RPi 5V PSU construction using Raspberry pi




A 5V power supply for the Raspberry Pi – Construction 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.

RPi 5V PSU construction
I have had problems with a cheap 5V/1A adapter from Ebay too (freezing, no LAN, etc.) … The adapter 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 [1], and that’s not good. The voltage drop might be partially caused by the cable, but I’ve used a branded Nokia cable that looks pretty solid. Anyway, we have to compensate for that too. Also there is some voltage drop on the polyfuse F3 (typically 0.1-0.2V, fuse should have about 0.2 Ohms [2]), hence don’t expect to get >=5.0V on TP1-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 [3] [4]. (There is also a 2A version available – 78S05)

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 fineΒ πŸ™‚

RPi 5V PSU construction

Starting from left, we have a transformer (protected by a fuse – F1) supplying about 7-12V 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[5] (depending on type/manufacturer) more on the input than on the output (I prefer using a little more, >=3V to be sure) for stable regulation, and it can be up to 35V (but a big difference between input and output voltage means “a lot work” for the regulator and a lot heating). In this case, 8-9V DC measured after the rectifier would be optimal. 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, as described before. D1 and D2 are for protection. A transil is used for over-voltage (peaks) protection on the output, a 5V8 type should be fine (5.8V reverse standoff voltage and approx. 6.2V breakdown voltage) – use P6KE6.8A or BZW06-5V8. For operation signaling (device on) a LED coupled with R3 is used. You may use another fuse on the output – F2.

 

For more detail: RPi 5V PSU construction


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