Parts List:
All resistors are carbon, 1/4 watt, 5% tolerance, unless otherwise indicated.
R1 = 500 ohm C1 = 0.1uF, ceramic
R2 = 3K C2 = 1uF/16 or 25 volt
R3 = 820 ohm C3 = 1000pF (1nF), ceramic
R4 = 15 Ohm D1 = 1N457
R5 = 230 Ohm Led1 = Red, 5mm, high brightness
R6 = 15K T1 = 2N2905
R7 = 0.2 ohm, 5watt, U1 = LM350T
wirewound U2 = LM301A
R8 = 820 ohm S1 = Switch, push type
This high-performance circuit first quickly starts (and holds) the charge at 2 amp, but as the voltage rises the current will consequently decrease.
When the current falls below 150mA, the charger automatically switches to a lower 'Float' voltage to prevent overcharging.
At the point that a full charge is reached, T1 will bias and the LED will light.
The LM301A is a 8-pin OpAmp. Transistor T1 is a PNP, Silicon, AF-Out type with a TO-39 metal case and can be substituted for a NTE or ECG129. Diode D1, a Si, GP Det. type, can be substituted with a NTE or ECG177. The LM350 (U1) needs to be cooled.
The input voltage should equal or about 18volts. This circuit is suitable as a field-charger for charging your flight box battery via your car-battery.
R2 & R5 are actually metal-film type resistors but carbon is okay to use. To get the 3K for R2 use two 1K5 (1500 ohm) resistors in series. For R5 use two 470 ohm resistors in parallel. Or whatever combination to get to these values.
R7, the 0.2 ohm resistor, is a 5 watt wire-wound type. Do not use the standard carbon type.
When the start switch is pushed, the output of the charger goes to 14.5 V. As the batttery approaches full charge, the charging current decreases and the output voltage is reduced form 14.5V to about 12.5V, terminating the charging process. Transistor T1 then lights the led as a visual indication of a full charge.
NOTE: For Gelled, lead-acid type 12volt batteries only!
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Copyright © 1996 - 2000 by Tony van Roon
