Bill,

Yes, that is exactly what I got. 1kV.  However, when I put this circuit int=
o
SPICE (actually TINA-TI), the answer that SPICE comes up with is 9.09kV.
I'm not sure why this is.  Is it a mistake in my (our) calculations, or is
it maybe with Zero ohms of series resistance, SPICE is confused?

Thanks and Regards,

Jim

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
William "Chops" Westfield
Sent: Sunday, January 29, 2012 4:26 PM
To: Microcontroller discussion list - Public.
Subject: Re: Calculus question


On Jan 29, 2012, at 2:02 PM, jim wrote:

> If there is a current of 10ma charging a 1uF capacitor, what is the
voltage
> across the capacitor at 100ms?

v(T) =3D 1/C (integrate(i(t), dt, t0, T)
     =3D 1e6 * 0.01 * .1
     =3D 1e3 V

Is that what you got?  Sanity check: if we has a 1e3 V power supply, then t=
o
limit current to 10mA we would use a 100k ohm resistor, and the RC time
constant would be
1e-6*1e5 =3D 0.1s.  Yep.  (This would not result in a constant current of
10mA, and the capacitor voltage would not be 1000V after 1 time constant,
but that it is the correct order of magnitude is reassuring.)

It does seem surprising, since 1uF and 10mA are values you'd expect to find
in a real circuit, and 1e5 V isn't.  However, pumping constant current into
a cap is not trivial to actually accomplish.  Most real circuits are closer
to voltage sources with resistance, and current into caps quickly falls to
small amounts.

BillW

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