Voltage on circuit input different than on power supply

Question

I'm using a really good 5V-35A switching power supply to power some mini PCs (Cubieboards if that matters) as below.

^{simulate this circuit – Schematic created using CircuitLab}

The weird thing is that when I measure the voltage on the PCs input terminals, they are below the power supply's voltage. If I measure it directly at the PS it's always 5.2V, no matter how many PCs I use on the circuit. However, If I measure it on the terminal that connects all the PCs, it gets lower and lower as I increase the number of PCs. Even though they are connected in parallel!!!

They don't have any other connection between them, only the power source.

The terrible thing about this is that after about 5 PCs the voltage gets so low that I can't power any PCs anymore (and I need to power around 20 of them using this source). I've tried using a different power supply and different connectors and the results were exactly the same.

What could possibly be happening? How can I solve it and what should be the best way to power all of those devices using the supply I have?

Thank you in advance,

Theo

Answer 1

As specified in comments, it is very likely that you are experiencing excessive loss in your cables. If you are using very long wire or wire of a too small gage, you are actually getting such an equivalent circuit:

^{simulate this circuit – Schematic created using CircuitLab}

Where the vertical resistors are the "computers" modeled as loads and the horizontal resistor are losses in the wire.

The resistor values are arbitrary, but a too small wire may cause the assertion of having a negligible resistance to be false. You can see on AWG page on Wikipédia the resistance of a cooper wire per meter as a result of it gage (diameter). You have to consider that the higher the current, the higher the resistance.

In order to solve your problem it is you case, you have to: 1) Use larger wire (smaller gage); 2) Use shorter wire (if possible); 3) Avoid daisy chaining your boards, as the first wire will have a larger amount of current flowing that the last one, resulting in larger losses.

You should prefer a star configuration:

^{simulate this circuit}

so that you have a different wire starting from your source to each of your board instead of a wire running from power supply to first board, then from first board to second one and so on. Same thing applies to your return path (ground).

You may or may not be able to apply all of the previous advices depending on your setup, though.