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Singin’ Solar!

Therese Peffer

©1994 Therese Peffer Over a year ago, my old boom

box died. It had provided radio

and taped tunes for me faithfully since high school graduation — eight years of service! When I searched for a replacement, I looked for one with a 12 Volt DC jack, like my old one. Why? Well, I wanted to use it in my 12 Volt solar electric system. And I discovered from talking to a few people (and looking inside my old box) that the ac power was just converted to DC anyway inside the stereo. Seems like a waste to convert DC to ac back to DC again especially when I’ve got DC solar power to begin with.

What a search! I did not find a boom box with a 12 VDC jack. But portable stereos take batteries, usually D cells, which provide DC electricity. So all I needed was a boom box that takes eight D cells. Why eight? Because each cell provides 1.5 Volt DC. When lined up head to toe in series, this adds up to (8 cells x 1.5 V) 12 Volts!

I finally settled on a portable stereo with AM/FM radio, dual cassette decks, and a compact disc (CD) player — a Sony CDF-40 rated at 21 watts. And I got cold feet. Yes, I got a little nervous about sawing holes in my brand new, $200 boom box. So I ran it in the office and made sure everything functioned properly for several months while I got involved with other projects (and the warranty expired). One snowy day recently, I decided to take the plunge.

Drilling Away

First, I unscrewed the five screws that held the the box together from the back. I ignored the warnings that said I could be shocked if I did this. Last year when I replaced the bands in my old boom box (hoping to bring it back to life), I asked about the warning. I was told that the only danger would be the capacitors, which store electric charge. I eased the two parts of the

104 Home Power #40 • April / May 1994

battery compartment


12 V jack –+

ac input

PV in


battery in

– +

10 Watt photovoltaic module

cigarette lighter for 12 Volt battery

My boom box can now play music using power from ac, a 12 Volt DC battery, or a 10 Watt PV module.

box apart (and was wary of small cylindrical knobs) The unit had not been used for several days, so I wasn’t too worried about shock, as capacitors lose their charge over time.

The boom box does not weigh much for its size, and I found out why. Much of the box contained empty space! I have a cigarette lighter plug with a set of interchangeable coaxial plugs — the common cylindrical ones with the inner pin. I originally thought that I could just add a coaxial jack to the box and plug it in. My boom box’s circuitry is protected from too much current by an internal two Amp fuse in the cigarette lighter plug. But I had forgotten about voltage.

I know the boom box can take 12 Volts DC or less — the eight D cells will drop down to 8 Volts when they are depleted. But what about voltages higher than 12 Volts? My 12 Volt lead acid battery can reach over 14 Volts when charged by my 60 Watt PV module. What if I used my boom box on a 12 Volt alkaline battery that can reach over 16 Volts under charge? Voltage higher than the circuitry is designed for can damage components. I certainly didn’t want to ruin my new boom box.

I had an electronics wiz, Richard, help me. Richard suggested measuring the DC voltage fed to the circuitry by the ac supply. So when I had the box apart, we traced the ac power supply as it was transformed, rectified, and filtered to DC power to feed the stereo. I carefully plugged it in, turned on the various functions, and we measured 13 to 16 Volts. This voltage range allows me to just plug right into my 12 Volt battery. If the voltage had been less than 16 V, I would have had to use a voltage regulator to protect the circuitry (see sidebar next page).

2 Amp fuse

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