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Model 428B
3-39. INCREASING THE ABSOLUTE SENSITIVITY
3-40. The sensitivity of the instrument can be increased by
looping the wire (carrying the dc current) several time
through the opening of the probe (see Figure 3-3). For
example, three turns increase the sensitivity three times.
With an increased sensitivity, however, the induced voltage
between the probe and the circuit under measurement will
increase also.
Figure 3-3. Increasing The Absolute Sensitivity.
3-41. CURRENT CHECK LOOPS.
3-42. In restricted situations such as printed circuit boards,
wire loops for the probe can be built into the circuit to allow
convenient current measurements with the Model 428B.
Here, currents can then be measured under operating
conditions with the same ease as voltage measurement.
3-43. Circuits can also be modified to accept an impromptu
loop for testing. As an example, to measure the collector
current of a transistor for troubleshooting purposes, the
collector lead can be removed from the board and a loop of
fine wire soldered between the collector lead and the board.
To measure current through a resistor, lift one lead and
install a series loop, clip the 428B probe around the loop and
measure current through the resistor. As an alternative, an
equivalent resistor with long leads can be installed to replace
the resistor in question.
3-44. NULLING CURRENTS.
3-45. The resolution of the 428B can be increased by nulling
one current against another and measuring the difference
between the two. To null the reading, clip the probe over
both wires at once with the wires so arranged that the
currents are going in opposite directions. The considerations
mentioned in Paragraph 3-39 also apply to current nulling.
For example, assume that a 0.6 A current source is to be
tested against a 0.4 A standard. The 0.6 A supply should be
looped twice through the probe jaws and the 0.4 A supply
should be looped three times through the jaws such that the
7
two currents oppose each other. It should be
remembered when making such a measurement,
that the absolute value of any deviations observed
have been multiplied. If, in the above example, the
0.6 A supply wavered by .01 A, the change would
be read as .02 A on the meter.
3-46. USE OF OUTPUT JACK.
3-47. The OUTPUT jack enables the 428B to be
used as a dc coupled: amplifier/I-E transducer
/isolator. The basic action of the 4 28B (considered
as an input/output device) is to sense the magnetic
field around a current carrying wire and deliver a
proportional voltage at the OUTPUT jack. The
value of the output can be varied by using the
OUTPUT LEVEL control to produce as much as 1
1/2 volts at 1 mA. While the 428B meter registers
average dc (ignoring ac), the output at the
OUTPUT jack contains both the dc and ac
components of the signal being measured.
3-48. With Oscilloscope.
3-49. To display the output of the 428B on an
oscilloscope:
a. If the oscilloscope is dc coupled, it can be
calibrated as in Paragraph 3-51.
b. Clip the probe around the wire which varies the
signal to be displayed.
c. Connect the oscilloscope input to the 428B
OUTPUT jack.
d. Adjust the 428B RANGE switch to the
appropriate range.
3-50. With Recorder.
3-51. To record the output of the 428B on a graphic
recorder:
a. Insure that the recorder's input impedance
exceeds 1400 ohms.
b. Connect the recorder input to the 428B
OUTPUT jack.
c. Zero the 428B on the 1 mA Range, turn
OUTPUT LEVEL to minimum output.
d. Zero the recorder.
e. Adjust the 428B ZERO control for full-scale on
the 428B meter.
f. Adjust the 428B OUTPUT LEVEL control for
full scale on the recorder.
g. Zero the 428B, switch to the appropriate range
and clamp the 428B probe around the wire
which carries the signal to be measured.
3-52. When recording current variations with the
428B, it should be borne in mind that the 428B
displays some long term zero drift. The 428B zero
drift normally amounts to about 300 µA (indicated)
per clay so periodic checks should be made to
determine whether or not the ZERO controls need
adjustment.
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Troubleshooting
