How digital multimeter works?

Introduction: It is a common & important laboratory instrument. It is used to measure AC/DC voltage, AC/DC current and resistance with digital display. It gives digital display, which is very accurate. It has an advantage of very high input resistance. It also provides over ranging indicator i.e. if the unknown electrical quantity increases beyond measuring capacity it shows 1— on the display.

The block diagram of DMM is given below. The working of each block to measure different types of electrical quantities is as follows.

How to measure resistance?

Connect an unknown resistor across its input probes. Keep rotary switch in the position-1 (refer block diagram above). The proportional current flows through the resistor, from constant current source. According to Ohm’s law voltage is produced across it. This voltage is directly proportional to its resistance. This voltage is buffered and fed to A-D converter, to get digital display in Ohms.

How to measure AC voltage?

Connect an unknown AC voltage across the input probes. Keep rotary switch in position-2. The voltage is attenuated, if it is above the selected range and then rectified to convert it into proportional DC voltage. It is then fed to A-D converter to get the digital display in Volts.

How to measure AC current?

Current is indirectly measured by converting it into proportional voltage. Connect an unknown AC current across input probes. Keep the switch in position-3. The current is converted into voltage proportionally with the help of I-V converter and then rectified. Now the voltage in terms of AC current is fed to A-D converter to get digital display in Amperes.

How to measure DC current?

The DC current is also measured indirectly. Connect an unknown DC current across input probes. Keep the switch in position-4. The current is converted into voltage proportionally with the help of I-V converter. Now the voltage in terms of DC current is fed to A-D converter to get the digital display in Amperes.

How to measure DC voltage?

Connect an unknown DC voltage across input probes. Keep the switch in position-5. The voltage is attenuated, if it is above the selected range and then directly fed to A-D converter to get the digital display in Volts.

Digital multimeter finds wide range of applications in the measurements of different electrical quantities. Remember that a meter capable of checking for voltage, current, and resistance is called a multimeter.

Remember that while measuring voltage, the DMM is connected in parallel. To measure voltage at a point in the circuit, first confirm the type of voltage, whether it is AC or DC. Also confirm the range of voltage (it is better to start with higher voltage range).

Now connect black probe of DMM to negative terminal of circuit power supply and then connect the RED probe to the point where you want to measure the voltage. Be careful not to touch the bare probe tip anywhere else in the circuit otherwise there may be the problem of short circuit, etc.

It is not important to be specific about the polarity of voltage in a circuit. You can connect RED and BLACK probes at random, in the circuit. The meter will automatically show a (–) sign for negative voltage and nothing for positive voltage.

Remember that while measuring current, the DMM is connected in series. To measure current flowing through a circuit or wire, first confirm the type of current, whether it is AC or DC. Also confirm the range of current (it is better to start with higher current range).

Now connect red and black probes at random in the circuit, to measure the current. Be careful about the connections between the circuit terminals and the probes. They should be tight, otherwise, there will be “makes” and “breaks” during measurement, which may lead to produce errors.

While measuring current, the connections of RED and BLACK probes may be, at random. If the current is positive no sign will be shown on the display. But if it is negative, (–) sign will displayed.

To measure the unknown resistance: If you are measuring the unknown value of a resistor already connected in a working circuit, then first of all, switch off the power supply and disconnect the resistor from the circuit.

This is very important, because if you measure the resistance without disconnecting it from the circuit, the voltage drop across it may damage the DMM permanently.

Now connect it across the probes keeping the DMM in resistance range. Fix the higher range first, say 10MΩ. Then reduce the range, until you get correct readout.

Remember that the multimeter has practically no resistance between its leads. This is essential during continuity testing, in particular. It is intended to allow electrons to flow through the meter with the least possible difficulty.

For example, while checking the continuity of a small piece of copper wire, its practical resistance should be zero. If the probe resistance were greater than zero, the meter would add extra resistance in the circuit and would display some errors during continuity testing and would also affect the current.

The elements of DMM

The former topic of DMM covered its fundamental concept. However the commercial DMM is more advanced and packed with many features. It has more precision also. Remember that any DMM internally works as digital voltmeter.

That is, any quantity under measurement is first converted into proportional DC voltage and then measured. For example:

• When we measure resistance, a constant current passes through the unknown resistance and proportional voltage is produced, which is then buffered, sampled and fed to the counter.
• When we measure current, it is converted into proportional voltage first. Then it is sampled and fed to the counter to obtain the equivalent reading in relevant unit.

Thus, every time the DMM converts the quantity under measurement into proportional DC voltage first and then the relevant reading is displayed. Now we shall understand the necessity and basic working of different blocks or elements used in DMM, as follows:

Attenuator: The commercial DMM has a rotary switch used selecting proper range with many steps in it. Now suppose the DMM is put in voltage range to measure AC or DC voltage. When unknown voltage is connected across its probes, first of all, it is checked for its magnitude within the specified range.

If voltage is high, then it is attenuated proportionally. The attenuator is a ladder of high wattage resistors, as shown in following figure. It has number of steps for attenuation from several volts to kilovolts. To select a particular range for measuring voltage, first switch to higher range.

If the resolution of the reading is less, then only you can switch over to successive lower range.

The process of sampling & gating: Once the input voltage under measurement is converted into DC voltage, it is further processed and sampled into a series of digital pulses, as shown below.

When unknown voltage is connected, at the start of measurement, the ramp voltage is initiated at point ‘a’. It is a negative going sawtooth voltage. The ramp voltage is constantly compared with unknown voltage. When magnitude of unknown voltage becomes equal to ramp voltage, at point ‘b’, at that instant the input comparator produces START pulse, and the gate is opened.

So digital pulses are fed to the counter and the counting is initiated. During this, the ramp voltage further falls. As it reaches to zero, at point ‘c’, the ground comparator produces STOP pulse and the gate is closed. So the digital pulses are disconnected from input of counter. The counting stops and result is displayed.

Current to voltage conversion: This circuit is built around special type of operational amplifier. A typical circuit of current to voltage converter is given in following circuit. We shall understand how it converts current into proportional voltage.

According to the theory of operational amplifiers, the output voltage of the circuit is given by the equation, as follows:

(Vo/Vi) = -(Rf/Ri) ………… ∴ Vo = -(Vi/R1).Rf

However as V1=0, V1=V2

(Vi/R1) = Iin and ∴ Vo = -(Iin.Rf)

So if we replace the Vi and R1 combination by a current source Iin of as shown in the above circuit, then the output voltage Vo becomes proportional to the input current Iin. Thus, we can say that the input current is converted into proportional output voltage.