Elenco 23 Range 20k/V VOM in Kit Form - Manual (Page 16)

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THEORY OF OPERATION
Introduction
Your multimeter is of professional quality using 1% precision resistors throughout the design. The accuracy at
full scale reading will be within 3% of full scale DC voltage or current (1,000V - 5%, 10A - 5%) and 4% of full
scale (for 1,000V - 5%) AC voltage. The accuracy of the ohms measurement is 3% of arc.
On the DC volts range, the loading impedance of the meter is 20,000 ohms per volt. This means that if the
range switch is on the 250V position, the loading to the circuit under test will be 20,000 x 250 = 5M
Ω.
The input loading of the meter is a very important factor to be considered when measuring the voltage of a high
resistance circuit. Take the example where two 1M
Ω resistors are connected in series across a 9V battery. The
voltage at the junction of the resistors will be 4.5V. When measured on the 10V scale, the input loading will be
about 200k
Ω (20,000 ohms/volt times 10V). The voltage at the junction will therefore drop to 1.28V and the
meter will read this voltage. If the meter is switched to the 50V position, the loading will be 1M
Ω and the meter
will read 3V. For reasonably accurate measurement, the circuit under test should have an impedance of less
than 100k
Ω or you should use the higher ranges. The loading on the 250V and 1,000V ranges will be 5MΩ and
20M
Ω respectively, but it will be hard to read 4.5V on these ranges.
DC Voltage Measurement
Figure 4 shows a simplified diagram of the DC voltage
measuring circuit. Here resistors are switched in series
with the meter to provide the desired ranges.
Figure 4
Buzzer Test
1. Set the range switch to the BUZZ position.
2. The buzzer will sound if there are 20
Ω or less across the leads.
DCV (NULL) Test
1. Set the range switch to either the ± 5 or ±25 scale in the DCV (NULL) position.
2. Adjust the 0
Ω ADJ pot for a zero center position
3. Plug the red test lead into the positive (+) socket and the black test lead into the –COM socket.
4. Connect voltage to the test leads and read the voltage.
Output Jack Test (allows measurement of AC voltage when superimposed on a DC voltage.)
1. Plug the red lead into the OUTPUT socket and the black lead into _COM.
2. Set the range switch to the appropriate ACV position. Touch the test leads to the power source and observe
the reading. Then, multiply by the appropriate scale factor.
Diode Tests
The diode forward current I f and reverse current I r are read LI scale. To check a diode in the forward direction
proceed as follows:
1) Plug the red test lead into the + socket and the black lead into the –COM socket.
2) Select the approximate forward current desired 150
μA, 1.5mA, 15mA or 150mA and set the range switch to
this position (blue markings in ohms range).
3) Short the test leads together and adjust the 0
ΩADJ pot for a zero reading on the ohms (top) scale.
4) Connect the red test lead to the cathode (striped end) of the diode and the black test lead to the anode of
the diode.
5) Read the forward current on the LI scale. The voltage drop across the diode is shown on the LV scale
immediately below the LI scale.