Although newer types of solar cell have come to the fore recently. amorphous cells are bound to be with us for some time to come, mainly because of their low cost. Most solar cells based on amorphous technology have a relatively high internal resistance. which results in large differences between the loaded and ‘open-circuit’ output voltages. Where a rechargeable battery is used as an energy storage device. a voltage regulator circuit arranges charging of the the battery when the cell output voltage is high. and forms a minimum load on the battery when the cell output voltage is low. For relatively small solar power systems, the parallel (or ‘shunt’) regulator is a viable alternative. Apart from a single (Schottky) diode. nothing is inserted between the solar cell and the battery. Since the supply voltage is furnished by the solar panel, the _regulator works always. even if the battery is fully discharged or not connected. This ensures the best possible protection against overvoltage of all circuits powered by the solar cell (or array of cells) The heart of the shunt regulator presented here is formed by a Type TL431LP preci-sion voltage regulator from National Semiconduc-tor. When the solar cell out-put voltage rises above the level set with preset Pi, a current starts to flow through R3-R2-D1. When this current has risen to about 5 mA. transistor T1 starts to con-duct. The transistor used here. a BD642, may be replaced by almost any other, simi-lar. power darlington, for example, the TIP147. The collector of the BD642 is con-veniently connected to ground. which means that the device can be bolted directly to a heat sink. since the regulator is capable of shunting luite high currents, it has separate sense nputs, ‘A’ and ‘B’. to monitor the battery ‘oltage. The solar cell is connected to ter-ninals’++’ and ‘- -‘. Resistor R4 limits the .urrent through D3 in the event of the ligh-power series diode. D2, breaking own. Not shown in the circuit diagram.
but required in the interest of safety, are fused in series with the battery and the load(s). Also, do not forget to connect a surge arrester in parallel with the solar cell. The Schottky diode used here, an SB530 from Conrad, is capable of passing up to 5 A. For panel output currents up to 3 A. it may be replaced by the more familiar 1N5401 (which, unfortunately, has a slightly higher forward drop voltage). Talking of ratings: the heat sink on to which the power transistor is bolted must have a thermal resistance of 1.5 K W-1 or smaller. in which case power dissipation levels up to 40 W can be handled without problems. For very high power applications. connect a number of power darling-tons in parallel, and interconnect their emitters via 0.2242 current distribution resistors. Obviously, to cope with the increased power dissipation. the size of the heat sink must be increased accordingly. The component values shown result in a battery voltage adjustment _range of 13.4-17.6 V. This is on the high side for most (lead-acid and gel-type) batteries. which dp not fare very well at.