Re: [CAD_CAM_EDM_DRO] Re: E-stop relay welds contacts for load dump resistor

Dan,

TBQH, I'm not sure how this would be beneficial for you.

However, at 60 volts, there will be no appreciable arcing on contact
"make" of the dump circuit. There may be some arcing at the contact if
it bounces, but a well designed and spec'd relay, in conjunction with a
proper dump resistor should prevent any arcing.

Relay contacts routinely arc upon release if they are carrying
appreciable current. As the contacts begin to separate, the area of
contact between the two poles gets smaller and smaller, concentrating
the current in a progressively smaller area. If the area conducting the
current gets small enough and hot enough, metal from the contact will
vaporize and make a conductive plasma, otherwise known as an arc.
Eventually the contacts get far enough apart that the plasma becomes
attenuated, and the arc is broken. All of this happens in micro or
milliseconds.

For contacts to arc at closure, there would have to be high enough
voltage across the contacts for electrons to flow through the air across
the contacts before they meet. Here, the arc would be extinguished when
the metallic faces finally meet. Roughly speaking, 10,000 volts will
only jump about half an inch in air. 60 volts just can't ionize air
very much, so an arc on contact make in your system is unlikely.

A dump relay that looks like the following would be good. Look around
-- there are lots of them up in different ratings and prices. This
particular offering states that the item offered is a SPDT relay, though
the photo shows a DPDT relay. You would want a DPDT unit.

http://cgi.ebay.com/NIB-MAGNECRAFT-W199X-2-POWER-RELAY-12-VDC-/370379614699

No connection to seller.

Dave

On 1/30/2011 2:48 PM, kzf817 wrote:
> I might not have described my proposed use of the condenser cap properly. I was thinking of wiring it like this:
>
> + Terminal on power supply Cap---> 7 ohm
> resistor---> + terminal on "condenser" Cap. - terminal on "condenser" cap goes to NC contacts on relay---> Ground buss bar. When the relay
> de-energizes, the Capacitor then has a path to ground, of course, to bleed off
> it's charge. I am speculating that the "condenser" cap will have to absorb current to charge, while it is simultaneously slowly passing current through through the relay contacts to ground. Kind of like a big hydraulic accumulator would absorb a pressure pulse or spike. Is my speculation correct or am I missing an obvious piece of electronics theory? Thanks!
>
> Dan.
> ===================================================================
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, David Speck<Dave@...> wrote:
>
>> If the condenser bridges the relay contacts, you will pass significant
>> AC current, and there will be some voltage applied to your transformer
>> even when the relay is open. A capacitor looks like a resistor to AC.
>> Worse than that, when the relay does close, the entire stored energy of
>> the cap will discharge across your contactor, so you will be back in
>> spot weld city again. Old point systems in auto ignitions had to be
>> replaced periodically, as the contacts burned away, but they were DC
>> systems, so there was no AC leakage through the cap.
>>
>> Dave
>
>
>
> ------------------------------------
>
> Addresses:
> FAQ: http://www.ktmarketing.com/faq.html
> FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
> Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
>
> Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
> Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
> List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com, wanliker@..., timg@...
> Moderator: pentam@... indigo_red@... davemucha@... [Moderators]
> URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
>
> OFF Topic POSTS: General Machining
> If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble.
> http://www.metalworking.com/news_servers.html
>
> http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list.
>
> NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
> bill
> List Mom
> List Owner
>
> Yahoo! Groups Links
>
>
>
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 10.0.1204 / Virus Database: 1435/3411 - Release Date: 01/29/11
>
>