# RPi 5V PSU construction

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## A 5V power supply for the Raspberry Pi - How To

Due to various problems with the power supply for the RaspberryPi, a home made PSU might be a solution for some of you. You will need some experience with construction of electronic circuits, appropriate tools and a multimeter.

I had problems with a cheap 5V/1A adapter from Ebay too (freezing, no LAN, etc.) ... The adaptor could not provide enough power. It had 5.0xV unloaded, but with RaspberryPi connected I've measured 4.78V and less - dropping to 4.5V on TP1 and TP2.

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.

The basic idea is shown on this schematic:

The resistors R1 and R2 serve as adjustment of the output voltage. The formula is: V out = V fixed + { R2 [ (V fixed/R1) + I standby] }, where V fixed=5V and I standby=V fixed/R1 (or just use 2.5mA). 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, 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 :)

Here is a schematic of a complete PSU including all components:
(a suggestion, with better filtering and protection)

Starting from left, we have a transformer (protected by a fuse - F1) supplying about 8-12V AC at 2A (use what you have at home or what is cheaper to buy). Next is a rectifier (or 4 diodes / >1A) with caps (C7-C10, for filtering). Now we should have approx. <AC voltage> x 1.41 - so if we have a 9V transformer, it will be about 12.69V. The 7805 needs at least 2V more on the input than on the output (I use >3V to be sure) for stable regulation, and it can be up to 35V. Main filtering is ensured by C4 (use at least 1000uF for each 1A drawn), another filtering after the regulator is C3 (100-470uF). C1-C6 serve the 7805 for stable function and HF filtering. R1 and R2 adjust the voltage to 5.25V. D1 and D2 are for protection. A transil is used for over-voltage (peaks) protection on the output, a 5V8 type (5.8V reverse standoff voltage and approx. 6.2V breakdown voltage) - use P6KE6.8A or BZW06-5V8. For operation signaling a LED coupled with R3 is used. You may use another fuse - F2 on the output.

This is a rather fancy circuit, you may simplify it if you like - by leaving out C7-C10, C5, C6, D1, D2, LED and R3, F2 (and the transil, if you don't want any protection). Your choice ;)

Also if you prefer to fine-tune the output voltage, you may replace R2 with a small pot (trimmer) as shown here:

With this values (R1=1k, R2=100) adjustment from 5V to approx. 5.75V is possible.

For more info on the 7805 regulator, google the datasheet and see the following resources.

The Adjustable Voltage Regulator,
Variable power supply using 7805

### The testing prototype

This is my testing prototype of this PSU, as on the first schematic. It's made of "what was found in the drawer". I've used an old 7805 regulator in TO3 package, C3 was taken from some broken mainboard, C4 is left out (it's not necessary, because the circuit is powered by a stable power supply from some old printer). The PSU supplies stable 5.25V and i have 5.82V on TP1-TP2. My RaspberryPi works OK now :)
///pictures will be added soon///
I will make another PSU (for permanent operation) later - new 7805 in TO-220 package, nice heatsink, 1k resistor for R1 and a small pot for R2, new "fresh" caps and a transil, usb connector on PCB ... and put it in some case.

~#Pinoccio

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