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