Re: [CAD_CAM_EDM_DRO] Re: Charging Capacitors

Larry,

I don't have all the original posts on this computer, but IIRC, you had
6 caps of 5600 uF each, for 33,600 uF total. I checked the cute little
on line RC time constant calculator at:

http://www.cvs1.uklinux.net/cgi-bin/calculators/time_const.cgi

and it indicates that a 125 ohm bleeder resistor across this bank would
drop its voltage to 10% of its original charge in about 10 seconds. If
you are feeding this with 200 volts, then you would be dissipating 320
watts in the bleeder, a nasty power drain, but perhaps worth it for
safety. If you are more brave, you could use a decently rated contactor
energized by the mains to the power transformer, with the contacts
connecting in the bleeder whenever the main power is deenergized. That
way, you would not have the bleeder powered all the time.

I'm not sure how your particular Fluke meter works. Your description
sounds contradictory. It sounds like a 3 3/4 digit meter, good up to
399.9 mA full scale. If it's a 300 mA full scale setting, then the
decimal point should look something like 300.0. I don't know if the
meter shifts the decimal point automatically, but a reading of 00.60 on
a 300 mA scale would suggest 0.6 milliamps, a negligible leakage current.

You can expect the current readings to bounce around with line voltage,
because when the line voltage sags a little, then the output voltage of
the transformer will be less than the stored voltage in the transformer,
and no current would be expected to flow. When the line rises a little,
then, the charging current would increase.

You can do a reality check on your meter with a simple setup. Connect a
9 volt transistor battery in series with a 1 K resistor and an ordinary
forward biased red LED, and your meter set on the 300 mA range in
question. You should see a scale reading of something like 5.0 to 7.0
for five to seven milliamps forward current if the LED is lit up. Any
other combination will tell you how the meter is scaling the current,
which you "know" has to be something like 5 to 7 mA, and you can work
from there.

HTH,
Dave

Larry Olson wrote:

> Not warm at all. That's why I'm questioning the MM reading. It's a fluke
> 75 on the 300ma dc setting and the readings get down to 00.60 to 00.20
> and bounce around considerably. I can see line voltage sags on the meter
> sometimes dropping to 00.00. I presume 1.00 on the meter is 10ma because
> it goes into overload (above 300ma) at 30.00 on the meter. I may be
> wrong here as I'm not a genius, just smart enough to be dangerous.
>
> I have a choice of 300W resisters I can use for discharge on the bench.
> A 20ohm that will be used on the soft start for the P/S to limit initial
> charge to a maximum of 10A draw on the mains. And a 10ohm that will
> eventually be used as a dump for the cap bank as well as any returning
> voltage from the servos as the tables are large and heavy. I've been
> discharging them with the 20ohm and than putting a strap across them. I
> notice even after total discharge there is a buildup of voltage again
> after some time. If I don't get the strap on right away I will see a
> small arc when installing it. Haven't been bit yet (knock on wood).
>
> I have 6 more of these caps in another set on hand as spares. I know
> they have a life cycle. I also have a set of higher voltage and
> capacitance that I can use as well.
>
>
>
>
>
> PS: I'm used to working around 470VDC when I have to re-tube my shop
> stereo amp about every 2500hrs. Or when it blows up.
>
>
>
> Thanks,
>
> Larry Olson
>
> _____
>
> From: David Speck [mailto:Dave@...]
> Sent: Wednesday, June 07, 2006 2:01 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] Re: Charging Capacitors
>
>
>
> Larry, if you are drawing 50 mA per cap, then it's 8.5 watts per cap,
> which might make me uncomfortable.
>
> Are the caps noticeably warm while being reformed? Turn off and unplug
> the Variac and let them self-discharge before you feel them. If you hit
> a metal spot, you may be connected to mains voltage. Be very careful
> with these as each cap is storing about 78 Joules at 170 volts -- enough
>
> to kill you several times over. You should install a discharge resistor
> across the bank. I'll have to get back to you on the proper resistor
> ohms and power rating -- it should discharge about 5 RC time constants
> in ten seconds or less, and I have to look up the formula again.
>
> You have to view electrolytic caps as a consumable commodity. They do
> have a finite lifetime. If yours are 30 years old, you might want to
> look for fresher ones with lower leakage.
>
> Larry Olson wrote:
>
>> Thanks for the info. A little more time on the voltage then as a unit
>> with all 6 in parallel.. That was 40-50ma per cap if I'm reading the
>>
> MM
>
>> right. Did I mention they are used and 30 yrs old?
>>
>>
>>
>>
>>
>> _____
>>
>> From: David Speck [mailto:Dave@...
>>
> <mailto:Dave%40DavidSpeckMD.org> ]
>
>> Sent: Wednesday, June 07, 2006 1:30 PM
>> To: CAD_CAM_EDM_DRO@yahoogroups.com
>>
> <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
>
>> Subject: Re: [CAD_CAM_EDM_DRO] Re: Charging Capacitors
>>
>>
>>
>> Larry,
>>
>> If all 6 caps together are leaking 50 mA at 170 volts, I'd say that is
>>
>
>
>> acceptable. It works out to about 8 watts dissipation over the 6 cans,
>>
>
>
>> which would not cause damaging heating.
>>
>> If it's 50 mA per cap, then you will have a little more heating, i.
>>
> e.,
>
>> 8 watts per cap. probably enough to make them warm to the touch, but
>> you might get away with it.
>>
>> I doubt that increasing the reforming voltage to 200 V would make any
>> significant improvement in their performance.
>>
>> You will have to check how hot the caps get in actual operation,
>>
> because
>
>> you will have the ripple currents to contend with. Now, with no load
>>
> on
>
>> them, the ripple is negligible, but in use, the caps will charge and
>> discharge with every peak of the line current, 120 times a second. The
>>
>
>
>> current flowing in and out will cause additional heating which may or
>> may not be a significant source of additional heat.
>>
>> I suspect that there may be a runaway phenomenon if the caps get too
>>
> hot
>
>> -- the hotter they get, the more they leak, which just makes them
>>
> hotter
>
>> still, until they fail explosively. Just be careful, and keep the caps
>>
>
>
>> in some sort of protective enclosure so that if one of them lets go,
>> then the electrolyte doesn't coat more expensive components. It
>>
> stinks,
>
>> and is hard to remove.
>>
>> Dave
>>
>> oldpayphones wrote:
>>
>>
>>> I've been charging (reforming)capacitors since Saturday and I just
>>>
>>>
>> have
>>
>>
>>> a couple of questions. First off I an using a 120V variac and a
>>>
> bridge
>
>>
>>> rectifier this is fed through my multimeter set for 0-300MA. I am
>>> slowly increasing voltage keeping the current limited to between 40-
>>> 100MA. I have managed to bring the voltage up to full line voltage
>>>
>>>
>> over
>>
>>
>>> a period of about 25-30 hours. This translates to about 170VDC. I
>>>
>>>
>> can't
>>
>>
>>> get the caps to fall below about 50MA of current draw at 170VDC. The
>>> caps are about the size of a soda can and are 200VDC @ 5400uf. Is
>>>
> this
>
>>
>>> a good current draw for this type of cap? Would it be advisable to
>>>
>>>
>> find
>>
>>
>>> a P/S that would deliver the full 200VDC rating for the cap to reform
>>>
>
>
>>> at their maximum voltage? Note: The caps (6ea.)will be operated at
>>> about 120-125VDC when they finally get installed in the PS that I am
>>> building. I am also building a 10A soft start for them.
>>>
>>> Thanks,
>>> Larry Olson
>>>
>
>
>
>
>
> [Non-text portions of this message have been removed]
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