Dennis Miles wrote:
Michael, Your ego is boasting,"I can design my own" but, your statements
show IMHO the fact is that you don't know "Real World Design." Perhaps you
should build your own and invest a dollar in your own design.
Yes, I would encourage him to go ahead and build his own. He'll learn a
lot along the way. Sometimes the journey is more interesting than the
destination! :-)
Michael: 17 x 3.6v = 61.2v nominal. Your pack could easily hit 70v peak
from charging or noise. I'd include a safety factor, and design your
circuit to survive 100v.
You want to power the circuit from the pack as a whole; not from a tap.
If you draw current from a tap, it unbalances the pack -- exactly the
situation a BMS is trying to avoid.
So to power your comparator from the pack, you need a power supply that
steps 50-100vdc down to under 24v (about the most you can apply to a
comparator IC like the LM393). This power supply probably won't be an IC
due to the high voltage. There are a few ICs that can do it, but you
won't like the price.
If it were me, I'd probably use 2 resistor, 2 zeners, and a bypass
capacitor. Bad ASCII circuit (view with a fixed width font):
+62v___________
pack+ |
>
3.9k >
>
|_________________
| | |
| | __|\|8 V+
24v _|_/ _|_ |+ \____
zener //_\ ___ 1uF | /
| | __|-/4 gnd (NOT chassis ground!)
| | |/| LM393 dual comparator
|_____|___________|
|
>
3.9k >
>
-62v_________|_____
pack-
This powers the comparator on about 5ma from the pack. The comparator's
V+ and GND pins are your floating power supply to power your voltage
comparing circuit.
The comparing circuit will basically be a "limit comparator" as shown on
the LM393 data sheet. The center tap of the pack goes to +Vin in that
circuit. The +Vref and -Vref voltages set your high and low limits.
You'll need more resistors and a precision reference to set these
voltages to something like +1v and -1v from +Vin.
If you want hysteresis, you'll need to add feedback resistors from input
to output of each comparator.
It will work... but as I say, it gets to be a lot of parts!
--
If you're not stubborn, you'll give up on experiments too soon.
And if you're not flexible, you'll pound your head against the wall
and you won't see a different solution to a problem you're trying
to solve. -- Jeff Bezos
--
Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm
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