Forums

How to discharge the capacitor faster

Started by Johnson Liuis January 19, 2005
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?\

Thanks in advance.

Johnson


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?\
Just add a resistor to the circuit so that the cap can discharge through the resistor. A 10K resistor will discharge the cap in about 1 second. vax, 9000
> > Thanks in advance. > > Johnson
Thanks. I think it is a good idea.
Johnson

"vax, 9000" <vax9000@gmail.com> wrote in message
news:csmdn4$8pi$1@charm.magnus.acs.ohio-state.edu...
> 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?\ > Just add a resistor to the circuit so that the cap can discharge through
the
> resistor. A 10K resistor will discharge the cap in about 1 second. > > vax, 9000 > > > > > Thanks in advance. > > > > Johnson >
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?\ > > Thanks in advance. > > Johnson
These caps are commonplace on PCBs with ICs on them (they are called decoupling caps). You're cap is taking a long time to drop below a certain threshold value because it's capacitance is far too high (RC time constant too high). Usually, 10 - 100nF is the standard for decoupling caps used on digital VCC/VDD inputs. Try 10nf first, if the ripple rejection is not high enough, step it up. I don't suggest using bleeder resistors of any sort until you have exhausted all other possibilities (resistors take up needless PCB real-estate and waste energy).
On Wed, 19 Jan 2005 12:44:20 -0700, "Johnson Liuis"
<gpsabove@yahoo.com> 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....
You are trying to solve the wrong problem. No matter what you use to discharge the capacitor, there will be some discharge time at which you can't re-apply power without a hangup. You should probably take another look at the "voltage-delicate" chipset and figure out why it hangs up during a brown-out. Perhaps the solution is to add a reset-detection chip, commonly used with microcontrollers to hold the chip in reset whenever the voltage is below a certain threshold. -Robert Scott Ypsilanti, Michigan (Reply through this forum, not by direct e-mail to me, as automatic reply address is fake.)
I believe it can wipe off the weed from the root, but I got to spend an
extra reset-detection chip and redraw part of my PCB. Is there any
reset-detection chip less than 50 cents each?

Johnson.


"Robert Scott" <no-one@dont-mail-me.com> wrote in message
news:41eec76e.11358494@news.provide.net...
> On Wed, 19 Jan 2005 12:44:20 -0700, "Johnson Liuis" > <gpsabove@yahoo.com> 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.... > > You are trying to solve the wrong problem. No matter what you use to > discharge the capacitor, there will be some discharge time at which > you can't re-apply power without a hangup. You should probably take > another look at the "voltage-delicate" chipset and figure out why it > hangs up during a brown-out. Perhaps the solution is to add a > reset-detection chip, commonly used with microcontrollers to hold the > chip in reset whenever the voltage is below a certain threshold. > > > -Robert Scott > Ypsilanti, Michigan > (Reply through this forum, not by direct e-mail to me, as automatic reply
address is fake.)
The sad thing is, I tried different capacitors, and only 100uF and 10uF
worked. All other capacitors less than 10 uF will still get the delicate
chip crazy.

Johnson



"K" <kiru.sengal@gmail.com> wrote in message
news:1106166059.317049.120790@z14g2000cwz.googlegroups.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?\ > > > > Thanks in advance. > > > > Johnson > > > These caps are commonplace on PCBs with ICs on them (they are called > decoupling caps). You're cap is taking a long time to drop below a > certain threshold value because it's capacitance is far too high (RC > time constant too high). Usually, 10 - 100nF is the standard for > decoupling caps used on digital VCC/VDD inputs. Try 10nf first, if the > ripple rejection is not high enough, step it up. I don't suggest using > bleeder resistors of any sort until you have exhausted all other > possibilities (resistors take up needless PCB real-estate and waste > energy). >
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
I suspect that the "voltage-delicate" chipset hangs up during a brown-out,
is possibly because the 100uF Cap works like a back-up battery during
brown-out. Until the cap get discharged, the chipset will not feel the power
cycle, thus it did not perform a reset.

Johnson


"Robert Scott" <no-one@dont-mail-me.com> wrote in message
news:41eec76e.11358494@news.provide.net...
> On Wed, 19 Jan 2005 12:44:20 -0700, "Johnson Liuis" > <gpsabove@yahoo.com> 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.... > > You are trying to solve the wrong problem. No matter what you use to > discharge the capacitor, there will be some discharge time at which > you can't re-apply power without a hangup. You should probably take > another look at the "voltage-delicate" chipset and figure out why it > hangs up during a brown-out. Perhaps the solution is to add a > reset-detection chip, commonly used with microcontrollers to hold the > chip in reset whenever the voltage is below a certain threshold. > > > -Robert Scott > Ypsilanti, Michigan > (Reply through this forum, not by direct e-mail to me, as automatic reply
address is fake.)
I strongly suggest using the full array of brown-out/monitoring ICs out
there to reset "all" major ICs on the PCB.  If your PCB has a
microcontroller on it, you would probably tie this circuitry in with
any watchdog.

If you don't want to go that far, I think the best solution is the have
a grace period before you assume all devices are fully reset.  Don't
just drop the rails and then turn them back on immediately.

I still find the fact that you needed a 100uF to get the ripple
rejection job done peculiar.  Are there are details of this PCB you can
post (possibly a partial schematic)?