vorange said:
1) What is the output from a microcontroller's pwm that is generating
square waves considered (+5v -> 0v -> +5v -> 0v...etc)? Is it
considered an AC or DC signal or a combination of both? After reading
all the replies, I'm left with the impression that "pulsed DC" is
considered as AC by some folks here even though it does not reverse
direction (go negative) when it goes low (0v).
DC has two common meanings. One is unidirectional current
or voltage, regardless of how it varies with time, and the
other is steady current or voltage. I think most circuit
designers (who are familiar with Fourier and LaPlace
analysis) tend to think in the frequency domain, at least
part of the time, and are more likely to think of DC as a
steady current or voltage, essentially a zero frequency signal.
By that frequency domain way of thinking, that pulsing PWM
unidirectional voltage has some DC component (the average
voltage, and a whole series of AC components with various
magnitudes and phases, relative to the pulse timing, that
are all harmonics of the pulse frequency. Filters with
various frequency responses will pass varying amounts of all
those components.
2) Should not the output from a capacitor be HIGH if it encouters a
steady DC current?
Very. Infinite, if you wait an infinitely long time. The
only way to get current through a capacitor is to have the
voltage across it change at some rate. The formula that
relates current to rate of change of voltage is :
I=C*(dv/dt) with I in amperes, C in farads, and dv/dt in
volts per second.
So, the only way to get 1 ampere of DC to pass through an
ideal 1 uF capacitor is to have the voltage across the
capacitor climb at 1 million volts per second, and keep
climbing at that rate for as long as the current must occur.
Since few of us have voltage sources, that can climb
toward infinity, sitting around, we generally think of
capacitors as devices that cannot pass DC.
I know you've said that capacitors 'block' steady
DC currents but why? Mentally, I imagine when the dc current first
hits the capacitor's plate like a tsunami, it charges up the plate and
pushes electrons on the opposite plate away.
That's good. But that sudden onset of voltage is not DC in
the frequency domain sense, but DC plus an infinity of AC
frequencies all added together. The sudden high rate of
change of voltage across the capacitor accounts for the
high, momentary current.
I imagine those (pushed
away) electrons then go "racing away" from the capacitor perhaps
towards a load which should be driven HIGH by those electrons so long
as the capacitor's opposite plate is charged (which is what a DC
current should keep doing). Why then is this not the case ?
Sounds pretty good to me. The output DC will be maintained,
if there is no load current. So static DC voltage is
possible through a capacitor. This is the same as the DC on
rubbed balloons. But you can't get a steady current from
this effect, only steady, insulated voltage.
Somehow
the mental model just does not fit. I can imagine how AC passes
after reading the descriptions you guys have provided but why DC does
not generate a contiuous high.
AC supplies repeated (and alternating) rate of change of
voltage, so alternating current can be passed.
If only I could watch cartoons of what the electrons were doing, it
would all be clear to me.
It is a shame we can't put on special glasses and watch the
little buggers. All this would be a lot less abstract.
