Thanks for the suggestion and I am some confuse on the inrush current
limiter. On the switch, do I use a PTC instead of NTC since I need to block
the high current first turn on and let the switch conduct fully.
Is the inrush current limiter is solder on the two connectors of the switch?
Thanks

Eric


"Ross Herbert" wrote in message
...
On Mon, 13 Sep 2004 10:08:12 -0500, "Eric"
wrote:

|Most of the old equipment has power supply switch problem. The contacts
arc
|by electric power and need to be replaced or sand down to refinish the
|surfaces to prevent resistance build up. After the resurfacing surface,
is
|there any chemical can prevent the arc again and the oxidation coming
back?
|I am using the de-ox cleaner now, any other suggestions.
|
|Thanks
|
|Eric
|
There are products such as De-Oxit and Stabilant 22 (very expensive)
which will prevent oxidation of switch contacts, but this doesn't
appear to be the main problem in your situation.

It seems that your problem is caused by arcing at the contacts which
results in pitting and carbonising deposits to form which can cause
erratic and unreliable contact switching. There is no "magic spray"
which can prevent arcing, and you will continue to have this problem
unless some means of reducing the heavy switch-on/switch-off surges is
employed.

Here is a brief explanation;

http://www.machinedesign.com/ASP/str...tedArticle.asp

Items such as high power audio power amps employ massive filter
capacitors and often use toroidal transformers. Toroidal transformers
inherently produce higher switch-on currents than conventional
laminated E-I core transformers and the large capacitors appear as a
near short circuit at the moment of switch-on. In order to reduce the
switch-on current you should have a surge limiting device such as a
PTC inrush current limiter or a power resistor in series with the
mains input to the transformer. In the case where a power resistor is
used a relay is used to short circuit the resistor several seconds
after switch-on.

To minimise arcing during switch-off you should have a "snubber" or
resistor of say 27 ohms in series with a capacitor of say 220nF (X2
rated or 270VAC) or thereabouts (the values will depend on the
magnitude of the surge currents) connected directly across the switch.
Note that the capacitor will have approximately 14K ohms impedance so
some minute current will flow through the transformer primary when the
switch is in the off position.

Here is an explanation;
http://www.allaboutcircuits.com/vol_4/chpt_4/2.html