■ FIGURE 4
full feature simulator that I am aware of
is LT Spice/SwitcherCad III from Linear
Technology. The downside is that the
library has only Linear parts in it, but you
can import other models or make your
own. Go to www.linear.com and download the program for free. If you have
trouble using LT Spice, the folks at the
Nuts & Volts online forum will help you
out. Spice originated at UC Berkley and
at one time was free. Probably still is.
number 682-0063; cost is $7.89.
QI have been looking everywhere for electronic simulation software for the PC, but
most of the software out there
is for educational institutions, and it costs
an arm and a leg for the software and
licensing fees. I want design software
which allows a simulation to view the
result of the circuit, i.e., an LED turning
on or off. It must have testing tools such
as an oscilloscope or multimeter. Could
you recommend a software similar to
SPICE but within a reasonable price
range for an electronics student?
QWhat is the best way to
figure the correct size of
electrolytic capacitors to use
on a DC power supply? I
have seen commercial power supplies
with both single and multiple sets of
capacitors and have always wondered
why there is a difference and what are
the benefits of both designs?
When multiple capacitors are
used, should they be of different
sizes? If this topic has already be
addressed in a previous issue, please
let me know where to look.
— Bill Blackburn
AA Google search for “free
schematic capture spice” will
turn up many possibilities, but
most of them are limited in circuit size or output in order to entice you
to buy the full-up version. The only free,
■ FIGURE 5
AI am sure the subject has
been covered before, but it
does not hurt to repeat it.
Filter caps perform two
important functions: smoothing the
input pulses from the rectifiers, and
absorbing pulses from the load.
Aluminum electrolytic capacitors are
good at 120 Hz, but not so good at 100
MHz. Ceramic or film capacitors work
best at high frequency, so two types
should be used in the power supply.
Short leads are important at high frequency; the film or ceramic caps should
be right at the power supply output
terminals. The size of the smoothing
cap depends on the output current. A
rule of thumb is 80,000 µF per amp for
100 mV ripple. Or, use this equation:
C = I*d T/dE
where C is the capacitance in farads, I is
the current in amps, dT is the time
between pulses in seconds, and dE is
the peak ripple voltage in volts. This
equation will also tell you how large a
bypass cap is needed for high frequency.