How to discharge the capacitor faster

"Johnson Liuis" <gpsabove@yahoo.com>

> I am dealing with a voltage-delicate chipset which will stop work if there
> is a ripple in Power Supply over 0.2V. I added a 100uF cap to the power
> circuit, and the ripple on power supply was sustained. However, I got a new
> problem, after powering down the device, I will have to wait for about 20
> seconds for the capacitor to discharge itself,

Don't you have a voltage regulator to remove the ripple?

> or the device will sometimes stop working during power on.

You need a proper reset circuit.  Try Maxim.  A cheap op-amp/
comparator can also do a good job.  You can use a dual amp for voltage
regulation and for reset, try an LM358; the extra current
draw of the op-amp will drain the cap faster.

Anybody make a combined voltage regulator/reset/voltage supervisor?

-- 
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer:  Electronics; Informatics; Photonics.
Remove spaces etc. to reply: n o lindan at net com dot com
psst.. want to buy an f-stop timer? nolindan.com/da/fstop/

"Johnson Liuis" <gpsabove@yahoo.com> wrote in message
news:357rklF4jimv0U1@individual.net...
>
> I am dealing with a voltage-delicate chipset which will stop work if there
> is a ripple in Power Supply over 0.2V. I added a 100uF cap to the power
> circuit, and the ripple on power supply was sustained. However, I got a
new
> problem, after powering down the device, I will have to wait for about 20
> seconds for the capacitor to discharge itself, or the device will
sometimes
> stop working during power on. (I find if I discharge the capacitor
manually,
> for example, by shorting the capacitor by a tweezer, I can save that 20
> seconds for wait). Does anybody know how to let the cap discharge faster
> while the ripple still need to be sustained?

As others have said, you're trying to fix the wrong problem. The clue is:
>>  I am dealing with a voltage-delicate chipset which will stop work if
there is a ripple in Power Supply over 0.2V. <<

WHY is there so much ripple on your rail? Solve this first, at source. Take
a long hard look at your power supply.

Secondly, beware of brownouts (again, as others have said). Use a decent
reset controller (e.g. MAX1232 or similar).

Steve
http://www.fivetrees.com

You haven't given any details whatsoever concerning your power-supply. 
If you are really craving assistance then start there.
Why be vague with "voltage-delicate chipset", say what it is.

Some questions:
- how are you measuring the ripple? (is it really there)
- what frequency is this ripple?
- what is the ripple like prior to the regulator?
- you are feeding dc into the regulator, aren't you?
- are you sure you are asking the right question?

Checklist:
- The input to the regulator should be filtered and at least 3V or more 
higher than the regulated voltage (assuming standard regulator etc)
- make sure your pcb layout is not the culprit (ground loops)

Sorry for the serve, I could give you the solution if I didn't have to 
guess.

*Peter*




Johnson Liuis wrote:
> I am dealing with a voltage-delicate chipset which will stop work if there
> is a ripple in Power Supply over 0.2V. I added a 100uF cap to the power
> circuit, and the ripple on power supply was sustained. However, I got a new
> problem, after powering down the device, I will have to wait for about 20
> seconds for the capacitor to discharge itself, or the device will sometimes
> stop working during power on. (I find if I discharge the capacitor manually,
> for example, by shorting the capacitor by a tweezer, I can save that 20
> seconds for wait). Does anybody know how to let the cap discharge faster
> while the ripple still need to be sustained?\

Mmmm, sounds you are right. I finally found a way to solve the problem with
a 1u Cap and a series resistor.

I am using Microsoft Outlook newsreader, not google group, so I cannot find
the buttons/links you mentioned. Any suggestion for posting on Microsoft
Outlook newsreader?

Johnson


"CBFalconer" <cbfalconer@yahoo.com> wrote in message
news:41EECA6C.BA5B35DC@yahoo.com...
> Johnson Liuis wrote:
> >
> > I am dealing with a voltage-delicate chipset which will stop work
> > if there is a ripple in Power Supply over 0.2V. I added a 100uF
> > cap to the power circuit, and the ripple on power supply was
> > sustained. However, I got a new problem, after powering down the
> > device, I will have to wait for about 20 seconds for the capacitor
> > to discharge itself, or the device will sometimes stop working
> > during power on. (I find if I discharge the capacitor manually,
> > for example, by shorting the capacitor by a tweezer, I can save
> > that 20 seconds for wait). Does anybody know how to let the cap
> > discharge faster while the ripple still need to be sustained?
>
> The brute capacitor may well be overkill.  A much smaller one might
> suffice with a series resistor, and then a relatively small bleeder
> can remove the power rapidly.  The first thing is to know the power
> requirements of that chipset.  You might also find that an active
> local regulator is the best solution.  Engineer it, don't beat it
> to death with a shovel.
>
> -- 
> "If you want to post a followup via groups.google.com, don't use
>  the broken "Reply" link at the bottom of the article.  Click on
>  "show options" at the top of the article, then click on the
>  "Reply" at the bottom of the article headers." - Keith Thompson
>
>

"Johnson Liuis" <gpsabove@yahoo.com> wrote in message 
news:35qktgF4pud77U1@individual.net...
> Mmmm, sounds you are right. I finally found a way to solve the problem 
> with
> a 1u Cap and a series resistor.
>
> I am using Microsoft Outlook newsreader, not google group, so I cannot 
> find
> the buttons/links you mentioned. Any suggestion for posting on Microsoft
> Outlook newsreader?
>
> Johnson

I have no idea why he uses that Google Groups exhortation (quoted below) as 
his tag line.  It has nothing to do with your question.

>> "If you want to post a followup via groups.google.com, don't use
>>  the broken "Reply" link at the bottom of the article.  Click on
>>  "show options" at the top of the article, then click on the
>>  "Reply" at the bottom of the article headers." - Keith Thompson