Re: DC switches - where do ya get 'em?

["Eric Chang" <ericchang@my-deja.com>]

 Hi Lee.  As usual, a very informative and
helpful post.

I have a question.  The snubber network placed
across the switch helps things out when the
switch is opened.  Are there any guidelines
about how the voltage rating of the switch is
affected as a function of how inductive the
load is?  Obviously, DC is harder to interrupt
than AC, and even if the load is not inductive
at all, the switch needs a higher rating.

What if the load is capacitive?  The reason I
ask is that I had a switch failure on my car.
This one is hard to blame on sub-standard
engineering, since I heard "zorch" noises
coming out of the switch long before it failed.
The interesting thing is that this switch
seemed to be making the noises as the switch
was making contact, not as the switch was
being turned off.  Also, rather than the
usual welding of contacts, the failure that
occurred was that the switch became permanently
open.  Upon close examination, the switch had
two small black spots near the end that the
active contacts was on.  A repair was made by
swapping the contact set (DPDT switch) and 
adding a relay with an RC snubber.

Since the "zorch" noises occurred when the
switch was being closed, I was wondering about
the effectiveness of the snubber in this
application.  The load?  It's almost too 
embarassing to mention, given your opinion on
this subject (which I am familiar with from
your earlier posts).  The load is made up of
small parallel connected DC-DC converters.
These are the ones that I was afraid white
powder would get into.  Taking a little look
inside the cases revealed no inrush current
limiters (of course, why would this be needed
if the supplies were to be used independently).
So, these loads would be essentially capacitive.

Eric
--

On Mon, 29 Nov 1999 20:18:07   Lee Hart wrote:
...
>
>You can put a "snubber" network across a contact to extend its life on
>DC (or AC, for that matter). The DC snubber is a resistor and capacitor
>in series, connected across the switch contact, plus a diode in parallel
>with the resistor and oriented so the capacitor charges thru the diode
>when the switch opens, and discharges through the resistor when the
>switch closes. The capacitor value should have a 1-10 millisecond time
>constant with the load resistance being switched. The resistor is chosen
>to limit switch current when the switch turns on and the capacitor is
>charged to the full DC supply voltage. The diode needs a current and
>voltage rating above that of the load being switched.
...



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