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-JULY NEWS LETTERWhat is it? The top Tuning section for the ISOTRON 160C. This article will also appear on our web site. WHAT CAN YOU DO WITH THOSE WALL WARTS? What is a Wall Wart? You know, it is the small power supplies that plug directly into an AC outlet. It charges our tablets, phones, shavers, tooth brush and more... On every Wall Wart its specifications are labeled on it. It states if it is an AC or DC supply. It will give you the voltage and the maximum current it can handle. It will give you the polarity of the connector if it is a DC supply. It may tell you if it is regulated. The devices come in a variety of voltages. From 1.5 volts to as high as 48 volts. Also, anywhere from 50 ma to several amps. The Warts can be used for a trickle charger as described below. To substitute for a battery, such as a 9 volt. It can run a cooling fan or a power supply for your proto boards. First determine the voltage you will need and the maximum current it will draw. Then select a Wall Wart that has a higher voltage and current rating. Select a regulator that has a lower voltage rating than what you need, or the exact one. Do not go higher than what is needed. The regulators can be increased in voltage, but not lowered. If you are substituting it for a 9V battery, a 7809 will work without the diode to ground (below). A 7812 will work for a 12V device, like a fan, the same way. If you need a higher voltage, then use the technique with a diode to raise the voltage. You can use more than one diode in series to increase voltage as needed. Most of the regulators will handle from 1 to 3 amps. However, you can mount them to a heat sink if you are near the limit of the device. CHANGING VOLTAGE ON A REGULATED SUPPLY The last two articles were on a fixed voltage regulator and a fixed current regulator. However, there are times when a different voltage is needed. There are a couple of ways this can be done. This article will discuss one of the two ways. For example, you want to use the regulator as a trickle charger for a 12 volt battery. Lawn tractors, generators and even cars and trucks that sit for a long time should have a charge on the batteries to keep it working year around. Select a LM7812. You can use a wall wart that can handle at least .5 amps. If the wall wart claims 12 volts, it will probably be around 18 volts measured. Connect it to the LM7812 as in the schematic. Your output with lead #2 grounded will give you a slightly lower reading than 12 volt. This is not enough for trickle charging a 12 volt battery. Therefore, how can you raise it to 13.1 volts that is needed? Simple, release lead #2 from ground. Then add a diode with the cathode to ground and anode to #2 lead. The forward direction of the diode will exhibit a resistance. This will increase the output voltage at lead #3. It will still be well regulated. A wall wart power supply can be picked for just a few dollars surplus. The regulator can be bought inexspenively. 2 filter capacitors will vary in cost, but will most likely be a small cost. The circuit can be built on a small board. You can use a fancy project box to house it, or a large shrink tube. You can build the whole charger in less than an hour. It will save the batteries that sit unused throughout the winter to be ready for use in the spring. 73, Ralph WD0EJA 02-18 BILAL COMPANY 137 MANCHESTER DR. FLORISSANT, CO. 80816 U.S.A PH/FX: 719/687-0650 |
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