Appendix Service Information
Common Mode Current Noise
193
Service
Information
1 Turn off the power supply and connect the output to be tested as shown above with
the current monitoring resistor 0.2
Ω (R
M2
) across output terminals. Connect a rms
voltmeter across the current monitoring resistor as shown on the previous page.
2 Turn on the power supply. Select the output to be tested and the low voltage range
(8V/3A)
†
, enable the output, and set the display to the limit mode. When the display
is in the limit mode, program the current to full scale rated value (3 A)
†
and the voltage
to the full scale rated value (8.0 V)
†
.
3 Divide the reading on the rms voltmeter by the load resistance to obtain rms current.
The readings should be within the limit of 4 mA.
4 Repeat steps (1) through (3) for the other output.
Common Mode Current Noise
The common mode current is that ac current component which exists between the
output or output lines and chassis ground. Common mode noise can be a problem for
very sensitive circuitry that is referenced to earth ground. When a circuit is referenced
to earth ground, a low level line-related ac current will flow from the output terminals
to earth ground. Any impedance to earth ground will create a voltage drop equal to
the output current flow multiplied by the impedance.
1 Turn off the power supply and connect a 100 k
Ω resistor (R
S
) and a 2200 pF capacitor
in parallel between the (-) terminal and chassis ground at the rear output terminals.
2 Connect a digital voltmeter across R
S
.
3 Turn on the power supply. Select the output to be tested and the high voltage range
(20V/1.5A)
†
, enable the output, and set the display to the limit mode. When the display
is in the limit mode, program the output to the full scale rated value (20 V and 1.5 A)
†
.
4 Record the voltage across R
S
and convert it to current by dividing by the resistance
(DVM reading/100 k
Ω). Note that the current is less than 1.5 μA.
5 Repeat steps (1) through (4) for the other output.
†
For E3646A model, and see Table A-3 for other models.
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