RE: [CAD_CAM_EDM_DRO] Re: Charging Capacitors

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
>

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