Combining current sources

Right then gents. I need some advice.
I have a project where I have a device that needs a power source of 5V at 1.5A. However I only have multiple 12v 1A sources available in the environment. How then would I go about combining the sources safely so I have 12V @ 2A (to then regulate to 5V at 1.5 -1.8A max)?
Any and all suggestions will be a help…

Cheers Hugh

No they are coming from another pcb via an 80 pin ribbon cable

Your 12V at 1A is going to be 2.4A at 5V so you should be OK with that as it will be more than your 1.5 to 1.8 you need.


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@ Dave McLaughlin - sure ? true with a switched mode regulator, with a linear regulator not… Or i am blond today? ;D

My worry is I can’t draw more than 1A through the ribbon cable as that may damage it, but because there are multiple 12V 1A sources I thought if I put them in parallel that would work but it doesn’t seem to be as simple as that. Unless I have the wrong end of the stick. :think:

@ andre.m - The source is DC I thought rectifiers only worked with AC source??

kind of like this?

Correct, you will lose some to heat with a linear regulator. I hardly ever use linear these days to drop 12V to 5V as they get too hot. I prefer those little TO220 switched mode ones. 3.3V from 5V is OK.

I also didn’t realise that you would only have 1 core for 12V and 1 core for GND so yes, trying to draw that much current may not work depending of the cable spec. Some I use are rated for 1A so 2 cores should be able to handle your 2A.

If you need to parallel the supplies, use a DIODE on each output to combine the 2 into 1. This will prevent any issues when 1 voltage is slightly different to the other but the current will be shared.

Bridge rectifiers are great if you want to have a non polarised supply input or want to use AC or DC input. I would only do this on something that has it’s own power supply and remember that the GROUND of the supply is not the same as the DC ground on the other side of the bridge rectifier. It can’t be used as a reference point. You have to have a separate ground connection taken from the bridge output. Hope that makes sense?

You can simplify this with a single diode in the positive rail will be more than enough. No need for the bridge rectifiers.

The GND’s can be commoned.

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Thanks Guys. This has really helped ;D. I will see how we get on…

@ HughB - can you share The final schematic, please?

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@ njbuch - yep I will at least the bits I am allowed to. It’s an actual work thing I’m doing and they get a bit funny about sharing stuff even though this will be an interface between two devices.

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@ HughB - Thats fine! In general I am often confronted with weird voltages and lack of current in the wrong places. So any generic pointers on this could be valuable to me. :wink: