This little circuit uses a LM317 variable voltage regulator to adjust the input voltage down to +9 volt, or whatever
else you need. Just a solid basic circuit without bells and whistles.
You can do with a 10uF capacitor for C1 if your battery is close to this circuit. If it is located more than
3 feet increase the value to 100uF or above. Without a coolrib it can easily handle 500mA. If you need more, or
the maximum current (1.5A), then a good coolrib is required.
Trimmer potentiometer R3 will vary the output voltage. Ceramic capacitor C2 improves frequency/transient response.
Can be omitted if not needed for your application. If you want extra protection in case the adjust pin is short
circuited, add an extra 1N4001 diode over the input and the output. Cathode to input. But normally only used if the
output is way over 25V.
R1 and R3 determine the output voltage. You can adapt them for your own needs and applications.
Use the following formula: (((R1+R3)/R2)+1)*1.25=V-out which comes to: (((560+1000)/220)+1)*1.25 = 10.11V
(assuming V-in is 12V).
Or vice-versa: ((V-out/1.25)-1)*R2=R1+R3 which comes to: ((9/1.25)-1)*220=1364. For 1364, you can make R1=560 and
R3=1K, which will give plenty of play.
After dozens of emails I have included the above circuit. The parts with the red 'X' are added and act to boost the
amperage. The NTE393 transistor can handle 25A with a sufficient coolrib.
Other power transistors, such as the TIP2955, or similar can be used also. The power transistor is used to boost the
extra needed current above the maximum allowable current provided via the regulator. Current up to 1500mA(1.5A) will
flow through the regulator, anything above that makes the regulator conduct and adding the extra needed current to the
It is no problem stacking power transistors for even more current. Both regulator and power transistor must be
mounted on an adequate heatsink, and if you intend to use lots of amps a fan would be nice too.
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