[R-390] MOV's and snubbers

[Charles Steinmetz]

Tisha  wrote:

>The switch contacts weld due to high inrush current when the power is
>applied when the current is at peak levels on the AC sine wave. In
>industrial applications this was remedied by "zero crossing" switching
>where current is at an absolute minimum on the sine wave.
>
>How we can address it is with either a device lilke a PTC (positive
>temperature coefficient) resistor like the CL-80 or CL-90, or with a
>soft-start setup with a relay, series load resistor and a small RC timing
>circuit on the coil side of the relay.

The fact remains that the vast majority of switch arcing occurs on 
the "break" operation, when the switch is turned OFF.  It is the 
damage to the contacts from this switch-off arcing that sets up the 
conditions for the switch to weld on a "make" cycle.  A PTC does not 
protect the switch contacts from arcing on the break cycle, so the 
contacts continue to be chewed away every time you switch the radio 
off.  Snubbers help mitigate this.

I'm not saying don't install a PTC.  Just that installing a PTC will 
not protect the microswitch contacts from erosion due to arcing on 
the break cycle and, consequently, will do little or nothing to 
prevent switches from welding themselves "ON."


Best regards,

Charles