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'[EE] Low Battery Voltage Detection (not uC)'
|> I'm working on a product that would use 6 series Ni-MH
> cells and would like a
> simple way to detect when the pack is at 6.0 and/or 5.4
> volts. This is a
> second generation version of my Battery Top Power Supply,
> so a minimalist solution is the only type that will work.
Ra supply to base.
Rb base to ground.
Transistor is on when divider supplies above ABOUT 0.6V.
Use that to do whatever.
So threshold is R/Rb = (Vsupply-0.6)/0.6
This is approximate due to the soft Vbe knee and what "on"
means to you. Also watch temperature variability.
BUT it's probably good for 10% accuracy (maybe better,
depending on ... ). Assuming it's for battery protection
even 20% would probably save your batteries well enough, as
cell voltage goes down very rapidly below about 1V under
normal loads. So a cell that is meant to cutout at say 1V
and actually does so at 0.9V may not bother you too much
(again "depending on ... ".)
At the risk of being labelled as a "NZ designer" I'll admit
to currently considering a similar scheme for a rough low
battery cutout. It has limitations but is also good enough
for some purposes.
If you want precision then a cheap op amp (eg LM358) and a
reference diode would do. Or even a 2 transistor long tailed
pair and a reference diode - small and under $US0.20 parts
cost all up (under $US0.10 in good volume). A zener MAY do
well enough as a reference depending on what precision you
want. As the cutout point is well defined the zener will
have a well defined feed voltage at cutout (infinite
regress) so it's better than in many zener applications.
|Den 2007-12-17 02:14:54 skrev Apptech <paradise.net.nz>:apptech
If you only have acpuple cells in seres that is OK.
But as cells are not identical, especially after half life, sone will be below 1V while other at 1.2V, and it is nice to cut of before causing unecessary stress oin the less god cells.
I believe it is less total cost to simply change that NPN for a three pin uC reset IC with open drain output.
For example, Currently I use STM1061 to pull a FET gate to GND when one voltage is low.
They usually need very little supply and reference current, which is a plus in your case.
And use to be pretty accurate. Some also have hysteresis and/or delay.
They aslo come with inverted logic variants. If not inverting, you can add hysteresis witha feedback R, and with series diode to that R make it latching.
Beware most such reset cirquits do not like more than 5V on the output.
If you want it to output positive current when batt is OK, consider a similar solution in positive side using LM385
(If i remember correctly...)
> If you only have acpuple cells in seres that is OK.
> But as cells are not identical, especially after half life, sone will be below 1V while other at 1.2V, and it is nice to cut of before causing unecessary stress oin the less god cells.
Four cells or below is safe.
Given you have regulated voltage output, then simply use a resistor divider
on the input to give you the same voltage as your output when you hit your
thresholds (eg 22k, 110k for 6V threshold, 5V output), then put that and
your output voltage into an op-amp.
On Dec 16, 2007 11:27 PM, Brooke Clarke <pacific.net> wrote: brooke
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