won’t help us very far, an adjustable scale still works very fine for most people.
One thing to remember, manual scale needs us to take extra caution to the maximum value we will get. If you think it will be over 500V then it is wise to use 1000V scale first. It will be disastrous if we use 100V to measure 500V.
Well, in conclusion, a digital multimeter is the absolute winner because of the convenience, speed, and practicality.
Digital Multimeter Diagram
Observe the digital multimeter block diagram below.
The block diagram above is for the common digital multimeter to measure basic quantities: resistance, current, and voltage.
The line connected to the input probe + is also connected to the rotary switch to select the electrical variables:
- Resistance,
- AC voltage (ACV),
- AC current (ACI),
- DC current (DCI),
- DC voltage (DCV),
The resistance line is connected to the constant current source to produce voltage that will be used as resistance measurement. After data passes the buffer amplifier, it will be converted by ADC (Analog to Digital Converter) before it is displayed by the digital display.
ACV and DCV are connected to the “calibrated attenuator”. This attenuator is used to reduce the signal’s power without disturbing its waveform. This step is taken to prevent power surge by the voltage measurement.
ACI and DCI are simply connected to the “current to voltage converter” or “I-V Converter”. This converter produces voltages proportional to the input current. The circuit is pretty simple, only an op-amp and a feedback resistor as shown below.
Observe further to the right, we find a rectifier circuit connected to the ACV and ACI. Why do we need a rectifier? The rectifier will convert the AC signals into DC signals for easier conversion.
All our measurement data from resistance, ACV, ACI, DCV, and DCI are converted by the “Analog to Digital Converter” (ADC) then displayed by the digital display consisting of their number, symbol, and unit.
Auto-ranging or auto-scaling a digital multimeter only needs us to use proper probe connection and let the multimeter handle the scaling, measuring, and displaying the result.
How Does a Multimeter Work
A digital multimeter can measure different electrical quantities such as:
- Voltage and current (DC and AC),
- Resistance, inductance, capacitance,
- Diode and continuity,
- Additional measurement (transistor gain, temperature, etc).
This multimeter comes with a pair of probes (red and black) for the hot line (or active line) and ground line (or negative line).
Digital multimeter is constructed with:
- Sensing element connected to probes,
- Analog to digital converter,
- Rectifier circuit,
- Range selector and converter,
- Digital display, and
- Amplifier.
Most digital multimeters are supplied by a pair of AA batteries.
How a Multimeter Measures Resistance
When the multimeter is used to measure resistance, it uses its reference voltage source inside it. The voltage is applied to the measured point and the voltage drop is generated. This voltage drop is calculated using a calibrated value to produce resistance on the measured points.
Sometimes a current source is used instead of voltage source. The result will be the same.
This current source with a fixed value will be used to measure the desired resistance. The current will flow through the resistor and produce a voltage. Using the basic Ohm’s Law (R=V/I), it will result in the resistance value and be displayed on the digital display.
How a Multimeter Measures Current
While doing current measurement in series, the probes will sense the current. If the measured current is DC then we will not have any problem, but if it is AC then it is converted to DC first by the AC to DC converter (rectifier circuit).
This measurement will convert the current into equivalent voltage from its internal resistance in the multimeter. Keep in mind that every measurement given by the multimeter is processed in the form of voltage.
Digital multimeter has a low resistance resistor with the set value. This low resistor acts as close as a conductor wire to minimize resistance that can disturb the measurement.
Later on, the current flows through this resistor and the multimeter measures the voltage across this resistor. This value is then calculated and calibrated by the multimeter using Ohm’s Law (I = V/R) and displayed digitally.
How a Multimeter Measures Voltage
Just as stated before, calibration beside the voltage will be calculated in the form of equivalent voltage, converted into a desired value equivalent to the voltage.
Now we are measuring voltage, then the process will be much simpler. The voltage measured by the multimeter will be calibrated and converted by the ADC (analog to digital converter) to show in the digital display.
How to Use Digital Multimeter
A digital multimeter or DMM is a device to measure various electrical quantities in a single handheld-size device. DMM is light, relatively small, operated by batteries, and multipurpose.
Even a DMM is a single device and some sensing probes, we still need to learn how to operate it based on what we want to measure.
Below is the complete step on how to use a digital multimeter to measure voltage, current, resistance, and diode.
The measurement for capacitance and inductance will be similar to the resistance measurement.
How to Measure Resistance
- Prepare the resistor or a circuit. If you need to measure a single resistor connected in a circuit then it is impossible to do because resistance in a circuit is already combined with other components.
- Plug the red probe’s terminal to the multimeter’s socket indicated by the symbol “Ω”. Plug the black probe’s terminal to the multimeter’s socket indicated by the ground symbol “⏚” or COM.
- Rotate the rotary switch to the symbol “Ω”. This symbol is Omega and used to represent Ohm in electricity.
- Use the bigger scale first (MΩ or kΩ) if you have no idea how big the resistance will be. You may lower it if the result shows zero in the first digit. That indicates that the scale is too big. Don’t worry, using a small scale won’t harm your multimeter.
- Turn off or cut off the power supply from the circuit.
- Remove the resistor you want to measure if it is already in the circuit to get an accurate result. Or if it has not been installed in the circuit, then we can measure it immediately.
- Touch one end of the resistor with the red probe and another end with the black probe. Since resistance doesn’t have polarity then we don’t need to think about which one should be connected to the red probe.
- Read the number in the digital display and take note of the scale you use. A measurement of 30 may be 30Ω, 300Ω, 3kΩ, or 3MΩ.
How to Measure Voltage
- Plug the red probe’s terminal to the multimeter’s socket indicated by the symbol “V”. Plug the black probe’s terminal to the multimeter’s socket indicated by the ground symbol “⏚” or COM.
- Determine whether the voltage is AC or DC. Some multimeters have auto-ranging indicated by DC and AC symbols in a single mode but some of them only measure mV.
- Assume that we don’t have auto-ranging then we need to manually choose DC or AC.
- Rotate the rotary switch to the voltage symbol.
- We start with the highest scale for safety if we don’t have an idea how high the voltage is.
- Connect the red probe to the hot line or positive line and black probe to the ground line or negative line.
- If the measured voltage is AC then we don’t need to specifically determine the polarity. If we measure DC voltage then connecting the wrong probe to the wrong polarity will show a negative value.
- Read the displayed value. Reduce the scale if the result is too small, indicating the scale is too big.
How to Measure Current
- Plug the red probe’s terminal to the multimeter’s socket indicated by the symbol “mA” for milliamps or “A” for Amps. Plug the black probe’s terminal to the multimeter’s socket indicated by the ground symbol “⏚” or COM.
- If you are not sure how high the measured current is, just use the “Amps” first. If the result is too low, maybe it is within milliamps and you need to use the mA.
- Determine whether the voltage is AC or DC. Some multimeters have auto-ranging indicated by DC and AC symbols in a single mode but some of them only measure mA.
- Assume that we don’t have auto-ranging then we need to manually choose DC or AC.
- Rotate the rotary switch to the “A” symbol.
- We start with the highest scale for safety if we don’t have an idea how high the voltage is.
- Turn off the circuit or cut off its power supply.
- Break the circuit at the point you want to measure the current.
- Connect the red probe to the positive side and black probe to the negative side. These two sides we connect with the probes are the result when we broke the circuit. Remember that an ammeter should be connected in series right?
- If the measured voltage is AC then we don’t need to specifically determine the polarity. If we measure DC voltage then connecting the wrong probe to the wrong polarity will show a negative value.
- Turn on the circuit or connect the power supply.
- Read the displayed value. Reduce the scale if the result is too small, indicating the scale is too big.
How to Test Diode
- Plug the red probe’s terminal to the multimeter’s socket indicated by the symbol diodes (an arrow). Plug the black probe’s terminal to the multimeter’s socket indicated by the ground symbol “⏚” or COM.
- Rotate the rotary switch to the diode symbol.
- Turn off the circuit or cut off its power supply.
- To test the forward bias, connect the red probe to the positive terminal of the diode and black probe to the negative terminal.
- If the displayed number is more than 0 then the forward bias is good (more than 0 less than 1). If the displayed number is OL (OverLoad) or 0 then the forward bias is bad.
- To test the reverse bias, connect the red probe to the negative terminal of the diode and black probe to the positive terminal.
- If the displayed number is 0 or OL then the reverse bias is good. If the displayed number is more than 0 and less than 1 then the reverse bias is bad.
- To conclude whether the diode is good or bad, the results for both forward bias and reverse bias should be good.
- Turn on the circuit or connect the power supply.
- Read the displayed value. Reduce the scale if the result is too small, indicating the scale is too big.
How to Test Continuity
We can use the same probe configuration as the resistance measurement. We simply connect the probes to the desired connection and if it shows any value in Ohm then it is good. If it shows OL then the continuity is bad.
Most of the multimeter will beep if the continuity is good. But if it is not beep that doesn’t mean the connection is bad, maybe the impedance is too high.
This test is useful for checking the fuse, switch, conductors, and many more.
