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Familiarization of Digital Multi meters and Analog Multi meters

Familiarization of Digital Multi meters and Analog Multi meters

The digital multi meter
The hand-held digital multi meter is used widely to make electrical measurements Of
1. voltage dc & ac
2. current dc & ac
3. resistance
4. continuity
5. other quantities, such as frequency, depending on meter’s features
Input impedance:
In the voltage mode, the input impedance of a digital multi meter is usually high enough (several M )
that it has negligible effect on the circuit being measured.
Continuity check:
Many models allow you to check continuity, emitting an audible beep so that you don’t need to look at
the meter while making the test.
Hooking up the meter:
Note that the meter is connected:
• in parallel to measure voltage or resistance
• in series to measure current.
See the figures to see how this is done.


Analogue multi meter
Analogue multi meters, like digital ones have a variety of ranges. They are described in terms of Full
Scale Deflection or FSD. This is the maximum that the range can read. In order to get the best reading, it
is necessary to have the scale reading somewhere between about a quarter and all of the FSD. In this way
the optimum accuracy and significant number of figures can be read. As a result of this meters have a
variety of ranges, that may appear to be reasonably close to each other.
A typical meter may have the following ranges (note that the figures indicate the FSD):
• DC Voltage: 2.5V, 10V, 25V, 100V, 250V, 1000V
• AC voltage: 10V, 25V, 100V, 250V, 1000V
• DC Current: 50uA, 1mA 10mW, 100mA
• Resistance: R, 100R, 10 000R
There are several points to note from this typical analogue multi meter specification:
1. The low voltage AC voltage, and in this example the 10V AC range may have a different scale to
the others. The reason for this is that at low voltages a bridge rectifier is non-linear and this needs
to be taken into consideration. It is also for this reason that no 2.5V AC range was included.
2. The 1000V or 1kV ranges will often use a different input connection to enable the reading to be
taken through a different shunt and kept away from the rotary switch that may not be able to
handle a voltage this high.
3. AC current is often not included in the lower end meters because of the difficulties of undertaking
the measurement without a transformer to step up any voltage across a series sensing resistor for
rectification.
4. Batteries inside the multi meter are used to provide a current for the resistance measurements. No
other readings require the use of battery power - the meter is passive from that viewpoint.
5. The three resistance ranges of varying sensitivity multiply the meter reading by 1, 100, or 10 000
dependent upon the range. This allows for low resistance measurements to be made as well as
very high ones. Typically the higher resistance ranges may use a higher voltage battery than the
one used for the low resistance ranges.
Multimeter sensitivity
One of the specifications for an analogue multimeter is its sensitivity. This comes about because the meter
must draw a certain amount of current from the circuit it is measuring in order for the meter to deflect.
Accordingly the meter appears as another resistor placed between the points being measured. The way
this is specified is in terms of a certain number of Ohms (or more usually kOhms) per volt. The figure
enables the effective resistance to be calculated for any given range.
Thus if a multimeter had a sensitivity of 20 kOhms per volt, then on the range having a full scale
deflection of 10 volts, it would appear as a resistance of 10 x 20 kohms, i.e. 200 kohms.
When making measurements the resistance of the meter should be at the very least ten times the
resistance of the circuit being measured. As a rough guide, this can be taken to be the highest resistor
value near where the meter is connected.
Normally the sensitivity of an analogue meter is much less on AC than DC. A meter with a DC
sensitivity of 20 kohms per volt on DC might only have a sensitivity of 1 kohm per volt on AC.
Multimeter operation
The operation of an analogue multimeter is quite easy. With a knowledge of how to make voltage, current
and resistance measurements , it is only necessary to know how to use the multi meter. If the meter is new
then it will obviously be necessary to install any battery or batteries needed for the resistance
measurements
.When using the meter it is possible to follow a number of simple steps:
1. Insert the probes into the correct connections - this is required because there may be a number of
different connections that can be used.
2. Set switch to the correct measurement type and range for the measurement to be made. When
selecting the range, ensure that the maximum range is above that anticipated. The range on the
multi meter can be reduced later if necessary. However by selecting a range that is too high, it
prevents the meter being overloaded and any possible damage to the movement of the meter
itself.
3. Optimise the range for the best reading. If possible adjust it so that the maximum deflection of the
meter can be gained. In this way the most accurate reading will be gained.
4. Once the reading is complete, it is a wise precaution to place the probes into the voltage
measurement sockets and turn the range to maximum voltage. In this way if the meter is
accidentally connected without thought for the range used, there is little chance of damage to the
meter. This may not be true if it left set for a current reading, and the meter is accidentally
connected across a high voltage point!
Schematic diagrams:
An ammeter measures current, a voltmeter measures the potential difference (voltage)
between two points, and an ohmmeter measures resistance. A multimeter combines these
functions and possibly some additional ones as well, into a single instrument.
The following diagrams show a multimeter can be used to measure current, voltage and
resistance:
Procedure:
1. To measure current, the circuit must be broken to allow the ammeter to be connected in series.
Ammeters must have a LOW resistance
2. To measure potential difference (voltage), the circuit is not changed: the voltmeter is connected in
parallel. voltmeters must have a HIGH resistance
3. An ohmmeter does not function with a circuit connected to a power supply. If you want to measure the
resistance of a particular component, you must take it out of the circuit altogether and test it separately.

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