Re: [CAD_CAM_EDM_DRO] Re: Capacitor safety questions

The discharge time is given by the formula:

Time = R C

So if you have a 1 000 ohm resistor, and a 12 000 micro Farad
capacitor, it would be:

1 000 x 0.012 = 12 seconds

(0.012 is 12 000 micro farads)

Here is the discharge rate:

Time Remaining charge
12 37%
24 13.5%
36 5%
48 2%
60 <1%

So, for a 100v power supply, after 36 seconds, it will be 5v or less.

To determine the size (power) resistor you need, use this formula:

E^2
------
R

that is:

voltage x voltage
------------------------
resistance

So in our 100v power supply example:

100 x 100
-------------- = 10 watts
1000

I would add a 20% or more safety factor and get, say, a 15 watt
resistor.

To calculate how much current the resistor will use from the supply:

voltage
----------
resistance

or

100
-------- = 0.100 amps
1000

Connect the resistor directly across the capacitor. I crimp ring lugs
on mine and connect directly to the screw terminals on the cap.

Also, when buying a capacitor, always add a safety margin to the
voltage rating of the cap. For a 100v supply, I would use 125v or more.
Since the output voltage of a transformer can rise above the stated
value when there is no load, blowing a fuse might cause the transformer
output to rise above the rating of the capacitor. You don't want to
blow or stress your cap if this happens. Of course you want to be extra
careful about the polarity of the cap too. Connecting one backwards can
cause quiet a mess when electrolyte spews everywhere.

Regards,
JJ

On Saturday, December 27, 2003, at 02:03 AM, Torsten wrote:

> --- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>>
>>
>> volitan712003 wrote:
>>
>>> Hello,
>>>
>>> I just ordered a 12,000uF x 100V capacitor for my power supply. I
> know
>>> capacitors can be deadly but I have no concept of what a micro
> farad
>>> is. If this cap is fully charged, is this thing big enough to kill?
>>>
>>>
>> At 100 V, if your fingers were wet or had cuts, and you jammed them
> onto
>> the terminals, yes, it could be fatal. This would be a pretty
> unusual
>> event.
>> But, that's why people put bleeder resistors on them, so they
> discharge in
>> a short amount of time.
>>
>
> Jon, I have a very similar setup and got buzzed a few times and
> like to find out more on the "bleeder resistors" you mensioned
> my system retains power when unpluged for more then 24 hours and
> I think this may be the answer to my problem.
> What I am looking for is something that will drain power after
> shutoff in about an hour or earlier so I can work on it.
> Will this bleeder resistor reduce power output during normal
> opperation?
> What size partno. where to get and how should it be wired up.
> Thanks for any info you could provide.
>
>
>
>
> 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
>
> To visit your group on the web, go to:
> http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/
>
> To unsubscribe from this group, send an email to:
> CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
>
> Your use of Yahoo! Groups is subject to:
> http://docs.yahoo.com/info/terms/
>
>