Q&A
need four 10F supercaps in series —
with balancing resistors. This will cost
you about $20.00, minimum.
A better solution for this application is a capacitance multiplier. The key
component (Figure 4) is C2, whose
capacitance is directly proportional to
the DC gain (hFE) of the transistor. The
TIP120 typically has a gain of 2,500
at four amps. Let’s say you want a
maximum ripple of 2 mV. Plugging the
numbers into the 8,300 uF thumb rule,
we need 14.5F. Divide that by the 2,500
gain of the TIP120, and C2 equals a
small 5,800 uF. I’d use a 6,300 uF, 10V
electrolytic. The input and output caps
are used to compensate for tempera-ture-related swings in the hFE.
the supply at up
to 12 amps.
— Emilio
Tancredi
Dumont, NJ
Capacitance Multiplier
Bridge
TIP120 + 6.3VDC@ 3.5A
AC In
6.3VAC
~+
~_
1k
+
+
1000uF
Better Than a
Banana Split
Q. I download a lot of executable
files (EXE) from an Internet cafe.
My problem is sometimes it doesn’t fit
on a 3. 5” floppy diskette. Is there a
software program that can chop large
files into 1.4 MB segments that will fit
on a floppy and be reconstructed to
its original single file when transferred
to a hard disk?
A. Oh, but you
do want pulse
power. Pulse modulation eliminates
heat that would
otherwise be generated by a linear voltage controller. That’s because the
switching transistor is either fully on or
off. The linear controller, on the other
hand, has the transistor acting like a
resistor. If the linear transistor is passing 12 amps with a voltage drop of 14
volts (half power to the tracks), it must
dissipate 168 watts of heat — more
than a 150W floodlamp. A pulse-modu-lated controller would generate about a
watt at the same 50% speed setting.
Moreover, you’ll get smoother throttle
response.
10,000uF
+
C2
6300uF
Figure 4
The circuit in Figure 5 is a down-and-dirty design that works best on
larger motors — like the ones I’m sure
your locomotives sport. The frequency varies in step with the duty cycle,
and there’s a bit of a “chirp” as
the dial approaches top speed (90%)
— which can be minimized by fine-tuning the .0001 capacitor. Critical to
the design is the smart IPS031
“Smart” FET. This device is optimized
for peak performance and reliability
in harsh auto environments, and is
enhanced to handle inductive loads —
— Unsigned
A. Try GSplit from G.D.G. Software
( www.gdgsoft.com/gsplit).
I’ve never used it, but I’ve heard good
things about it.
Reader feedback: GSplit is
great! Thanks a lot.
Thomas the Tank
Engine
Q. I have a 28-volt, 15-amp switching power supply that I would like
to use for my 36-inch Garden
Railroad. To do this, I need to vary the
voltage to the tracks. Can you show
me a schematic that would use, say,
some 2N3055 transistors and a pot?
The locomotive doesn’t need any
kind of pulse power. I just want to control the power to the tracks to control
the speed of the trains. The voltage
would vary from zero to the max of
JULY 2005
19
