piclist 2001\11\10\065941a >
Thread: ~ 3.3V from 7805
picon face BY : Russell McMahon email (remove spam text)

> Is it possible to achieve near about 3.3 volts out of a 7805 plus a couple
> of IN4007 (1 volt drop) on the output pin?

A slightly better result than using diodes at not too much more cost is to
use a single transistor and 2 resistors.

BC337 or similar small TO92 NPN transistor
Collector to 5 volts
Emitter is the 3V3 output
1K2 resistor to base from 5 volts.
3K9 resistor from base to ground.

The two resistors provide about 3.8V to the transistor base.
The emitter is about 0.6v below this at about 3.2V.
Juggle the resistor values to provide different output voltages.
(eg 1K, 3K9 = about 3.4V)
The resistor values can be multiplied by 10 for small output currents (say
12K & 39K)

A transistor and 2 resistors doesn't cost much more than 2 diodes
Adding an output capacitor (10uF to 100 uF)  would be a very good idea in
either case.


                Russell McMahon

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See also: www.piclist.com/techref/power.htm?key=7805
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