Solar Panels? - Why PWM Works
Thomas McGahee email (remove spam text)
If you try to draw current from a battery with a high impedance,
the internal resistance of the battery will limit the maximum
current you can draw. If you place a capacitor in parallel with
the same battery, the battery will charge the capacitor up
to the full voltage within a certain time period determined
by the product R*C. The capacitor will initially charge
according to the standard charge curve. Assuming that the
capacitor has no leakage, the power system as a whole still
has the same total energy capacity it had before. However,
since the impedance of the capacitor can be MUCH lower than that of
the battery, it can supply high currents for short periods.
*After* being discharged, there will be a time delay before
the capacitor fully charges again. That is the tradeoff.
Every time you use a decoupling cap from an IC power pin to ground,
you are doing the same thing. This techniques is also useful
when driving relays: they have an initial need for a good stiff
voltage source *until they are closed* then, they only require
a certain minimum holding current. After releasing such a relay,
there is a certain minimum recharge time that must be observed
before you can re-activate the relay.
With relays that can tolerate this wait time between activations
you can actually add a resistor between the battery and the
capacitor that will power the relay. This greatly reduces the chance that
activating the relay will produce noise on the power bus.
Hope this helps.
Fr. Tom McGahee
See also: www.piclist.com/techref/io/pwm/index.htm?key=pwm
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