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I am designing a device and I'm confused on calculating my total amperage required.

I have designed this device to have 3 main circuits in parallel that all take 5 V from the same power source:

  • First is some LEDs,
  • second is a microcontroller circuit to control those LEDs, and
  • third is a few USB charging ports for phones etc.

I've estimated I will need at most 600 mA for the LEDs, the microcontroller circuit will take next to nothing but I am using 200 mA as the max current it will pull, and the charging devices I think will at most need 12A. (I have 4 in parallel that take 0-3A input, is 12A correct?)

So am I correct in thinking this will need 13A max?

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    \$\begingroup\$ short answer: yes. \$\endgroup\$ – Marcus Müller Jan 7 '17 at 2:28
  • \$\begingroup\$ Thanks, does that mean I need a power supply that can always provide 13A? \$\endgroup\$ – dhalvin Jan 7 '17 at 4:23
  • \$\begingroup\$ Yes, unless you can guarantee that some of the sub-elements of your system are not used at the simultaneously. For example, if you know there will never be more than two charging ports used at a given time, you could set the supply requirements to 7A only. Oterwise, yes, it is 13A. Note: 13A is a lot. Use thick wires. Note 2: for a 5V supply, a requirement of 13A means you need a supply rated for at least 65W. \$\endgroup\$ – dim Jan 7 '17 at 4:55
  • \$\begingroup\$ Be aware that not all USB ports are rated for 3A \$\endgroup\$ – HighInBC Jan 7 '17 at 5:23
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    \$\begingroup\$ A resistor does not put a hard limit on the current. It will just "eat up" some voltage from the supply, in proportion to the current actually drawn by the device. Which means, basically, that it will transform your nice, steady, supply rail into a crappy, unstable supply rail from which it's missing some voltage, but it won't achieve the effect you want. So don't do that. \$\endgroup\$ – dim Jan 8 '17 at 19:49
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You are correct on the math, however designing for the max current could be a more expensive design than necessary. For example, if you are not going to have more than two devices charging, and each actually needs 2 amps, then you just need a 5 amp supply, which would be a lot cheaper (and smaller) than the 13 amp supply.

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