Agilent Technologies manual N6700 Modular Power System Scope

2 Installation

Protecting Sensitive Loads from AC Power Switching Transients

This only applies if you are connecting loads that are highly sensitive to voltage

NOTE or current transients to the output of the modular power system. If your load is connected directly to the output of the power system and is not connected to chassis ground in any way, you do not need to worry about AC power switching transients appearing at the output of the modular power system.

Operating the AC line switch can inject common mode current spikes into the DC output leads, resulting in voltage spikes, which may damage sensitive loads. Note that any electronic device meeting international standards for EMI compliance is likely to generate similar current spikes. This situation arises from the presence of EMI filters at both the AC input and the DC output of the modular power system. These filters typically include common mode capacitors connected to the power system chassis. Since the AC input has an earth ground, any load that is also earth-grounded provides a possible return path for common mode currents.

The following figure illustrates a typical situation where a load that might otherwise be floating becomes grounded, thereby providing a return path for any injected currents. In this case, the return path is created by the low side of the scope probe, which is connected to the load circuit common and also to the scope’s chassis. For this and similar cases, the following steps by order of preference, will help mitigate common mode current spikes appearing at the output when the modular power system is turned on or off by the AC line switch:

1Install the ferrite core on the power cord as described under "Snap-On Core". This inserts impedance in the current path.

2Install a separate “bonding” wire from the load’s common point, to the ground terminal of the modular power system. This provides a lower impedance path that helps direct injected currents away from the DC output leads (and the sensitive load).

3Break the return path through the external equipment. For example, instead of the single-ended scope shown in the figure, you can use a differential scope with a floating input or you can connect an isolated measuring instrument to the load.

1

N6700 Modular

Power System

-S

+

+S

2

+

Scope

probe

3

NOTE

Disconnecting the load from the output before switching the modular power system on or off will always protect the load from common mode currents.

28 Series N6700 User’s Guide