Adam Bryant wrote:
>
> Forgive my limited electronics background, but what is the purpose of the
> RC component in this diagram?  I understand the purpose of the diodes,
> but why would you use the RC component?  I would guess as a method to
> reduce spikes and "slow down" the rate of change of the incoming signal?
>
> Thanks in advance for increasing my electronics knowledge,
> Adam
>

Adam, any capacitor has its internal "resistance", the difficulty the
electrons have to flow into and out of the capacitor plates. Better
capacitors has low ESR (resistance) so they are the ones to be chosen to
be used at power supply filters, mainly when high demand of current is
required by motors and low impedance short pulses.  So, it is somehow
clear that low ESR is good for capacitors, but not always.  A low ESR
capacitor can "suck" fast voltage transitions, ac signals horsing dc
levels, and this is the main usage in power supplies, removing the ac
component from the dc, as a filter.

A capacitor also has the use as an integrator, creating a non linear
ramp of voltage over time.  This time is calculated as "factor RC". The
formula is "R x C", R in Ohms, C in Farads, the result is the transient
time to reach aprox 60% of the value of the charging voltage.  It is
widely used as a timer in lots of circuits that demand time control.  If
you use a R = 10,000 Ohms, a C = 0.0001 Farads (100uF), connect both in
series, and apply 10 Volts to them, after (10,000 x 0.0001) 1 second,
the capacitor would has aprox 6 Volts.  Larger values of R or C increase
the time, smaller reduce it.  We can call it "RC time".

If you think that if instead of apply a steady 10Vdc over the RC, you
apply 10VAC, it means that the AC component would try to charge the cap
with its polarity, when the ac polarity reverses, it would try to charge
the cap with that polarity. To do that, it first need to discharge the
cap from the previous (other polarity) loaded voltage, then try to
charge the cap, and it repeats itself.  If the period of time the AC
signal stays in each polarity is long enough to charge the cap to its
60% or more, it means that the cap would be "saturated" before that
polarity period of time ends, and the cap would not "suck" current from
that voltage or circuit, as if it do not exist anymore.

If the "RC Time" is very small and the AC period is long, the RC
"filtering" would be very small and would not interfere much into the AC
signal.  By the other way, if the RC time is long and the AC period is
short, the RC filtering would make great effect over the AC signal,
"sucking it" in great effect, so representing a heavy filter to the AC
signal.

Imagine a transformer 110 x 10 Vac powered with 60 Hz (16ms AC period).
If you apply a RC with "RC time" longer than 150ms to the 10Vac
transformer secondary, it probably would drop the 10Vac significatively
(R=150 Ohms, C=1000 uF). Capacitor would need to be non polarized, or
two polarized capacitors (2000uF each) connected in reversed serial.

Now, if the AC signal is mounted over a DC voltage, the capacitor would
never receive reversed polarization, but electrons would enter and exit
the capacitor, with a large amount of charge at the plates caused by the
DC component.  This is what happens in a power supply, AC (top of the
wave) over the DC generated by the diodes and capacitors.  To have the
best filtering, you need to use resistors as low as possible, so "no
resistor" is the best one at the power supplies.  Some "PI" filtering
with two capacitors connected by a resistor is used as a "low pass"
filter.  For better efficiency, an inductor (coil) can replace the
resistor, since its impedance is higher to AC then DC.

A low pass filter allows only signals below certain frequency to cross
it. The "RC time" just rules about what would be AC frequency to pass
and which not.  There are plenty of formulas and models of low pass
filters around.  One of them is just as you saw and asked about the ADC
input, a plain resistor and a capacitor. The main reason at that drawing
is just reduce the AC component at the ADC input, so yes, it is a plain
filtering, an integrating RC.

--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site:  http://www.ustr.net
Microcontrollers Survey:  http://www.ustr.net/tellme.htm