15
3.4
Polarity Reversing
The function switch provides a convenient means to reverse polarity of the internal
battery to facilitate testing semiconductor devices. The +DC and -DC positions
transpose internal connections to the COMMON and +jacks and therefore can
also be used to reverse polarity of external voltages and currents connected to
these jacks (only). Because polarity reversing momentarily interrupts the circuit, it
should not be done while measuring current or voltage without first turning off the
circuit power. When making measurements on the 50
μ
A/250mV, 1 Volt, or 10
Amp range, polarity can be corrected only by reversing the test leads.
3.5
Measurement of Unknown Voltage or Current
Sometimes the approximate voltage or current to be measured is known and the
correct range may easily be selected. Often, however, particularly when working
on faulty equipment, the voltage or current is either unknown or may be much
higher than normal and anticipated. When working upon unfamiliar or unknown
equipment, always begin a measurement using the highest range available on the
Instrument. Once it is determined that the voltage is within the limits of a lower
range, change to the lower range.
3.6
Test Leads
These are provided with accessory screw-on alligator clips which may be attached
to the probe tips. As well as eliminating the need to hand-hold test prods to a
circuit for extended periods, the test clips provide a means of reducing hand prox-
imity to a high voltage circuit while energized. Circuit power must be turned off
and any charged capacitors discharged before connecting or disconnecting clips
to or from the current.
3.7
DC Voltage Measurement
Before making voltage measurements, review the SAFETY PRECAUTIONS listed
in paragraph 3.2. Also, when using the 260 as a millivoltmeter, care must be taken
to prevent damage to the indicating instrument from excessive voltage.
3.7.1
DC Voltage Measurement 0-250 mV Range
Before using the 250 millivolt range, use the 2.5V DC range to determine that the
voltage measured is not greater than 250 mV (or .25 V DC).
a.
Set the function switch at +DC.
b.
Plug the black test lead in the -COMMON jack and the red test lead into the
+50
μ
A/250mV jack.
c.
Set the range switch at 50
μ
A common position with 50V.
d.
Connect the black test lead to the negative side of the circuit being measured
and the red test lead to the positive side of the circuit.
e.
Read the voltage on the black scale marked DC and use the figures marked
0-250. Read directly in millivolts.
NOTE:
For polarity reversing see paragraph 3.4.
!