Array Direct Regulator Chris Greacen c.1992 Chris Greacen Want to run a radio or cassette player directly off your photovoltaic panel? You can get away with running some 12 Volt equipment "array direct" Ä that is with no battery. But if the equipment is electronic, plugging it directly into a twelve Volt panel can be risky business. The instant you hook your appliance to an illuminated PV panel it's at its open circuit voltage, as high as 21 Volts. This can fry electronics. The circuits below can attach directly to the frame of photovoltaic panel. At the flip of a switch, the panel's voltage is regulated to anywhere between 5 and 15 Volts. Low Power Simple Circuit If your panel puts out less than an 1.5 Amperes, you can use the circuit in Figure 1. The LM317 voltage regulator chip takes any incoming voltage up to 37 Volts, and puts out a steady adjustable voltage up to two volts less than the incoming voltage. The 317 does what it can to maintain a voltage drop of 1.25 Volts from Vout to Adj. You adjust the output voltage of the chip by adjusting the voltage of the adjust pin with the voltage divider formed by the 240 ê resistor and the 2Kê potentiometer. Figure 1: LM317 regulator good to 1.5 Amperes INSERT Just317PICT Power Steering Higher amperage versions of the LM317 are available: the LM350 (3A), the LM338 (5A), and the LM396 (10A). But another way to increase the amperage of this regulator is to use an "outboard transistor" (figure 2). In this circuit the LM317 is limited to only 100mA Ä a very easy job for it. If the load draws more than 100mA, the voltage drop across the 6ê resistor approaches 0.6 Volts, beginning to turn on the PNP transistor. Now when the 317 draws a little more current to supply the load a constant voltage, the PNP transistor turns more "on". If the load draws less, then the 317 decreases it's draw, which in turn, turns the transistor more "off". The transistor works like power steering, with the 317 in the driver's seat concerned with staying on the "constant voltage road". This method has some advantages of the LM350 approach. Parts are cheaper, and power transistors have a much higher allowable chip operating temperature than the voltage regulators, so heat sinks don't need to be as good. Figure 2: Array-direct regulator good to at least 10 Amperes INSERT ArrayDirectPICT A big capacitor on the input works like a battery, storing up power for short-duration surges the load might draw. If your load surges are especially big Äsay you've got a single sideband ham radio which draws little power in receive mode, but draws more than your panel can put out on transmit Ä then use even more capacitors. Building the Circuit: This circuit dissipates excess voltage as heat. For example, if the regulator is connected to a 16 Volt PV module and set to 10 volts at three amperes, the regulator dissipates 6 Volts. Six Volts times three Amperes is 18 Watts of power dissipated as heat. It's important to get rid of this heat. Heat sink the LM317 and MJ2955 on a piece of metal that can be cooled by air. Don't build the circuit inside the PV's junction box. If you put it together and find the chips too hot to touch, then you need a better heat sink. Access Author: Chris Greacen, c/o Home Power, POB 520, Ashland, OR 97520 ù 916-475-3179