Appendix Service Information
Common Mode Current Noise
178
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.
1Turn off the power supply and connect a 100 k9 resistor (RS) and a 2200 pF
capacitor in parallel between the (-) terminal and chassis ground at the rear
output terminals.
2Connect a digital voltmeter across RS.
3Turn on the power supply. Select 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 output to the full scale rated value (8.0 V and 3 A)†.
4Record the voltage across RS and convert it to current by dividing by the
resistance (DVM reading/100 k9). Note that the current is less than 1.5 mA.
†For E3640A model, and see Table A-3 for other models