June 2017 7
A quick tour shows that Q1 is the temperature sensor.
This should be physically attached to the device that you
want to be cooled. R2 controls how much gain there is
for the thermal sensing. Q3 and Q4 form a threshold
comparator, and R3 sets the comparison voltage. D1 is the
In this way, you can set the point at which the circuit
starts to react. Q3 is also a common-emitter amplifier for
the thermal signal, and that controls the current source
made by Q2: a Darlington pair in a single package. The
collector of Q2 goes to the fan, controlling its speed.
NiCd to NiMH
QNow that Ni-Cad batteries are becoming an endangered species, is there any way to convert their chargers to work on the newer and ecologically sounder Ni-Metal Hydride
batteries? I read that the Ni-Metal batteries have very
different charging requirements rather than the constant
current chargers for Ni-Cads.
I have several bench chargers, not to mention a
multitude of wall warts that I would like to modify.
La Quinta, CA
ANickel-cadmium or NiCd batteries are certainly becoming more scarce. It’s unfortunate in some sense because they have some significant advantages over nickel-metal hydride (or
NiMH) batteries. While NiCd batteries exhibit lower
energy density, they generally have lower self-discharge
rates. There are some NiMH batteries that are specifically
designed for low self-discharge, however.
NiCd batteries are also very robust. They can handle
high discharge currents for longer, and can generally take a
lot of abuse. I have a NiCd battery pack on a Handi-Talkie
that is probably about 25 years old and still going strong.
I use that Handi-Talkie on a weekly basis. I always worry
about how warm the battery pack gets when I charge it,
is the toxicity
of the cadmium
making up the
Cadmium is a
metal that is
toxic to most
It is also more
expensive than alternatives. Because of its toxicity, the EU
has completely banned them for consumer use, though
they still find applications in commercial and medical gear
because of their desirable characteristics.
Unfortunately, the charging requirements for NiCd
batteries are different than NiMH. For a NiCd, as you
mention, a constant current is generally adequate, as long
as the current is kept relatively low. Charging at a rate like
1C (or the capacity of the battery in one hour) can cause
overheating, but lower rates are fine. Regardless, this is
different from NiMH charging methods.
NiMH batteries generally are charged with two
different methods: delta-V and delta-T. For delta-V
charging, the voltage is monitored while a constant current
is applied. When the voltage drops slightly, this is an
indication that the battery is fully charged (see Figure 2).
Note that NiCd batteries can be charged this way as well,
but it really depends on the charger, and the voltage drop
characteristic is slightly different. It’s not uncommon for a
NiCd charger to just continue to apply current even after
reaching full charge. NiCd batteries will not be damaged by
the heat generated from this overcharging. NiMH batteries,
however, will eventually be damaged.
For delta-T charging, the battery temperature is
monitored. As a NiMH cell is charged, energy is stored
in the chemical state within the battery. As full charge is
reached, the excess energy not going into the chemistry is
released as heat instead. By monitoring the temperature,
the full charge state can be detected and charging
terminated. Many chargers for NiMH batteries use both
methods to avoid overcharging. As above, overcharging is
not good for that chemistry.
The wind up is that NiCd chargers generally do not
have the necessary sensing to charge NiMH batteries
properly. There are many chargers for sale that will charge
both types (Maha Powerex makes several), but I think it
would be difficult to repurpose NiCd chargers for NiMH.
In general, it’s a good idea to use chargers appropriate for
the battery chemistry involved. As I’ve mentioned before in
this column, improper battery charging can cause heating,
outgassing of flammable or explosive gasses, and even
fires. Please be careful.
Clean Automotive Power
QMany hobbyists (like myself) are playing with circuits used in automobile applications. The voltage in a 12V automotive environment is particularly dirty. I would like to see an article
on how dirty the automotive electrical voltage supply is
and why, and I’d also like to see ways of cleaning and
stabilizing the 12V power in an automobile for circuit
QUESTIONS and ANSWERS
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n FIGURE 2. NiMH charge curve
with dip. (Credit: Wikipedia CC 3.0