and 2,200 mAH. The 1,800 mAH cells
I have state right on the case of the
battery that they should be charged
with 450 mA for 5 hours or 180 mA for
14 hours — after which they have to be
disconnected from the charger.
However, these currents and times
assume the cell is fully discharged. If
it’s only half-used, then these times will
overcharge the battery. Once a battery
is fully charged, it produces gas —
creating a high internal pressure and a
sudden rise in temperature. At this
point, the battery will begin to vent and
release its electrolyte — which severely
shortens the life of the cell.
As a rule, a NiCd battery charger
can be used to charge a NiMH battery
— provided the charger has a two-step
charging routine. This class of chargers first applies a fast charge of C/4 to
4C (the total capacity of a battery is
defined as C), then switches off (or
applies a timed trickle charge).
There are two recommended
methods of detecting charge termination: using a temperature sensor in
the battery pack or using negative ∆V.
The temperature technique relies on
detecting the sudden rise in battery
temperature to shut off the charge.
The negative ∆V system relies on
the fact that the NiCd/NiMH battery
voltage peaks and drops about 20
mV per cell when fully charged.
Quality chargers use a mix of
both methods, called ∆V/∆T. Cheap
battery chargers can’t afford this
much circuitry and simply place the
battery on prolonged trickle charge
(typically C/10) in the hope that it
doesn’t do much damage.
Eventually, the thermostat detects
this and the fan starts. To correct the
problem, I built 555 timer to cycle the
fan. This helped a lot, but ran the fan
at the wrong time in the summer.
I then built a differential temperature device using two thermistors, an
op-amp, and a relay. Unfortunately,
my design was not sensitive enough
— the temperature spread was too
great. I kept changing feedback resistors, but was unable to get it right. I
would like to build a controller that I
can comfortably adjust to a 1 to 4°F
C. P. Furney, Jr.
A. Maybe your problem is that thermistors are non-linear. That is,
the resistance doesn’t stay in step
with the temperature. Let’s replace
the thermistors with the venerable
LM34 temperature sensor (shop
around for best price; it can vary
widely). The LM34 is a precision
Fahrenheit temperature sensor with a
guaranteed 10 mV/°F linear output
( 80 mV at 80°F, 100 mV at 100°F).
Now, if you place two LM34
sensors on the differential inputs of
an op-amp (any garden variety will
work), you automatically get a
voltage that reflects the difference in
temperature between the upstairs and
downstairs (Figure 4).
This voltage is first multiplied by
10 and then fed to a comparator with
an adjustable temperature differential
(SET) up to 10°F. This should give
you enough range between upstairs
and downstairs so that the rooms
don’t become uncomfortable.
The 1M resistor provides a slight
amount of hysteresis to prevent the
fan from hunting — constantly going
on and off. If you find the fan hunting,
lower the value of the 1M resistor. To
clarify: The SET determines the
temperature difference between the
basement and upper house. The 1M
(feedback) resistor determines the
temperature difference between fan
on and fan off — a dead band that
Grid Dip Revisited
Q. About the “Dip Oscillator Meter”
in the January 2005 issue ... I
pretty much have most of the parts,
except for the variable capacitor. Do
you know where I can find those variable capacitors?
5 - 40 Watts
8Bit Set $ 89
This Old House
Q. My house has an open stairwell
between the main floor and the
basement, where I have an office and
my electronics workshop. When the
weather is neither too hot nor too
cold, the circulating fan on the heat
pump stays off for long periods. The
cold air gravitates down and the hot
air goes up, creating an uncomfortable condition in both places.
glasses allow for extra $789
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