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I was wondering what precautions I should be taking when I am making a PSU. I managed to find an old dc adapter. These are the parts I currently have.

| no | name           | specs              |
|----+----------------+--------------------|
|  1 | dc adaptor     | 12v ~5A            |
|  2 | 7805           | 5v ~ 2A            |
|  4 | capacitors     | 1500uf,100nf,470uf |
|  5 | female usb pin |                    |
|  6 | resistors      | 10,330 Ohm         |
|----+----------------+--------------------|

Here is the circuit that I am referencing. link http://elinux.org/images/d/d7/PSU_7805_v01.jpg

enter image description here

lenik
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feverDream
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3 Answers3

16

  1. schematics is plain wrong, 7805 does not have "Adjustment" pin, it's supposed to be tied to the ground, the resistors R1 and R10 (using binary encoding, aren't we? =) are unnecessary.

  2. if you convert 12V to 5V using 7805, expect to dissipate about (12-5) = 7V * 1A of heat, that would require a good radiator, or 7805 will overheat and cut the power.

I would definitely recommend to get $5 power adapter from the authorized Raspberry Pi resellers instead.

UPDATE: R1/R10 may have been introduced into this schematics to raise output voltage from 5V to 5.25V, still I would strongly advise against using them, firstly because your Pi is not designed to handle 5.25V -- 5V would be perfectly enough, and secondly because using these resistors will introduce constant leak current about 5V/340ohm=15mA for no particular reason. If you need voltage adjustment there are different voltage regulators with dedicated adjustment pin that do not require small resistors/large leak currents.

lenik
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6

7805 can easily be destroyed by reverse current. That means if the 12V supply is off, but there is 5V from another source - HDMI, powered USB hub or GPIO the 7805 can die quite effortlessly.

You could prevent that by adding a reversed biased diode between the input and output, but you'll still need a decent heatsink which will likely cost more than a 5V power adapter if you can't scavange one.

Overall it's a very wasteful (energywise) design. It's still good to try this type of thing for learning.

Also consider the LM2596 based regulator modules from ebay for a low cost voltage reduction. They have variable output, but you can set it to 5V easily if you have a multimeter.

John La Rooy
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3

If your intention is to make your own power supply, take one of the step-down switching regulators, like LT1076-5 or LM2593, which can handle 2A at 5.0V. Those are around 5-7EUR for Minimum Order Quantity = 1 (plus a few EURos for essential components). As already mentioned, linear regulators have big thermal issues, specially when large voltage drop (input to output voltage difference) is present.

TomiL
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  • I ran into these while I was reading about power supplies. I am not from the EU so I will have to take a look what the local dealers have to offer here in india. Efficiency wise these seem great! – feverDream Jul 30 '13 at 13:04