I bought a switching power dupply (SMPS) rated at 5V | 20A | 100W. Power Supply

But it seems I don't understand how power supply works. I need total of 18A on output, but that seems to be over the limit of standard 16A | 220V wall outlet's limit.

How would I power this supply without burning my house down? I bought fuses and relays but that can only handle max at 10A.

Reason for such high amperage needed:

I was going to power 300 LEDs that require 5v 60mA per LED. I'm planning to make it in 2 parallel structure, giving me 150 leds each side. It turns out I need 5v each side with 9A flowing each side, totaling 18A. So does two 9A merge on outlet's side?

  • 2
    \$\begingroup\$ ... What's 5V times 18A? What's 220V times 16A? \$\endgroup\$ Commented Jul 18, 2017 at 15:39
  • \$\begingroup\$ 18A at 5V is 90W. Which is about half an Amp from 220V outlet. \$\endgroup\$
    – Eugene Sh.
    Commented Jul 18, 2017 at 15:39
  • 2
    \$\begingroup\$ It will consume ~0.5 A on the primary (mains) side att full load. Up to 10 A/mm^2 is fine heating-wise, but you will be limited by maximum allowed voltage drop in your 5 V system. \$\endgroup\$
    – winny
    Commented Jul 18, 2017 at 15:39
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    \$\begingroup\$ @Sysnaptic You need to consider the high current on the output, of course and use the proper cable. \$\endgroup\$
    – Eugene Sh.
    Commented Jul 18, 2017 at 15:44
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    \$\begingroup\$ @Sysnaptic Note that you do need heavy wires capable of supporting 20A on the 5V side! \$\endgroup\$
    – marcelm
    Commented Jul 18, 2017 at 16:37

1 Answer 1


If you only draw 90 watts from the supply, it will only draw perhaps 100 watts from the AC source - a power supply passes power (voltage times current), not just voltage or just amperage. (The "extra" input power is lost, usually as heat, due to inefficiency in the conversion process).

  • \$\begingroup\$ Thank you for the comment! Now I understand it only pulls 100W / 220V = 0.5A \$\endgroup\$
    – Sysnaptic
    Commented Jul 18, 2017 at 15:47
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    \$\begingroup\$ nitpicking: a switching power supply, or a transformer, transfer power. A linear power supply does keep current constant, and heats up quite a bit in the process. \$\endgroup\$ Commented Jul 18, 2017 at 17:02
  • \$\begingroup\$ I'd say a linear power supply does transfer power, but with less efficiency than a plain transformer or a switching supply. If you have a 12 volt 20 amp linear supply, it certainly won't draw anything near 20 amps at 120 or 240 volts AC input. \$\endgroup\$ Commented Jul 19, 2017 at 1:01
  • \$\begingroup\$ A linear voltage regulator does draw as much current as it delivers, but a linear power supply will have a transformer before the linear regulator - the transformer will do the voltage/current conversion and deliver an appropriate voltage to the input of the regulator. \$\endgroup\$ Commented Jul 19, 2017 at 15:39

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