Q&A
10 ohms
10 watts
RadioShack
270-054
Ground Loop
Isolator
and some attitude.
560 I also put a 10 Ω, 10
watt resistor (RS 271-132)
R. OUT in series with each speaker
(Figure 7). This has the
560 effect of lowering the bass
L. OUT cut-off frequency from
90 Hz to 40 Hz. While the
four inch speakers in the
TV are useless at 40 Hz,
the lower frequencies are
now available at the rear
jacks, so my massive
home stereo can make use of them. I
was wary about the (unpublished)
impedance of the ground loop isolator
— lest it load down the lower frequencies
— but it tested out well on my scope
and eats up about 1.5 dB at 40 Hz
after the 560 Ω protection resistors
that are in series with the Sanyo
audio-out jacks.
In short, the frequency response
is flat within 1.5 dB from 40 Hz to 50
kHz. When the 560 Ω resistor was
not included, the frequency response
of the ground loop isolator was dead
flat from 20 Hz to 20 kHz and well
beyond.
the people asking for help in “Q & A”
are novices and they should be guided
with solutions that reflect proper
design practices.
10 ohms
10 watts
Daniel Bernes
via Internet
Sanyo TV Sound
Modification
Figure 7
the Microsemi (www.microsemi.
com/datasheets/SA7-29.PDF)
datasheet.
Reader’s Circuit
Response: That circuit was
designed for a specific application,
where space was at a premium
and the motor current requirements
were well within the specs of the
reed relay — even without the EMF
damper diode. You’re right, though,
larger motors require larger relays
and EMF protection. Here is the
circuit to use for those applications
(Figure 8).
— TJ
I bought a 31” Sanyo TV for
$300.00 only to discover that the
audio output jacks did not respond
to the volume control. I phoned
Sanyo and found that they had a
“hum” problem and had decided to
use a fixed amplitude output instead
of using a couple of transformers to
send the speaker drive to the external
jacks. In this way, they offered to
provide me with the version with
volume-controlled rear jacks for only
$300.00 more.
I took the TV apart, removed the
coupling caps from the fixed
amplitude drive, jumpered the speaker
drive to the output jacks, and
promptly produced the hum problem.
Then I added a RadioShack 270-054
ground loop isolator and have the
completed $600.00 version —
working perfectly — for the initial
$300.00, plus an additional $16.00,
C. L. Larson
via Internet
MAILBAG
Dear TJ,
I enjoy your “Q & A” column
very much, but there is a problem
with Figure 4 (“SSR Meets MCU”) in
the March 2004 issue. The 2N2222A
won’t work reliably with loads
approaching 800 mA. I looked up
the specs for this transistor and
it’s rated at 500 mA, with 200 mA
preferred. For larger load currents,
substitute an NPN Darlington transistor
in a power package — such as a
TIP-140 — and attach a small
heatsink to the transistor tab for
currents greater than 400 mA.
Ernie Worley
via Internet NV
F
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NUTS & VOLTS
E
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5 - 48VDC
Figure 8
10k
Level
Sensors
2N4410
Motor
100k
Bilge/Sump Pump Sensor
Dear TJ,
I looked at your solution for “Sink
the Bismarck” (February 2004) and,
quite frankly, the solution is worse
than the problem. The contacts of a
compact reed relay like RadioShack’s
P/N 275-232 are not designed to
carry heavy currents — such as the
current demands of a DC motor.
The reason for this is that the
contacts will “arc weld” themselves
together after a few operations,
especially with the sparks generated
because of the lack of a diode in
parallel with the motor to limit the
back EMF generated when the relay
turns off and, without a diode in
parallel with the relay coil, the back
EMF generated when the transistor
turns off will destroy the poor
MPSA14.
Please remember that most of
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96
JULY 2004