AC Power Switching (was Electronic components sources)

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 resistance]. 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 President Dynamic Concepts