Using 2 power supplies for one circuit board

Thread Starter


Joined Sep 16, 2021
Hello all,
I am attempting to salvage an all-in-one HHD & SD card reading device which was unfortunately a victim of a flood a few months back. The original power supply rated at 12v 850mA, which didn't make it, has 3 wires outputting 12v+, 5v+, & a negative to the device. I have a 24v bench top power supply which I set to 12v 1A but need something to supply the 5v. I found an old phone charger with 5v 800mA I was hoping might work.

I used a breadboard and added 12v+ & 12v- to one rail and then 5v+ & 5v-, to another; I didn't add the device's circuit board. Using my MM the 2 rails correctly measured their voltage but with only one negative to the device I'm unsure what to do. So I measured the 5v+ with the 12v- and I'm getting what I presume is an average of the 2, ~8v. This tells me this isn't likely to be a solution. One thought was putting the 2 negatives on the same rail but my guess is that would not end well.

The bench top power supply has a USB A plug providing 5v and I was hoping it may work but when testing 12v+ with 5v- or 5v+ with 12v-, my MM is displaying AC reading and there is no continuity between the two negatives. I tried using a potentiometer I have from an old circuit board but stopped when I saw smoke.

I have found several articles and video suggesting a LM7805 but I don't have one and other suggestions only output ~100mA. I am sure it was overly obvious I don't qualify as even a rookie when you read the 1st paragraph.

Anything I can try to quickly determine is this flood victim will even turn on before I build the LM7805 suggested circuit?


Joined Nov 6, 2012
Unless You just really need a challenge to keep You busy,
go buy a new one,
it should be less than ~$20 bucks, and it will be new and reliable.


Joined Jun 5, 2013
The two different power supplies. If this is to be connected to a computer, its ground needs also to be connected.

It is very common to use multiple voltages in a circuit with the grounds connected together. That is necessary if you want to get signals from one part to the other without using an opto-isolator or similar device.