Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!sdd.hp.com!decwrl!shelby!riacs!agate!darkstar!ucscc.UCSC.EDU!haynes
From: haynes@ucscc.UCSC.EDU (99700000)
Newsgroups: sci.electronics
Subject: Re: Are switches supposed to spark?
Message-ID: <6233@darkstar.ucsc.edu>
Date: 22 Aug 90 07:05:48 GMT
References: <27155@pasteur.Berkeley.EDU>
Sender: usenet@darkstar.ucsc.edu
Reply-To: haynes@ucscc.UCSC.EDU.UUCP (Jim Haynes)
Organization: University of California, Santa Cruz  CATS
Lines: 24

Sparking is practically unavoidable in switches because the contacts
open while the circuit is carrying current; and with the inductive
load the collapsing current will generate a voltage as high as necessary
to strike a spark.  The snubber circuit allow the current to continue
through the capacitor, so it doesn't have to go through the arc across
the switch contacts.  The resistor serves two purposes: it makes the
circuit dissipative so the current dies out rather than oscillating
in the L-C circuit; and it limits the current that can flow from the
capacitor into the switch contacts when they are suddenly closed.

Switching DC is more of a problem than switching AC, because with AC
the current will go to zero anyway; whereas with DC it will continue
as long as the arc exists at the switch contacts.  Which means the
contacts have to open really wide.

So a snubber might help some, but probably a better solution is a more
heavy-duty switch.

haynes@ucscc.ucsc.edu
haynes@ucscc.bitnet
..ucbvax!ucscc!haynes

"Any clod can have the facts, but having opinions is an Art."
        Charles McCabe, San Francisco Chronicle