10

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

  • 3
    7 Watts of heat. It's going to get hot and toasty :-) – Gerben Jul 29 '13 at 14:52
  • Yup.I guess I have to get a good transformer that brings down the voltage to something that is bit efficient. – feverDream Jul 30 '13 at 13:00
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.

15
  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.

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.

  • 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

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