V G wrote:
> Let's say that there's a power source with variable voltage. Across
> its terminals is a resistor of 10 Ohms. The power supply can only
> supply a maximum of 1 Amp.
>
> If the voltage is gradually increased, to the point where I =3D V/R > 1
> Amp, what will happen? Will Ohm's "law" not be violated?

No, since the power supply can't supply more than one amp according to your
definition, its output voltage will cease to increase, regardless of how
much you tell it it should.

Most bench supplies have both a constant voltage and constant current mode.
What that really means is the output will be where the voltage or current
limit is met, whichever is lower.  You can use Ohms law to compute the
resistive load at which the power supply switches between current and
voltage regulation mode.  Ohms =3D Volts/Amps, so the voltage setting divid=
ed
by the current setting is the resistance at which the supply will cross
over.  For lower resistances it will be in current limit mode, for higher
resistances in voltage limit mode.

> In the case of the battery charger example:
>
> A NiMH battery should be trickle charged (assume constant current of a
> few milliamps, rather than a high current pulse). The battery has a
> relatively low internal resistance. How can the current supplied to
> the battery be controlled precisely when the applied voltage is
> relatively constant?

By controlling the current directly.  There is generally a signal inside a
power supply that makes it put out more when higher and less when lower.  I=
n
voltage mode, this signal is driven from feedback that compares the actual
output voltage with the desired output voltage.  It can just as well be
driven by feedback that compares the actual output current with the desired
output current.  In bench supplies, both such feedback paths are active
simultaneously, and the internal signal is driven from the lesser of the
two.

Did you look at the battery charger/maintainer I pointed you at?  You can
see current feedback and a constant current regulator for each battery.


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