I don't know much at all about electronics, so this is a very basic question that I'm struggling to figure out, even with the help of Google.

So the scenario I want to achieve is this:

  • I have a 12V/1A converter plug (which can go lower in voltage, same amps I think. I guess I could also get a different one).
  • There are two devices that need powering from the same source. One [5V, 2.1-3A] and one [12V, 1A].

Could I just split the output of the wall converter and feed one 'branch' into a Step Down module, to transform the power supply to something that fits the 5V device?

The module I'm looking at (Amazon link) has these specs:

  • Input: DC 3V-35V (Input voltage must be higher than the output voltage of 1.5V or more).
  • Output: DC 1.5V-35V the voltage is continuously adjustable, the max output current is 3A.

I've read that if I make the voltage lower, then the amps should go up in linear relation to that? Would that be true for a down stepper too?

I also think I read somewhere that doing this kind of thing can cause power supply fluctuations on the other 'branch' that might be damaging. Is this correct?

Grateful for any advice!

  • 1
    \$\begingroup\$ Yes you could, but getting 5V 3A from the step down (buck convertor) requires about 1.5A at 12V, so you need at least 2.5A (make it 3A) at 12V overall. \$\endgroup\$
    – user16324
    Nov 6, 2020 at 23:31
  • \$\begingroup\$ Ok so when I split the output of the wall converter I also split the Ampere? Splitting the ampere but the voltage stays the same? \$\endgroup\$ Nov 7, 2020 at 1:12

3 Answers 3


Power = voltage x current

Calculate the power available:-

12 V x 1 A = 12 W

Calculate the power required:-

12 V x 1 A = 12 W

5 V x 3 A / 0.8 (estimated converter efficiency) = 18.75 W

Total power required:-

12 W + 18.75 W = 30.75 W

Clearly you not have enough power to run both devices, nor even enough to run the 5 V device at its maximum rated current draw.

So what do you need?

30.75 W / 12 V = ~2.6 A

You need an adapter which can deliver at least 2.6 A at 12 V, preferably more.

  • \$\begingroup\$ Great! But if I get a higher Amp adapter which I then split in two then the 12v device will get too much power as the current will be higher. Maybe that's not a huge problem though? (its a ex-laptop LCD panel) \$\endgroup\$ Nov 7, 2020 at 1:20
  • \$\begingroup\$ The Amperage is the maximum current the converter can supply NOT the current actually supplied to a given device attached to it. \$\endgroup\$
    – mhaselup
    Nov 7, 2020 at 1:25

You can certainly do this but you'll need a beefier 12V output converter.

The currents to the downstream devices of the 12V converter must add up to no more than it maximum output. One of your devices requires 12V at 1A which immediately uses up your current budget.

As a rule of thumb you can assume that the DC-DC converter is likely to be 90+ % efficient. So for 5V at 3A output (i.e. matching power) you will require an input current of

5 * 3/(12 * 0.9) = 1.4A let's call this 2A for some additional headroom

So you need your 12V converter to supply 1+2 =3A total.

  • \$\begingroup\$ Awesome thanks guys!! This has been super helpful and I've learned some new things as well!! :) \$\endgroup\$ Nov 7, 2020 at 1:22

It's always preferable and maybe even cheaper in some cases to have two separate power supplies connected to main. The only advantage of using a step down converter is to run low voltage lines like 12V for safety and make a 5V "branch" near the 5V device. if both devices are near the main outlet, then it's better to use two separate AC/DC supplies.


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