"John Fields" wrote in message
...
On Thu, 14 Jun 2007 18:08:08 -0600, "starfire"
wrote:


"John Fields" wrote in message
. ..
On Thu, 14 Jun 2007 15:06:52 -0600, "starfire"
wrote:


"starfire" wrote in message
...
I'm posting the associated circuit and traces here, per request from
Paul
Hovnarian. Thanks for the suggestion, Paul.


I'm posting some traces from some of the implemented suggested
modifications:

trace 1: no load from switched 3.6VDC to ground
trace 2: 1K-ohm load from switched 3.6VDC to ground
trace 3: 100-ohm load from switched 3.6VDC to ground
trace 5: 10-ohm load from switched 3.6VDC to ground

I installed 10uF caps across the exisitng input 1uF caps on the input
and
output of the TC1108 voltage regulator.
I also installed a 10uF cap across the existing 0.1uF cap on the Vcc and
ground leads of the PIC right at the power pins.
I also removed C30 (the 0.1uF cap on the drain of the N-Channel MOSFET
and
the gate of the P-Channel MOSFET.

As can be seen in the traces, the dip seems to be unrelated to the
amount
of
switched current. The dip is also reduced significantly in amplitude
now.
The switching action works now every time I tried it. This was all very
welcome news!

I used 10uF caps because that's what I had on hand and they were small
enough to fit nicely across the pins of the voltage regulator. I will
try
to find some 22uF (or so) caps to see if I can reduce the dip even
further.
This is very hopeful...

I tried the series diode with a BFC from the PIC Vcc to ground on a
separate
breadboard to convince myself this will work, also. It looks like this
approach would work, too. I'm a little reluctant in going that way if I
need to get analog inputs from an accelerometer (which is a proposed
add-on
to this system). If I use a Schottky diode drop from the raw battery to
the
PIC (along with a BFC to ground), the PIC Vcc could vary significantly
dependant on the current load going into the PIC and the charge status
of
the battery. I will keep this one in the back pocket, though...

---
Since the ADC in the PIC is ratiometric, (I believe) there will be
no errors due to changes in Vcc if you excite the accelerometer with
Vcc.


--
JF

Good point. I will try an experiment with the breadboard system and look
at
that. In addition to the accelerometer inputs, I have an op-amp
configured
as a voltage follower monitoring the center tap between a pair of 100K 1%
resistors in series. One end of the string goes to the raw 3.6VDC input.
The other end is tied to ground. I will see how this voltage changes with
varying raw input voltage.

---
It should always be half of the raw input voltage, +/- 1% worst
case, since for:


. E1
. |
. [R1|
. |
. +---E2
. |
. [R2]
. |
. GND



E1 * R2
E2 = ---------
R1 + R2



--
JF

Thanks, John.

I was thinking more of how the PIC's ADC would convert (what code was
produced) with the varying input voltage and the series Schottky in line
with the Vcc of the PIC. I would think it should be fairly stable
independent of raw voltage since both the voltage divider string and the PIC
are riding the raw changes. As long as the current draw from the PIC
doesn't vary much (due to loads switching on or off causing more current to
be drawn into the Vcc pin). This could potentially cause a variation in the
voltage drop across the Schottky diode. That could shift the code for the
center point. At least that's the theory...

Dave


Dave