I have a 1980s chess computer to which I have added a Raspberry Pi 2 Zero to provide display and speech capability via a Python. It all works via serial messages and the only electrical connection between the two is an RS232 to USB adapter.

The chess computer power supply is 9 volts 300mA. The Pi 2 Zero supply is 5.1 volts 2.5A.

The Pi has a USB HAT with a USB sound card driving two tiny speakers and a 4 character 14 segment LCD display. The 5.1 volt 2.5A supply (genuine Rapsberry Pi model) powers it without issues.

I want to have a single power supply, but about the only thing I know about electricity is that I know too little to be safe.

So in my head a 3A 9v 27w AC/DC Regulated Switch Mode Power Adaptor with correct polarity would be fine (if overkill) for powering the chess computer. Could I take a parallel feed from that same supply and through some magic provide 5.1 volts with enough amps for the Raspberry Pi?

Despite my lack of knowledge my soldering skills are pretty good. I'm hoping somebody can help with the missing magic.

  • \$\begingroup\$ I'll add a +1 to help move you along towards being able to upvote answers. I think you get it at +15. Which won't be far away, now. \$\endgroup\$
    – jonk
    Jun 15, 2022 at 22:59

1 Answer 1


There are plenty of 5V DC-DC converters that will happily take 9V in. Something in the 3-5A range is recommended and will probably be around $25 from a reputable source. With current ratings you are almost always fine overspecifying the source and it's generally recommended not to run them close to 100%.

  • 1
    \$\begingroup\$ The original supplies can give a maximum of 15.45W. Unless the DC-DC converter is massively inefficient a single 27W supply is only going to be running at just over half power. \$\endgroup\$
    – Finbarr
    Jun 15, 2022 at 22:16
  • \$\begingroup\$ Of course! I forgot the power conversion; let me edit. \$\endgroup\$
    – vir
    Jun 15, 2022 at 22:18
  • \$\begingroup\$ Thanks vir. And Finbarr - 15.45W - I see how you got that number! I have an answer and learned something along the way. \$\endgroup\$ Jun 15, 2022 at 22:44
  • \$\begingroup\$ It seems my reputation will not allow me to click the up arrow. \$\endgroup\$ Jun 15, 2022 at 22:45
  • \$\begingroup\$ @clarke-baxter If the answer fully answers your question you should accept it. \$\endgroup\$
    – Finbarr
    Jun 15, 2022 at 22:53

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