still a bit confused about the ANSEL
and CMCON command. Can you
shed a little light on this? Thanks.
AI use PICBASIC PRO from
microEngineering Labs, Inc.
You can download a demo
version plus MicroCode
Studio, plus the PICBASIC PRO
manual from: www.mecanique.co.
uk/products/compiler/ pbp.html. You
can download just the manual which
will answer all your questions. I’m not
the one to ask because I am a newbie
QI am hard of hearing and
have a difficult time hearing
on the telephone. Headsets
help but most cover only
one ear. I have a set of headphones
for my computer that cover both
ears and have a boom microphone.
I would love to be able to plug these
into my phone.
Can you recommend a circuit
that would allow some amplification
for incoming audio and provide
phantom power, as well as interfacing
for the condenser mic which is what
I think these computer mics are?
Good job continuing the Q&A
column! I enjoy reading it.
One comment: one reader asked
you about a power supply for a battery
charger, and you guessed the oddball
transformer in question was a ferro-resonant regulating transformer. Seems like
a good guess to me. But at the end of your
answer you said "batteries don't care about
being pulsed; in fact, it may be better than
This seems not to be true of laptop
batteries. If you look at the Maxim 8731A
charger chip, the Intersil 88731, the TI
bq24702, or any of the other modern
chips, they all use buck converters and the
battery charge current is regulated via the
duty cycle of a power FET that controls the
current into the coil of the buck converter.
The output of that buck converter is,
of course, a non-smooth DC waveform.
The charger chip makers all warn you to
use the proper ripple-absorbing capacitors
in parallel with the battery and go to great
lengths to show you what types of caps
work, which don't, what values the caps
should have, and where they should be
placed. Some of them specifically warn
you not to let the batteries absorb the
ripple themselves because it heats them
— Bob Colwell
Thanks for the feedback, Bob; I was thinking of automotive type batteries which
absorb pulses easily. A capacitor in parallel
with the battery will only work if the battery has high impedance which may be the
case in those types. Certainly a temperature
rise due to internal losses would not be
good in a fast charger that determines full
charge by a temperature rise.
Sioux City, IA
ANot knowing anything
about your phone or the
microphone, I cannot come
up with a circuit, but I do
have a solution to your problem. I
have a Panasonic model KX-TG5672
portable telephone. The handset
has a speaker phone button which
increases the volume so it is audible
across the room. My wife loves it; she
is hard of hearing, also. The handset
has a phone jack on the side that
accepts a 2.5 mm plug, in case you
want some privacy. You no doubt will
have to change the 3. 5 mm plug that
is on your headphones, look for
Mouser part number 17PP053-EX.
30 January 2009
Regarding the "Battery Charger"
inquiry by Michael Craft in the October 08
N&V issue, page 24...my take on the
question is this: The charger is used to
charge and to maintain a fully charged
state of a string of lead-acid cells while
simultaneously supporting an external load.
The reader describes a float voltage of 36
volts. The "float" or recharge voltage under
which a lead acid cell will experience long
life is 2.25 volts; this suggests a battery
composed of 16 cells, having a float
voltage of 36.0 volts, a fully charged open
circuit voltage of 35.2 volts, and an end-of-discharge voltage of 32.0 volts.
The load characteristic of a lead acid
battery — that is, the impedance presented
to the charger — is non-linear with voltage.
As the battery charges (from a discharged
state), the voltage impressed across the
battery terminals rises so that the charging
current — if delivered from a constant
voltage source — decreases. As the float
voltage point is approached, the charging
current drops to a very small value, and it
is important not to force charging at this
point, else electrolysis will occur and water
will be lost from the electrolyte solution.
The voltage and current charging
characteristics look like (Figure A); the
charge current dropping off markedly as
end-of-charge conditions are approached.
Thus, a lead acid battery charger needs to
be a two step device, delivering current
(without regard to battery terminal voltage)
at the onset of charge, and a constant float
voltage at the end-of-charge point.
Transformer T2 operates from the
unregulated AC line voltage. The voltage
delivered to the rectifiers is the sum of the
T2 secondary voltage plus the two secondary winding voltages on T1. We can
surmise that T2 is designed such that
float voltage will be provided out of the
rectifiers with little or no contribution by
T1. On the other hand, at the start of
charge, the battery terminal voltage is significantly smaller than the voltage out of
the rectifiers, and without the moderating
influence of T1, this could result in forcing
excessive charge current into the battery.
T1 is effectively fed by a current
source — the series-connected inductor
L1 — and this limits the battery charging
current because T1 is a series-connected
member of the charging circuit.
T1 also provides a degree of line
regulation. L1 and C1 form a 60 Hz
resonant tank circuit and for constant
load, the output voltage from T1 remains
relatively constant. If the AC line voltage
decreases below design center, the output
voltage from T2 will decrease proportionally. The battery current, however, will
remain nearly constant during charge.
Once the charging event has completed
and float conditions are obtained, T1
will compensate for variations in T2's secondary voltage caused by corresponding
variations in AC line voltage.
Inductor L2 makes the battery appear
as a current fed load. L2 also acts to
remove the 120 Hz ripples from the
battery bus, which suggests that this
charger might be part of a telephone
system in which it's important to isolate
a hum voltage component from the 36
■ FIGURE A
— Peter A. Goodwin
Thanks for the feedback; I forwarded
your comments to Michael Craft.