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Okay, I'm a total noob and don't know if I'm even wording this question correctly, but I'll try.

My understanding is that the power supply (an USB power supply in my case) determines the current. I have a 2.5A 5V supply, and a USB-to-DC cable that connects that power supply to an USB hub (5V as well). Now that hub would get a total input of 900mA from its data USB 3.0 connection to the PC + 2500 mA from the power supply. Does it just always get that current, regardless of whether it actually needs it? In other words, if I just have one little USB drive in the hub, will that USB drive get all the 3400 mA and burst into flames?

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The power supply works as a voltage source up to 2.5A current. You cannot combine the current of the externals supply and the PC's port - that would require expensive adaptive circuitry.

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  • \$\begingroup\$ So what determines the actual flowing current? The current-drawing device (like the usb drive)? \$\endgroup\$ – Krateng Aug 10 '16 at 15:27
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    \$\begingroup\$ Elementary physics class: The load determines the current in a voltage source circuit. \$\endgroup\$ – Turbo J Aug 10 '16 at 15:31
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Your power supply can deliver up to 2.5 Amps at 5 Volts, but does not force 2.5 Amps into a load. Any load will only "ask" the power supply for the current it requires.

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  • \$\begingroup\$ Is that a rule of electrics in general or a feature of the power supply (it analyzes the current and adjusts its output)? I was always under the impression that a circuit without any load would just have insanely high current? \$\endgroup\$ – Krateng Aug 10 '16 at 15:43
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    \$\begingroup\$ That is a general rule - any "normal" (constant voltage) power supply or battery will only deliver the current required by the load. An exception is a "constant current power supply" which will adjust its output voltage in an attempt to deliver its rated current to the load. Some "LED Drivers" are constant current supplies. \$\endgroup\$ – Peter Bennett Aug 10 '16 at 15:54
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Most power supplies regulate voltage. And can supply enough current to maintain that voltage up to a certain limit. The insight needed here is to uderstand that only enough current is supplied by a voltage regulating power supply to maintain the voltage.

In math terms voltage is equal to the current times the resistance of the load. For a given voltage, something that need a lot of current to run will have a low resistance. The usb power supply's job is to make sure that the load's resistance times the supplied current is always 5 volts.

So, remember this equation and you should be able to solve these types of questions from now on using math:

V = I * R

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  • \$\begingroup\$ So basically, the power supply changes its current to always supply the voltage of 5V, dependent on what current is 'used'? \$\endgroup\$ – Krateng Aug 10 '16 at 15:29
  • \$\begingroup\$ Yes. That's a good way of saying it. In math terms voltage is equal to the current times the resistance of the load. Something that need a lot of current to run will have a low resistance. The usb power supply's job is to make sure that the load's resistance times the supplied current is always 5 volts! \$\endgroup\$ – st2000 Aug 11 '16 at 0:08

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