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QUESTIONS & ANSWERS
reduced in voltage, but the 60 Hz
AC is being passed through the
opto-isolator and is turning on-off the
2N2222A transistor. Removing the
C3 capacitor will actually worsen the
problem. PLC inputs are optically
coupled and have an average
response of 1-5 kHz; 60 Hz will
definitely turn on/off the input.
There are parameters that can be
adjusted to limit input frequency
response, but none that reject 60
Hz. I recommend he install a half
wave rectifier at the R3-C2 top
junction pin 1 side; (1N4004),
C2 = 47 µF. This should get a +only
DC input to the H11AA1. PLC
inputs are in the 1-10 mA range.
Rich Wright, Senior Design Eng.
Since I knew nothing about
PLCs, I appreciate your feedback. I
forwarded your email to Al Bochter.
Thanks for writing.
Re: NiMH Battery Charger,
February 2013, pages 24-26.
You say that the battery
temperature is used to indicate full
charge. I have some questions: What
causes the battery to heat up during
charging? And, why do you resume
the charging after the battery cools
down if it is already charged?
Thanks for the questions. The
battery temperature does not rise
during charging; it starts to rise when
it is fully charged. The reason is
that it starts to generate gas which
could cause it to explode, so the
manufacturer puts in a catalyst that
causes the gas to re-combine. That is
an exothermic process, so the battery
heats up. The charging resumes
when the voltage drops, and when it
cools down. The battery is charged
to over 100% when the charging
stops and starts charging when the
charge drops to 100% (due to
internal leakage), so it is always at
least fully charged.
Re: Bench Top Power Supply,
January 2013, pages 24 and 25.
Ed (no last name) writes to
say that R2 in Figure 2 could be
a 25 ohm five watt pot instead of
the 50 ohm 12. 5 watt pot that I
specified, at lower cost. Digi-Key part
CT21152-ND is $4.63. The same
model pot could be used for R4.
Thanks for the input, Ed.
May 2013 25