On Tue, 12 Aug 2014 22:16:35 -0700, Robert Baer wrote:

EngineeringGuy wrote:
Kevin wrote in
:

In ,
Jim
wrote:

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


Jim... I haven't vanished, just out of town. I don't quite understand
what clairification you require... 10 caps in series to give 27VDC
total operating voltage.. each with "something" in parallel to prevent
over voltage... overvoltage will cause "super capacitors" to short and
fail..
that "something" must be such that it will not discharge the capacitors
when the 25 volts is removed, turned off, disconnected, not charging,
etc.

Bill
...with NOTHING to protect each one from reverse voltage?

Reverse polarity protection is important too, but what he is asking about
is individual cell overvoltage protection, for which he needs a 2.7 volt
shunt regulator across each cell, which will bypass a fully charged cell
while allowing the rest of the string to fully charge. The forward
polarity diode string others have suggested is a crude, high tempco shunt
regulator of sorts when used as suggested, but a much more precise and
stable voltage limit could be set with IC shunt regulators.

By replacing the resistor string with any variety of shunt regulator you
do also need to consider replacing the other feature of a resistor
string: preventing individual cell reverse polarity when the capacitor
string is fully discharged.