QUESTIONS & ANSWERS
when I tried to use it for a passive
circuit, I got an error message: No IC
in the circuit. I did a Google search
and found 5spice which looks to be
easy to use but costs $199. There is a
free demo but it is not useful. I also
found Opus spice which is free but I
have not evaluated it or figured out
how to install it.
LTspice has made some
modifications that minimize “time
step too small” errors in switching
circuits so it is good for those types
of circuits. In any simulator, the
results are only as good as the
model, but the more complex the
model, the more likely that the
program will fail. Always use the
simplest model that will give the
results you need and keep in mind
that if you use an IC in an atypical
way, it may not work because of
assumptions that the model designer
made.
Purchased heat sources are mega
bucks and I’d like to do this cheaper.
Would you please help me?
— Mark Hoffman
be much less than that if it is well
insulated. I will use the step down
circuit of the datasheet and calculate
the parameters using the Design
Formulas from the datasheet.
ASince the 78S40 has been doing the job, we may as well continue with that. I don’t know how much
power is required but with 12 volts
and 9. 5 ohms, it can’t be much more
than 10 watts. I expect the power to
Vo=10V (that’s the most we can get with
12V supply)
Vd = 1.5V (diode drop at 1 amp)
Vi=12V (battery)
Vsat=1.3V (transistor drop at 1 amp)
Ton/Toff = 12 (from datasheet formula)
Set Fmin to 20Khz (so you can’t hear it)
BLACK BODY
HEAT SOURCE
QI need to calibrate some infrared thermometers. I hacked an old black body heat source, modifying the
well to accommodate the IR
thermometers and an NIST traceable
RTD thermometer. My issue is that
the source only produces a single
(100 deg C) temperature. My goal is
to be able to construct a stable
multiple point heat source (e.g., 50,
100, 150 deg C) or a variable source
of the same range.
Here are some particulars about
the old black body heat source,
some of which I want to reuse: The
heat is provided by a coil of resistive
wire which measures 9. 5 ohms at 75
deg F, which is wound around a
copper bobbin with a well painted
flat black. It is powered by a 12V 7
AH SLA battery (nice because it
makes it quite portable) and it uses a
temperature sensor which measures
~800 ohms at 75 deg F, and uses a
uA78S40PC IC to do the control.
The only components that I want
to reuse are the well with the heating
coil and the 12V power source.
October 2010 27