So I recently made the acquisition of pic related, which is an adapter for a 9.7" LCD panel.
It needs 5V 1.37A so I'm currently using a 2A USB phone charger to power it since I read 3.0 USB deliver up to 0.9A and 2.0 USB 0.5A
So here's my question, since you can mount in parallel batteries to "raise" their amps, could I do the same with USB ports? (two 3.0 ports or three 2.0 ports)
You'll need to put small resistors in series with the device and USB ports to balance them (a couple ohms). If you put two voltage sources in parallel without that one will supply a slightly higher voltage and all the current will be drawn from that one. This will make your PC shut down the higher USB port for drawing too much current, then the second one for drawing too much current. Batteries have their own internal resistance.
Pardon my shitty schematic and my stupid question, but I assume you mean putting a resistor where the green arrow is?
Or one for each port where the blue arrows are?
And I'm quite new with resistors, what would a good value for it?
>(a couple ohms)
dont listen to this idiot.
a 'couple' of ohms at 1 amp will drop a couple of volts.
meaning burned resistors and low voltage for your board.
usb is current regulated, should be fine.
external hardrives divide their load over multiple usb drives all the time.
Watch out with that assumption, many computers have USB ports that share their power connection, so you can't just use two ports because they are both sharing the same 0.9A power supply.
Like pic related? Yes. And in the right situation it will actually work. But in many situations it will make no difference because the ports that are plugged into are shared, because of power protection built-in modern chipsets, etc.
> This will make your PC shut down the higher USB port for drawing too much current
Not exactly. USB ports usually have over voltage protection, not over current. Pull too much voltage, it will shut down the port. Try and pull too many amps.... and it just won't deliver.
Imagine an arduino powered off of USB. Attach a long string of programmable LEDs to it, program it to print a message over serial on boot, and then turn on one led at a time. It never draws too much voltage or current, and will happily work away. Update it to do one at a time, then two, n+1... It will reboot and start over after ten or so at a time, because it can't draw enough current to light all the LEDs and run the atmel chip simultaneously. But it never drew too much voltage, so the computer never shuts down the USB port.
>USB ports usually have over voltage protection
You can't "pull too much voltage" from a USB port. It's a 5v voltage source. There is no way for any circuit to tell a USB port to provide more or less than 5v. The current is what changes based on the load.
>But you can push too much
Well sure, if you overvolt it.
>For which there is protection
In my experience? Nah, if you put 12v to a 5v USB you're gonna see smoke.
5v is 5v (unless it's regulated and you're drawing more than the regulated current, in which case voltage drops).
>But in many situations it will make no difference because the ports that are plugged into are share
That was my thougths at first, but it doesnt make sense for them to be shared. USB standard says they all should be able to supple 500ma. if one fatass usb device is drawing >=500ma then no other usb device would even function.
I dont think they could monitor the current going to ground because sometiems if not always the whole damn case is ground and a usb device could just dump into that instead. couldnt a diode or capacitor or something be placed in the + wire to not allow the chip to have a real idea of what current is really flowing through both ports combine?
>USB standard says they all should be able to supple 500ma
One thing that I think should be clarified that hasn't yet - that spec from the standards is a MINIMUM. In other words, a USB port is GUARANTEED to be capable of delivering up to 500mA. Any supply capable of more will still be compliant, but any accessory that draws more than 500mA isn't guaranteed to work with all USB ports.
In any case, your 2A supply is 2A (which is more than 1.37), so I don't see any problem.
make sure the ribbon connector is installed properly. I ended up burning out my adapter for the polish display port to ipad lcd adapter. the idiot had designed his circuit with the ribbon connector wired backwards so that the ribbon should be installed upside down.
They monitor the actual voltage at the output (like an ATX PSU does), and if the voltage has dropped too far, they conclude that too short a load must be connected.
There's an intermediate power supply so one USB port can't drop the PC's entire 5v rail. How much it can deliver determines how much the port(s) it's powering can deliver.