Measuring Resistance; Diode And Continuity Testing - Fluke 27/FM Operator's Manual

114
Es = Source voltage
RL = Load resistance -1- Source resistance
Im = Measured current (display reading in mA)
Es = Burden voltage (calculated). i.e.. Display reading
expressed as a % of full-scale (100 x
READING
) times
FULL-SCALE
full-scale burden voltage for selected range. See Table:
TYPICAL
BURDEN VOLTAGE
320 pA
3200 pA
32 mA
320 mA
10A
Maximum current error due to Burden Voltage
Es
fl
Of
100 x
Es - Es
Es x I m
Es -Es
Example: Es = 15V, R L = 500,1m = 270 mA.
270
Es = 100 x
320
x 1.8 (from Table)
84.4% x 1.8 = 1.519V
1.519
1.519
Error in % — 100
— 100
= 11.3%
15 - 1.519
13.481
Increase displayed current by 11.3% to obtain true current
Error in mA = 1 . 519 x 270 , 410 s_ 30.41 mA
15- 1.519 13.481
Increase displayed current by 30 mA to obtain true current.
RANGE
0.16V
1.6V
.18V
1.8V
0.5V
in mA
Resistance, Diode and Continuity Testing
formula in Figure 5. Approximate terminal resistances for the current ranges
are: 0.05 ohms for A, 5.5 ohms for mA, and 500 ohms for uA.
Figure 5. Current Measurement Error Calculations
Measuring Resistance
CAUTION
Turn test circuit power off and discharge all capacitors
before attempting in-circuit resistance measurements.
The multimeter features six resistance ranges and a conductance range. All
ranges employ a two-wire measurement technique. As a result, test lead
resistance may influence measurement accuracy on the 320-ohm range. To
determine the error, short the test leads together and read the lead
resistance. Correct the measurement by subtracting the lead resistance from
the measurement, or use the Relative (REL A) mode to zero the display. The
error is usually 0.1 to 0.2 ohms for a standard pair of test leads.
Some in-circuit resistance measurements can be made without removing diodes
and transistors from the circuit. The full-scale measurement voltage
produced on ranges below 32 megohms does not strongly forward bias silicon
diodes or transistor junctions. Use the highest range you can (except 32
megohm) to minimize the possibility of turning on diodes or transistor
junctions. Full scale measurement voltage in the 32-megohm range does
strongly forward bias a diode or transistor.
Diode and Continuity Testing
In diode test, there is only one range: 0 to +2.08 volts. Voltage is
developed across the component(s) under test by a test current output from
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