Except for use as a normal Battery Charger, this circuit is perfect to
'constant-charge' a 12-Volt Lead-Acid Battery, like the one in your
flight box, and keep it in optimum charged condition. This circuit is
not recommended for GEL-TYPE batteries since it draws to much current.
The above circuit is a precision voltage source, and contains a
temperature sensor with a negative temperature coλficient. Meaning,
whenever the surrounding or battery temperature increases the voltage
will automatically decrease. Temperature coλficient for this circuit is
-8mV per °Celcius. A normal transistor (Q1) is used as a temperature
sensor. This Battery Charger is centered around the LM350 integrated,
3-amp, adjustable stabilizer IC. Output voltage can be adjusted with P1
between 13.5 and 14.5 volt.
T2
was added to prevent battery discharge via R1 if no power present. P1
can adjust the output voltage between 13.5 and 14.5 volts. R4's value
can be adjusted to accommodate a bit larger or smaller window. D1 is a
large power-diode, 100V PRV @ 3 amp. Bigger is best but I don't
recommend going smaller. The LM350's 'adjust' pin will try to keep the
voltage drop between its pin and the output pin at a constant value of
1.25V. So there is a constant current flow through R1. Q1 act here as a
temperature sensor with the help of components P1/R3/R4 who more or less
control the base of Q1. Since the emitter/base connection of Q1, just
like any other semiconductor, contains a temperature coλficient of
-2mV/°C, the output voltage will also show a negative temperature
coλficient.
That
one is only a factor of 4 larger, because of the variation of the
emitter/basis of Q1 multiplied by the division factor of P1/R3/R4. Which
results in approximately -8mV/°C. To prevent that sensor Q1 is warmed
up by its own current draw, I recommend adding a cooling rib of sorts.
(If you wish to compensate for the battery-temperature itself, then Q1
should be mounted as close on the battery as possible) The red led (D2)
indicates the presence of input power.Depending on what type of
transistor you use for Q1, the pads on the circuit board may not fit
exactly (in case of the BD140).
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