atn@cory.Berkeley.EDU (Alan Nishioka) (08/22/90)
Are switches supposed to spark when you switch them? Usually they have opaque cases so I can't tell if they are sparking. I just took apart a vacuum cleaner switch which had failed by fusing two of the contacts together. I separated them and put the switch back together, and the switch worked, but the case is translucent and I can see it spark when I switch the vacuum off. Horowitz and Hill in The Art of Electronics show a "snubber" circuit for use with inductive circuits, a 100 ohm resistor in series with a 0.05uF capacitor, the combination in parallel with the load. But they don't mention why they chose those values. Should I add something like this to the vacuum? It's probably no big deal, but I don't like to leave loose ends and I have a feeling if I leave it, it will simply fail in the same way again. ----------------------------------------------------------------------------- Alan Nishioka KC6KHV atn@cory.berkeley.edu ...!ucbvax!cory!atn
haynes@ucscc.UCSC.EDU (99700000) (08/22/90)
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 Chroniclehenry@zoo.toronto.edu (Henry Spencer) (08/23/90)
In article <27155@pasteur.Berkeley.EDU> atn@cory.Berkeley.EDU (Alan Nishioka) writes: >Are switches supposed to spark when you switch them? ... It's pretty much inevitable when they are carrying noticeable currents. (Breaking the arc is the dominant design problem of really big circuit breakers, in fact -- when you try to cut off a circuit carrying, say, 1000 A at 50 kV, you get some impressive sparks! :-)) -- Committees do harm merely by existing. | Henry Spencer at U of Toronto Zoology -Freeman Dyson | henry@zoo.toronto.edu utzoo!henry
tomb@hplsla.HP.COM (Tom Bruhns) (08/23/90)
>Horowitz and Hill in The Art of Electronics show a "snubber" circuit >for use with inductive circuits, a 100 ohm resistor in series with a 0.05uF >capacitor, the combination in parallel with the load. But they don't >mention why they chose those values. RCA thyristor manuals from the late 70's at least included an ap note on snubber networks. They have some nice design graphs, along with (as I recall; it's been years) the philosophy. Concise, practical info. You will probably have to extrapolate a little for application to mechanical switches, but it's a good starting point.
larry@kitty.UUCP (Larry Lippman) (08/23/90)
In article <1990Aug22.171934.24558@zoo.toronto.edu>, henry@zoo.toronto.edu (Henry Spencer) writes: > >Are switches supposed to spark when you switch them? ... > > It's pretty much inevitable when they are carrying noticeable currents. > (Breaking the arc is the dominant design problem of really big circuit > breakers, in fact -- when you try to cut off a circuit carrying, say, > 1000 A at 50 kV, you get some impressive sparks! :-)) Outdoor high voltage switches usually contain a "horn gap" which spreads as the switch is opened, thereby specifically directing the arc away from the actual current-carrying switch contacts. Other types of high voltage switches (non-oil filled type), such as those contained in metal clad switchgear, sometimes use a source of compressed air to blow out the arc at the time the switch is opened. The compressed air valve is triggered by the switch controller. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 {utzoo, uunet}!/ \aerion!larry
bkoball@cup.portal.com (Bruce R Koball) (08/23/90)
re: the comments of other posters on strategies for arc suppression on switch contact opening....contactors (large switches used in industrial applications, usually for switching large inductive loads) have a number of interesting approaches.... arc chutes -- these are shaped pathways designed to extinguish large arcs and contain the hot gases generated. one strategy involves breaking the large arc into numerous smaller ones with an array of parallel plates in the chute blow out coils -- these are used in DC circuits. the current being carried is also routed through a short, heavy-gauge coil wound around a "blowout core." this core has pole piece extensions which bracket the contact area. when the contact is opened the magnetic field of the blowout coil forces the arc perpendicular to the direction of it's current flow (the ol' right hand rule) thus extinguishing it. Bruce Koball Motion West 2210 Sixth Street Berkeley, CA 94710 415-540-7503 bkoball@cup.portal.com