Nuts & Volts

USB
+5V
Data
GND
1N5819
+
USB Power Boost
ripple effect over again. Well, the only
way to turn off an SCR is to cut the
power, which is what the relay does.
The Q9 output of the 4017 is normally
low, which engages the relay via the
2N3906 transistor. When Q9 goes high,
the relay opens and turns off the SCRs.
The 4017 is clocked with a pair of
NAND gates — in an astable configuration
— that turns on the lamps a
half second apart. To adjust the speed
of the timing sequence, replace the
100K resistor with a 250K pot. Since I
had an unused gate, I used it to drive
a TIP110 Darlington transistor that
flashes the front turn light. It’s optional
and not critical to the circuit. That’s
it. As simple as the original Ford
version, but without the cams.
USB POWER BOOST
■ FIGURE 9
1F
5V
To Drive
QI have several 2.5” hard drive
enclosures that claim to be
USB powered. But, as you
know, some machines just
don’t have sufficient current capabilities
on their USB ports to power these devices.
What I have discovered is that once
the drive is up to speed, there is usually
sufficient current to keep it running. I
would like to build an in-line power supply
using AAA or AA batteries to provide
the boost necessary to get the drive spinning.
The power supply should be invisible
to the host PC, while simultaneously
being paralleled with the USB power,
with maybe a (automatic) switch to
remove the “power box” from the picture
after the drive has spun up. Can you help?
— David Draper
Bakersfield, CA
AYou have need for concern.
Although the USB port is
rated for five volts at 500 mA,
many notebook USB ports
can only deliver 100 mA. While a
battery boost would help, I suggest
using a supercap instead. Because
the supercap has a lower internal
resistance than an AA cell does, it can
provide a bigger jolt of current.
In fact, the motor and capacitor
curves match up perfectly. The motor demands
a larger amount of current to start
from a stall than it takes to operate at
full speed, whereas the capacitor has the
most current to give at the beginning of
its discharge curve and loses energy as
the cap discharges. By balancing the
charge in the capacitor with the needs of
the motor, the voltage will remain high
enough for long enough for the motor to
wind up and let the USB port take over.
A one Farad supercap can store
enough energy to power most disk drives
for three seconds with little sag in
the voltage (less than a volt) as the cap
discharges — plenty of time for most
drives to be in high gear — at which
point the cap recharges and floats. The
design requires nothing more than a
supercap and Schottky diode, as shown
in Figure 9. The diode prevents the supercap
from manhandling the USB port.
OUTPUT TRANSFORMER
RESISTANCE
QI’m in the process of restoring
a pair of 60-watt theater
amps (Simplex AM-1026)
and I have a concern about
the output transformers. On one amp,
the DC winding resistance of the primary
is 75 ohms from B+ to one plate,
and 75 ohms from B+ to the other
plate: 150 ohms plate-to-plate. On the
other amp, the corresponding readings
are 75 ohms and 140 ohms, respectively,
with 2, 215 ohms plate-to-plate!
My thinking is that if high voltage
shorted some windings, shouldn’t the
resistance decrease? Otherwise, the 140
MAILBAG
Dear TJ,
Regarding paging feedback to Dan Elliot in the
January ‘06 column ... the device you suggested is
good, and filtering bad frequencies is an answer if
they are consistent. In our building, we’ve installed a
delay to totally break the feedback loop with 100%
success, and I would suggest Mr. Elliot use the delay
element on the Shark unit before worrying about the
notch filters.
— Greg
Dear TJ,
In the October ‘05 column, under “The Hounds
of Baskervilles” (concerning the defecating dogs),
to keep the next-door neighbors happy, I would
recommend an environmentally friendly approach
over the loud horn. How about an oscillating lawn
sprinkler or two turned on by the existing sensors? I
don’t think the dogs would be comfortable answering
the call of nature with water streams pulsating
around them. A junked washing machine would be a
good source of electric water valves.
— Bud Fuechtmann
Dear TJ,
One thing you neglected to mention in the January
‘06 “Tube Amp Power Regulation” answer is to put
a filter choke and bypass caps on the input to the
filament regulator. In any switcher, noise will couple
back to the power line through the transformer
interwinding capacitance. It’s easy enough to do — just
cannibalize the parts from a busted ATX power supply.
The first time I built a flyback switcher I knocked out
every AM radio within a few hundred feet!
— Jack Walton
Short Hills, NJ
28 April 2006