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I have a bare-bones Arduino (basically an Atmega328P) and a Raspberry Pi 2. I have them communicating with each other over I2C and a 5v/3.3v logic level converter.

Right now I power the Pi (micro-usb) and Atmega (5v from FTDI) separately. I have a 12V 6A DC PSU (5.5mm barrel jack) that I would like to use to power both the Pi and the Atmega (and my LED strip lighting).

What's a good way to go about doing this? I have a few LM7805 chips on hand if I could use those, but I'm open to ordering new stuff if I have to.

Do I convert to 5v from 12v and then use that to power both Pi via GPIO and Atmega or is there current draw issues with doing that?

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  • \$\begingroup\$ If you don't want to fry your RPi and ATMega, then yes, you should really consider stepping down your voltage. \$\endgroup\$
    – user103380
    Commented Oct 12, 2018 at 15:57
  • \$\begingroup\$ Yes... I know that. But how exactly? Is a single simple Lm7805 circuit good enough to power both pi and atmega? \$\endgroup\$
    – darkadept
    Commented Oct 12, 2018 at 15:58
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    \$\begingroup\$ My suggestion: don't bother with the 7805, it will get HOT. Also don't bother making your own DCDC converter circuit. Instead, get a module like this one based on the LM2596: nl.aliexpress.com/item/… Buy it, apply 12 V at the input, adjust screw to get 5 V at output, enjoy. If the link doesn't work well for you, just Google "LM2596 module", they're sold in many shops but cheapest from China. \$\endgroup\$
    – Bimpelrekkie
    Commented Oct 12, 2018 at 15:59
  • \$\begingroup\$ I was about to answer suggesting a buck converter but it looks like someone was a step ahead of me ;) \$\endgroup\$
    – user103380
    Commented Oct 12, 2018 at 16:01
  • \$\begingroup\$ ok. I think that's exactly what I need @Bimpelrekkie! So I can power both the Pi to GPIO and Atmega straight from the output of that buck convertor? Looks simple enough. Do you want to make that an answer so I can close this question? \$\endgroup\$
    – darkadept
    Commented Oct 12, 2018 at 16:12

4 Answers 4

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It looks from the comments you stated that 1.5A at 5v is not enough for you so It would be better to look for some of the higher power usually isolated DC/DC converters for example : 15w dc-dc converter will give you (3A at 5v) and are readily available though not cheap around 15-30$ meanwell SKM15A-05 .

However using a linear regulator is next to impossible , since the heat loss is around (12-5)*2 = 14W!! for 2A load , and (12-5)*3=21W!! for 3A load . and thats alot of power , you will need a heatsink the size of the respberry Pi and a fan to cool that .

There also the option to use 2 stage , 1 DC/DC converter 6v for example followed by a 5v linear regulator if you need to have clean power rail with less noise ..

have fun :)

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  • \$\begingroup\$ What the difference between the SKM15A-05 and a LM2596 based board? \$\endgroup\$
    – darkadept
    Commented Oct 12, 2018 at 19:51
  • \$\begingroup\$ the isolation, SKM is a flyback converter ( isolated , safer , EMI tested ) very professional product , just take a look at the data sheet and see the details of the effceincy , protection under voltage current limit .... LM2596 is non-isolated buck converter which will work for you but you get what you pay for .. \$\endgroup\$
    – Sarah
    Commented Oct 12, 2018 at 20:04
  • \$\begingroup\$ hopefully this answers your question and help you in making the decision \$\endgroup\$
    – Sarah
    Commented Oct 12, 2018 at 20:04
  • \$\begingroup\$ It helps! The amount of information learning about electronics is astounding! \$\endgroup\$
    – darkadept
    Commented Oct 12, 2018 at 20:06
  • \$\begingroup\$ you are welcome , glad i could help:) \$\endgroup\$
    – Sarah
    Commented Oct 12, 2018 at 20:09
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You want a DC-DC converter; linear regulators will waste a lot of power and require very large heatsinks in this application. A quick search on Digi-key found cheap this self-contained module (a Recom Power R-78E5.0-0.5) that will do what you need, but I'd advise browsing around a bit in the power supply section of the distributor of your choice. You might need more power than the Recom part can provide, for example, or you might find something cheaper or in a more convenient package.

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  • \$\begingroup\$ These don't seem to support higher than 1A of current. The Raspberry Pi 2 Model B requires 1.8A. \$\endgroup\$
    – darkadept
    Commented Oct 12, 2018 at 19:56
  • \$\begingroup\$ Hence the advice to browse around. I'm not aware of what the current demand is, so I just picked a cheap one. Digikey has a dedicated section for DC-DC board-mount modules, with parametric search, so you should be able to find one that meets your needs easily enough. \$\endgroup\$
    – Hearth
    Commented Oct 12, 2018 at 20:33
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itA switching power supply is clearly the only option especially for the power hungry Rpi. There are plenty of pre-made modules that will make it an easy task. If you are unsure on the current rating, just plug your circuits to a 5V power supply and measure the power consumption, multiply that figure by 1.5, and get a module for that ballpark figure or higher.

Unless there is a good reason for using a 12V power supply, I suggest that you buy a 5V power supply which can deliver at least 2A.

This might also be the cheapest option.

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  • \$\begingroup\$ I needed the 12v for my LED strips. \$\endgroup\$
    – darkadept
    Commented Oct 12, 2018 at 18:39
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Digikey carries these 12V to 5V switching regulators that would work well

https://power.murata.com/data/power/oki-78sr.pdf

Pololu also carries similar switching regulators.

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    \$\begingroup\$ It looks like these only output 1.5A at 5V so it wouldn't be enough for a Raspberry Pi 2. \$\endgroup\$
    – darkadept
    Commented Oct 12, 2018 at 18:49

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