AC Power Switching (was Electronic components sources)

From: David V. Corbin <>
Date: Wed Mar 24 19:25:51 2004

To all that have posted....

My original comment was made specifically in reference to 2-wire (therefore
single phase) power sources WITHOUT an independent ground. Note: This is
ALWAYS the case with a device that uses an external power supply wich just
supplys a low voltage to the actual device (e.g. Wall Warts).

The comment is totally in-applicable (IMHO) when there is a dedicated
(non-switched) ground connection OR the power is multi-phase.

With this as the configuration there are a number of issues that can (and
undoubtably will) arise if both of the legs are disconnected in a power off
condition. It is necessary to realize that this condition is EXACTLY
equivilant to the device being unplugged!

The first condition is discharge. This can be the result of either static or
an external application of charge. [Oversimplifing things a wee bit] Charge
will always seek to evenly distribute itself. It will do this through the
path of least resistance. If the Ground of the circuit is available, this
will typically be the most commong route, and is usuaslly designed to handle
larger currents than paths between signals [which typically also have higher

Consider normal static protection. It is necessary (or at least good
practice) to be VERY careful when dealing with loose chips (which have their
power and ground paths disconnected). Also good to be careful with board
level assemblies. Not so important when dealing with completely assembled
units (while unplugged).

The reason for the dramatic drop in static mortality in dealing with
assembled units is the fact that the power supply is intact and can
absorb/buffer most of the transients involved. If the power supply is
external (closer to the actual source of power than the switch), it
effectively does not exist.

If the power supply can dump the charge via the external connection, so much
the better. There is no chance of long term build-up (which would discharge
when the connections are completed).

Therefore, I (and most of the other design engineers I know) fully recommed
ONLY switching the hot side of a 2 legged connection.

David V. Corbin
Dynamic Concepts
Received on Wed Mar 24 2004 - 19:25:51 GMT

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