Short Circuit 35 Ampere Regulator Chris Greacen c.1992 Chris Greacen Wind, hydro and photovoltaic panels can sometimes produce more electricity than your batteries can handle. Overcharging batteries causes water loss, decreases battery life, and produces potentially explosive hydrogen and oxygen. Without regulation, the system voltage can rise to levels which can fry 12 Volt electronics, including some inverters. Series and Shunt Regulators There are two kinds of regulators: series regulators and shunt regulators. Both regulators have a sensing circuit which regulates current flow into batteries when the voltage exceeds a threshold level. Series regulators work by switching off current from the charging source (panels, etc) when voltage climbs too high. With the charging source disconnected like this the battery voltage sinks to its standing voltage (lower than its voltage under charge). Series regulators are simple, but they have two disadvantages: first, not all power potentially produced by the power source passes through the system. If you monitor the total power put out by your solar panels with an ampere-hour meter, you will see a lower reading when the regulator is 'refusing power.' More critical, when the load is switched off, generators used in hydro or wind systems can spin too quickly and destroy themselves. This does not apply to solar cells - running them open circuit, or short circuit for that matter, does not hurt them. INSERT SERIES ART Shunt regulators work by diverting the current from the charging source into some other load. In effect they waste electricity, but wasting electricity is much better than overcharging batteries - or destroying electronics. Of course if you do have a way to put those electrons to a good use, by all means do it: run that washing machine or vacuum cleaner, make some toast in the toaster. INSERT SHUNT ART This short circuit regulator is a shunt regulator, but with no load! No bulky, heat producing power resistors are needed. The regulator makes use of the fact that when short circuited, PVs and permanent magnet generators (used in many wind and microhydro turbines) produce a limited amount of current. A short-circuited Kyocera K51 PV panel, for example, is rated at 3.25 Amperes. For comparison, a single 6 V deep cycle battery, when short circuited, produces an unmanageable amount of current (over 6,000 Amps for a few seconds). A power diode between the panel and the battery assures that only the charging source is short circuited and not the battery! How Many PV Panels? This regulator as designed will handle 35 Amps. Specifying the regulator for PVs is easy: divide 35 Amps by the short circuit current (Isc) of the panel to find the maximum number of panels you can regulate. Actually, under extreme conditions (a cold bright snowy day), a photovoltaic panel will put out as much as 125% of its rated current. If you live where it is ever bright and cold, take this into consideration. Permanent Magnet Wind and Microhydro regulation Specifying this regulator for a micro hydro or a wind turbine driven permanent magnet motor is more difficult. Consider that short circuiting the generator will quickly slow down the turbine perhaps putting mechanical stress on parts which were not designed to take it. Consult the manufacturer to be sure that short circuit regulation harm the machine. Then determine the peak short circuit current by running the device at normal operating voltage (so it gets up so operating speed), and manually short circuiting it (make sure you don't short-circuit the battery!), and measuring peak current and continuous current using a shunt and a recording multimeter. Use this regulator only if: ¥ the peak short circuit current does not exceed the maximum drain current (IDM) of the FET you use (160 Amps for the IRFZ40), and ¥ the continuous short circuit current does not exceed the continuous drain current (ID Cont) of the FET (51.0 Amps for the IRFZ40). We tested this regulator on an Energy Systems and Design hydro turbine putting out 2.1 Amperes at 13 VDC, and found that when shorted the turbine put out as much as 6.8 Amperes for 1 ms. How it Works The threshold voltage is determined by the potentiometer on the voltage divider into pin 5 of the 723. When this voltage exceeds the 723's internal reference voltage (pin 6), pin 9 goes high saturating Q1. The 10k½ resistor and 0.1µF capacitor assure that the power FET does not make the ON-OFF-ON transition too fast. When the capacitor on pin 2 is discharged to 1/3 Vcc the 555 is triggered and pin 3 stays high for a time period determined by the resistor to Vcc and capacitor to ground on pins 6 and 7: time Å 1.1RC Å 1.1 seconds. (Hydro folks, use a 100 K½ resistor so the permanent magnet motor doesn't lurch). Pin 3 feeds the gate of the FET, protected against over voltage (Vgatemax= 20 Volts) by an 18 Volt zener. When the gate goes high the FET conducts, short circuiting the PV panel (well, almost: the drain to source resistance is 0.028½). Total power dissipation in the IRFZ40 FET for 35 Amps is 34.3 Watts. This isn't much considering you're regulating 8 or 9 panels, but make sure that the FET is well heat sunk. A 5 Watt, 21 Volt zener diode protects against voltage spikes from inductive loads. The two LEDs help see what the regulator is doing. The green LED is on whenever power is on. The amber LED lights under regulation, when the power source is short- circuited. Construction and Use Information If you leave out the power diode, or put in it backwards, you will torch the FET in the first millisecond you turn the circuit on, guaranteed. In the schematic dark lines indicate high current carrying wires. To avoid over heating on these use 16 gauge or lower. When hooking up your regulator make sure the system's voltage is below 16 VDC since the NE555 is rated at a maximum 16 Volts. The regulator will regulate voltages from 12.0 to 15.8 Volts. Access Author: Chris Greacen c/o Home Power, POB 130, Hornbrook, CA 96044 ¥ 916-475-3179 Power FETs and diodes: get the Digi-Key catalog: Digi-key, 701 Brooks Ave. South P.O. Box 677, Thief River Falls, MN 56701-0677¥ 1-800-DIGI- KEY). The IRFZ40 is made by International Rectifier, and costs $4.08 a piece, $34.68 for 10. Schottky Rectifiers: two 16 Amp Schottky diodes in a TO-220 case made by Philips Rectifiers, Digikey part PBYR2545CTPH-ND, $3.02 a piece.