On Fri, 7 Dec 2001, Robert Downs wrote:
> polarity of a cell. I have never seen this happen, but know it is possible.
> However, since TTL circuitry (which I'm pretty certain the power supply of
> the Newton is) requires .7 volts or more to activate, you should never be
> able to drain any particular cell in a newton's battery pack below that
> level.
While it's true that the threshold voltage of a transistor might be .7V,
most Integrated Circuits need voltages higher then that to operate.
Typical values these days are 2.5V, 3.3V, etc.
The power supplies in the Newton are the limiting factor as to if the pack
voltage is enough or not for it to run. The Newton no doubt supplies 3.3V
to devices like the memories & FlashROM, so the Newton needs at least 3.3V
in to do this properly.
> If you wish to drain a battery outside the newton, determine, using
> ohms law, what resistor will provide a current draw from the pack that the
> newton usually pulls under a full charge, combine that with a diode (LED)
> and connect that combination to the terminals of the pack and let it go
> until you can no longer see light from the led. (unfortunately this method
This might work. Stacking LEDs too would make this easier. Some LEDs will
turn on with 1.2V forward bias, so as the pack voltage drops, if you put a
few of these LEDs in series, at some point the LEDs won't turn on and will
prevent current from being drawn (neglecting leakage).
> is NOT a constant current method and can take days to discharge. If anyone
> knows a simple constant current method, speak up.) The pack should be
> discharged to .7 volts. About as far as you should go. If you are
Well the pack would fall to the forward bias of the LED. Most LEDs need a
bias higher than .7V though.
A simple constant current scheme can be developed using a "current
mirror". This can be done with BJTs or FETs, but I prefer using n-channel
FETs. Try a search engine or if you have a copy "The Art of Electronics"
by Horowitz & Hill. Also any college text that covers electronic circuits
(basic design with BJTs & MOSFETs) will contained a detailed explanation.
There are op-amp circuits that work as voltage to constant-current
convertors, if you want more info, contact me directly.
Thanks,
Speedy2.
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