Remember that in the real world a constant current source may fail at 5V or=
 100V ( or whatever ).
Before failing, the constant current source may stop being constant.   The =
capacitor may stop being a capacitor.
Maybe SPICE does not believe there a voltages higher than .999KV and goes c=
razy.
Try a series resistance of 1 ohm.  This will introduce an error factor of m=
aybe .01V .
99guspuppet

> On Jan 29, 2012, at 3:40 PM, jim wrote:
>=20
>=20
> Bill,
>=20
> Yes, that is exactly what I got. 1kV.  However, when I put this circuit i=
nto
> 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 i=
s
> it maybe with Zero ohms of series resistance, SPICE is confused?
>=20
> Thanks and Regards,
>=20
> Jim
>=20
> -----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
>=20
>=20
> On Jan 29, 2012, at 2:02 PM, jim wrote:
>=20
>> If there is a current of 10ma charging a 1uF capacitor, what is the
> voltage
>> across the capacitor at 100ms?
>=20
> v(T) =3D 1/C (integrate(i(t), dt, t0, T)
>     =3D 1e6 * 0.01 * .1
>     =3D 1e3 V
>=20
> Is that what you got?  Sanity check: if we has a 1e3 V power supply, then=
 to
> 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.)
>=20
> It does seem surprising, since 1uF and 10mA are values you'd expect to fi=
nd
> in a real circuit, and 1e5 V isn't.  However, pumping constant current in=
to
> a cap is not trivial to actually accomplish.  Most real circuits are clos=
er
> to voltage sources with resistance, and current into caps quickly falls t=
o
> small amounts.
>=20
> BillW
>=20

--=20
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