In article ,
Jim Thompson
wrote:

On Sun, 10 Aug 2014 16:19:02 -0400, rickman wrote:

On 8/9/2014 6:09 PM, Robert Baer wrote:
Jon Elson wrote:
EngineeringGuy wrote:

When connecting "super capacitors" in series to increase the working
voltage above the level of the individual capacitors, does anyone
have an
active voltage clamp that would not discharge the capacitors when the
power supply is disconnected? Most "super" capacitors have a working
voltage of 2.5 or 2.7 VDC... I would like to connect 10 capacitors in
series for a 25
VDC capacitor stack. Most suggestions call for connecting equalizing
resistors in parallel with each capacitor, but I would rather have a
circuit that would not discharge the capacitors when the power is off...
anyone have any ideas?
Well, a couple diodes across each cap would not leak a whole lot
when forward biased below the forward conduction voltage.
With plain Si diodes, that might be 5 diodes in series. they would
start leaking somewhere below 3 V, and conduct pretty strongly
at 3.5 V.

You could also look a Schottky diodes and see what values of Vf you
see. If they start to conduct at .4 V, then 7 in series would
give you 2.8 V turn-on. You can get surface mount SOT23 dual diodes
wired in series, so that would only take 4 parts.

Jon
Sounds silly to me; why use the diodes in the forward bias mode,when
the reverse bias mode seems far better?
That way, the capacitors are always reverse polarity protected.

Because the reverse breakdown voltage is not well specified. It is
guaranteed to be above some value, but where exactly will that be? How
many diodes do you know of that will protect a 2.7 volt cap when the
diode is reverse biased?

LEDs might be a good choice for forward biased protection. Some of them
work at 2.7 volts or so. Add a single Si diode and you will get a very
small amount of current flow in the cap working voltage range with the
knee in the I/V curve somewhere above 2.7 volts depending on the color
used.

You're whistling somewhere inappropriate if you think diodes are going
to match and track well enough.

Late generation high efficiency LEDs have incredibly sharp conduction
knees, amazing matching between components, and low internal resistance.
LED manufacturers have put lots of black magic into those chips to break
past normal efficiency limitations. Their only imperfection as a shunt
regulator is that the voltage drops with temperature.

The problem of handling full inrush current after the caps have
self-discharged to different voltages remains unresolved. I still think
resistors would balance the best.


This is one of those annoying cases where the OP has vanished after
the initial post and is not answering questions. Without more info,
it's a wasted effort.

Something along the lines of Field's relay method post... maybe with
MOSFET's... is most likely to succeed... just needs the right controls
:-}

...Jim Thompson