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Cheap PSU Cheap PSU - inside
So instead of looking for another PSU (or cell phone charger), I decided to make my own
PSU with the popular 7805 - 5V/1A regulator [3] [4] . (There is also a 2A version available
- 78S05)
The basic idea is shown on this schematic:
The resistors R1 and R2 serve as adjustment of the output voltage (~ 5.25V). The formula
is: V out = V fixed + { R2 [ (V fixed/R1) + I standby] }, where V fixed=5V and I
standby=2.5mA (for 7805). I calculated for resistors that I had at home, but for best
results R1 should be about 470ohm to 1k. Remember that resistors have some tolerance,
so results may vary slightly, always measure. Value of C3 is not critical, I recommend
100-470uF. Same for C4, where for every 1A drawn, use 1000uF of capacity (and add
some reserve). Don't forget to put C1 and C2 as close as possible to the regulator. And a
heatsink for the regulator is necessary too.
I've used an old 9.5V/1500mA power supply from an printer as the source for this
regulator, so no transformer and rectifier etc. was needed in my case. And it works just
fine :)