>>>READER-TO-READER QUESTIONS AND ANSWERS
overload a low capacity circuit breaker
when charging away from home.
Ideally, it would be better to
charge each battery separately so
they are properly balanced, but using
13 chargers seems a bit too bulky.
The best way to accomplish this
is through stringent monitoring of
voltage, temperature, and current.
Battery performance is largely a function of temperature and temperature
gradient (temperature difference)
across the battery itself, since it causes
internal voltage differences which, in
turn, causes an increase in self-discharge. Another big issue is a bad
cell from manufacturing (metal chips
or other defects have been in the
news lately together with "thermal"
events). Therefore, monitoring the
string battery by battery is essential,
and equalization of temperature is
equally as important.
The voltages of each battery need
to be measured. One way is to use a
voltage controlled current source and
convert battery voltage to current (a
few hundred microamps), then use a
dropping resistor against ground to
convert back to a voltage again and
measure it with an A/D converter.
Simply using a PNP 300 V TO- 92
transistor and two resistors per battery
can do this. Measuring a fixed voltage
with the same arrangement can compensate the temperature dependence
One or two smaller converters for
10-20% (for the maximum expected
difference per battery) can then
equalize the charges from battery to
battery. This allows using the small
converters during the charge and
discharge cycle, which could be a
benefit, because in my experience
there is always a weak cell somewhere
going into reversal. This voltage
reversal could be prevented as far as
possible and battery life maximized.
The measurement will tell you
whether you are dealing with a
systemic problem or one that moves
[#1082 - January 2008]
Does anyone know of a circuit that can produce pulses
at a constant (but adjustable) duty cycle (around 3% or
less) from 20 Hz to 20 kHz? I have seen many designs
that can vary duty cycle or frequency, but not
both without one affecting the other. This is for a strobe
#1 I'd suggest an eight pin Microchip PIC. Depending
on the resolution needed, an external crystal of up to 20
MHz could be used. However, using the internal clock can
get you up to 8 MHz, depending on the part. This should
do fine. Then I'd hook a variable resistor voltage divider to
one of the analog inputs for the duty cycle adjust. And, use
a similar schema of one or two dials to set the frequency.
I know I have seen these boards available in the ads of
this magazine. I think CCS has (or had one); ccsinfo.com.
#2 The basic principle of this circuit (figure 3) is to generate a triangular waveform ( 4-8 volts) with well-defined
and stable upper and lower switching points. Then, a comparator uses this waveform to switch on and off at the
upper part of the wave. This allows the independent adjustment of frequency and duty cycle.
U1 and U2 form a VCO (voltage controlled oscillator)
with triangle and square wave output. U4 provides a 1/2
supply voltage for the VCO, and U3 is the comparator. A
small amount of positive feedback is added through R9 and
pulls the duty cycle adjust voltage slightly up or down to
provide a small amount of hysteresis for stable switching.
The LT1213 is a high speed, precision, and single supply
op-amp from Linear Technology. The circuit was modeled
on their Switchercad III (free), the VCO is a National Semi
application, as well as the comparator (for LM324, which
will only work for lower frequencies).
Another approach is to use a microcontroller with two
internal timers (excellent, if crystal oscillator is used) or add
another comparator to the 555 timing circuit with lower
switching points, but this circuit will not be linearly
Walter J Heissenberger
#3 A circuit with independent duty cycle and frequency
control is easily created in logic hardware or coded into a
microcontroller (µC). The µC is a preferred solution with
much fewer hardware parts, but both operate on the same
Assuming the duty cycle resolution is in one percent
increments, an oscillator is used with one hundred times
the desired output frequency. This can be made to adjust
over the desired frequency range, and only needs to
generate trigger pulses to a binary counter longer than
one hundred counts (i.e., a seven stage binary counter is
2^ 7 = 128).
Two digital word comparators are fed from the counter. One detects the value ‘ 100’ and resets the counter
while the other detects the desired duty cycle number
from 1 to 99. The pulse output is taken from a latch that is
reset by the first comparator (at 100 counts when the
counter resets) and set by the second comparator at the
count equal to the duty cycle value, thus making it a