Nuts & Volts - July 2004

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. ^OUTin series with each speaker (Figure 7). This has the

⁵⁶⁰effect 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 I was wary about the (unpublished) impedance of the ground loop isolator — lest it load down the lower frequencies — but it tested out well on my scope and eats up about 1.5 dB at 40 Hz after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 o r

E l e c t r o n i c s

NUTS & VOLTS E v e r y t h i n g

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.