How To Run A 4.5 Volt Radio Off A 12 Volt Battery (A Beginner's One Piece DC Power Supply) Joel Chinkes c.1992 Joel Chinkes DISCLAIMER: This is designed for the complete beginner, but not for the complete klutz. It is possible to hurt yourself and/or damage your property if you are not careful. If you try this stunt at home, be prepared to take responsibility for your own actions. Need to run a 4.5 Volt (or 9 Volt) radio off a 12 or 24 Volt battery? Ever wish for a circuit diagram with only one piece in it? Try this! It's as uncomplicated as one bean salad: "Open can; eat." I now run my 4.5 Volt shortwave radio directly from my 12 Volt photovoltaic battery system. Maybe you have a similar problem (opportunity?) with an appliance that runs on batteries or DC wall-cube adapter. This method will not work if your appliance uses low voltage ac. If your wall cube adapter says 120 volts ac input, OK, but if it says any number of ac volts output, this solution is not for you. You can also use a voltmeter to see what is coming out of it. A wall cube is a plastic box with a small transformer in it, meant to be plugged into an ac outlet. Its DC output is usually delivered through a wire with a barrel connector on the end. The barrel slips into a little hole on the side of the appliance (see Figure 1). The barrel connector makes contact with both the inside (pin) and the outside (wall) of the hole, for plus and minus. INSERT FIGURE 1 My radio's cube eats normal 120 volts ac house current, and creates 4.5 Volts DC. On solar, it seems a little silly to run an inverter to boost 12 Volts DC to 120 volts ac, and then transform back to 4.5 Volts DC. The conversion from DC to ac to DC wastes energy. The second-hand DC has ac ripples and noise in it. Also my inverter has a radio frequency howl when running that interferes with shortwave reception. Next to the power supply hole is a symbol molded into the plastic. Your symbol probably looks like Figure 1, except that plus and minus might be reversed - so watch out! There is no industry standard on polarity. My radio also runs on 3 "D" cells, nominally rated at 1.5 Volts each (3 times 1.5 Volts = 4.5 Volts). Battery powered circuitry cannot be picky about input voltage. New batteries start out peppy, but their voltage gradually fades. The radio must be designed to run on a wide range of voltages, to accommodate the waning batteries. A new battery might measure 1.65 Volts. So you don't have to be fussy when building your homebrew DC to DC adapter. Home-Brew My own high-current adapter uses an off-the-shelf voltage regulator. A voltage regulator is installed at the tail end of a power supply. The power supply industry routinely takes 120 volt ac grid power and creates low- voltage DC output to run computers, TV's, anything with microchips. They guarantee that power supply circuits produce the rated voltage regardless of (moderate) variations in input voltage or output current load. Voltage regulators always need a higher voltage input than they show at the output. They cannot be used to "step up" a DC voltage. To pick the right voltage regulator for the job, I went up to an electronics parts store. The professional electronics store has a comprehensive catalog of parts called the "ECG¨ Semiconductors Master Replacement Guide". It turns out that semiconductors are cheap as sand, which is what they are made from in the first place. Regulators come in two flavors, fixed and adjustable. With adjustable ones you have to build a circuit around them to adjust the voltage.I wanted to simulate a few "D" cells of approximate voltage, so I just picked the closest available fixed voltage regulator. This is called "engineering". Use What Works Look in the ECG¨ Catalog under Linear IC (Integrated Circuit) for the Voltage Regulator Selector Guide. You will find a variety of useful output voltages to match any need, and the input voltages they will run on. In my case I chose the ECG309K. Rated at 1.5 Amps, 20 Watts, it eats DC voltages between 7.5 VDC to 35 VDC, and puts out exactly 5 volts DC. It cost me $4.50, but I saw one in a mail-order catalog priced at $1.39 + post. It has two leads, input and output. Its metal case serves as negative ground. The LM309K is an equivalent part made by another company. If you want to run a 9 Volt appliance, you can use the LM7808 or the ECG964. Why pick a 5 Volt regulator, when my radio needed 4.5 volts? I guessed that my radio was designed to operate on anything from 3.5 to 5 volts DC with no problem, because it is also battery powered. It has worked well in the four years since I built the circuit . The ECG¨ Catalog assumes that you are building a power supply to convert ac to DC. Since your battery powered input is smooth DC to begin with, you can safely ignore all their unnecessary filtering advice. Sizing The Sink To see an electric circuit designed to run red-hot, take a peek inside your toaster. You do not want your voltage regulator to glow. The regulator will get warm, making it a "source" of heat. That heat needs a "sink", to keep the it from melting. Engineers use the word "sink" to mean "place where stuff disappears or gets used up". The opposite word is "source" which is where stuff comes from. You can describe most of the known universe in terms of sinks and sources. You must have a heat sink to get the full rated wattage out of a voltage regulator. If you use a regulator with twice or ten times the needed power rating, it will not break a sweat. Just install your regulator and the shiny metal box you bolt it onto will allow what little warmth there is to radiate safely away. If you cut it close on the amperage or wattage of your regulator, you might need more of a heat sink. How hot is hot? Test your heat sink by trying to hold your finger on the voltage regulator with your circuit operating. If it's painfully hot, either use a bigger heat sink or select a regulator with a higher amperage rating. Or install cooling fans. Final Assembly Bolt your regulator to your metal box with star washers, which dig into the metal box. Attach a solder lug (a washer-like thing you can solder to) to the bolt. Solder the ground wires from the 12 V and 5 V cords to the solder lug (see HP18, page 35 "How to Solder - the basics"). The metal box, the metal case of the regulator, and two of your four wires must all make good contact. Use rubber grommets to keep the input wires from chafing in the side holes. INSERT FIGURE 2 Twelve Volts, or whatever, goes in one pin of the regulator, and five Volts, or whatever, comes off the other pin. In the "positive TO-3 case style" for the ECG309K, the shell of the regulator is common ground, and the two pins of the regulator are +12 and +5 Volts. The catalog description of each regulator will tell you which pins to use. Be sure to look these up for the regulator you use. You will experience SPD (Sudden Parts Death) if you are sloppy. Use a large enough metal box, so the wires inside will be well separated. The larger your box, the cooler the regulator will run. You can use lamp cord for your input and output leads. Make sure to put a fuse in your positive input lead, and be careful not to mix up plus and minus when attaching your connector plugs. A mistake here will also cause SPD. Solder a barrel plug to the output end, watching out for polarity. When done, take a voltmeter and measure the voltage and polarity you are about to feed to your appliance. You could add an on-off switch and pilot light, on the input and output side. You could add panel voltmeters and ammeters to track results or tail fins too, but be sure to call them "heat sinks". Parts List: Voltage Regulator, sized to fit your application Voltage regulator mounting kit (optional if you are handy with a drill) Input power connector (probably male cigarette lighter plug) Output power connector (probably male barrel plug) Shiny metal box (also acts as heat sink) Rubber grommets to protect wire entry to box Wire, nuts, bolts, solder Access: Author: Joel Chinkes, c/o Home Power, POB 130, Hornbrook, CA 96044 The author has been living "off the grid" since Feb. 1987. He knows all the words to "Ramblin' Reck From Georgia Tech" and is a Life Member of Mensa. Maker: Phillips ECG Inc., 1025 Westminster Dr., POB 3277, Williamsport, PA 17701 Ą 800-233-8767