Kevin McMurtrie wrote in
:

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


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