[Amps] Apparent Arcing (Roger Parsons)

[Manfred Mornhinweg]

Roger,

> Obviously I'm missing something! Any ideas what?

If I follow your story correctly, the noise appears only when there is 
actually some current flowing through the soft start circuitry. This 
points at arcing at the relay contact, even if you cleaned at and even 
if it looks bright.

Power relays use a silver alloy, or sometimes pure silver, on the 
contacts. This surface sulfidizes and becomes insulating. A high contact 
pressure, and mechanical wiping action during contact closure, can 
largely break down this layer. But in many power relays, specially ones 
that have been in storage for a long time, or that have spent time in a 
place with a high level of sulfur pollution (traffic, power plants, 
industry), this layer can be thick enough to be hard to rub off.

Such a sulfide layer on the contact is an insulator at low voltage, but 
arcs through at higher voltage. So it does conduct current, and causes a 
voltage drop that might be too small to be obvious, but will cause noise.

I suggest this test: Prepare a piece of wire, with freshly tinned ends. 
Also re-tin the relay connections. Power up the circuit, and after the 
step start has switched, short out the relay contact with this wire, 
making contact between freshly tinned surfaces only. If that short 
silences the arcing noise, then you know the relay contacts are the culprit.

Manfred


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