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Topic: extra USB power (Read 6597 times) previous topic - next topic
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Re: extra USB power

Reply #25 –
It is only a simple circuit board with (originally) four input USB sockets in parallel and one output cable. There is no room for any chip unless it is hidden inside one of the USB sockets, which seem unlikely. The sockets re pretty simple.

While advertised as a 4 port hub it never worked with more than one device plugged into it at a time. The second device would either not be seen or would immediately replace the first (in Windows Explorer) so that the PC always saw only one device whether one or four were plugged in -- useless as a hub.

As I stated, I tested it well before modifying it, with flash drives and hard drive, both with and without the USB charger attached. In all cases it worked as though the device were plugged directly into the computer's USB socket.

With the V+ disconnected it did not work but had some strange connection to the computer, as I described.

Re-attach the V+ and it is back to normal operation -- so no mistakes. Maybe tomorrow I will have time to remove the V- too and see if that makes a difference. As I tried to explain last post, neither V+ or V- are connected to the output USB cable on the powered enclosure, so apparently they are not necessary as long as the drive gets it power elsewhere.

Re: extra USB power

Reply #26 –
Removing the V- lead did not work. It might be said that it was even worse because the drive didn't even register in the devices that could be ejected but the difference hardly matters.

I opened the powered USB enclosure to get at power connections more directly. Its V- is connected to the output USB cable but there is no connection to the cable from either the +12 or the +5. Why the modified  hub doesn't work with attached V- is mysterious to me. How is it any different than the enclosure or even really different than the powered hub with the V- cut off electronically?

There is a connection on the powered hub between the V- and V+ . With the power supply attached I read about 0.01V but 0.00V with the power supply unplugged. My modified hub has straight 0V because there is no connection. The powered enclosure also has 0V because there is no connection. Maybe I could achieve something useful with a large resistor is series with the unpowered hub's V+? About 500 ohms should drop the V+ to around 0.01V.

Re: extra USB power

Reply #27 –
splice, on 11/23/16 you wrote
Quote
In the Y cable plus USB power source scenario, bad things can happen.
- If the voltage from the external supply is lower than the voltage from the device, the device will still supply most of the current required and may be overloaded.
- If the voltage from the external supply is higher, it will be fed back into the device USB port. Consider what happens if you power off the device - the external supply will try and power the device. Depending on the internal circuitry of the device, bad things can happen.

I ordered, and shortly received
https://www.amazon.com/DROK-300043-Multimeter-Voltmeter-Capacitance/dp/B00J3JSEG6/ref=pd_cp_147_1/168-4598109-1125900?_encoding=UTF8&pd_rd_i=B00J3JSEG6&pd_rd_r=YCNS5R1ES6SV5GH74A4S&pd_rd_w=7dY0p&pd_rd_wg=KEl3t&psc=1&refRID=YCNS5R1ES6SV5GH74A4S
While somewhat more expensive than those on e-bay, it has good reviews and did not have to come on a slow boat from China.

What I found, using a USB charger on Y cable to power a hard drive, tested on three different computers and two different brands of video recorder interfaces,  is
** with the meter on the host USB port, it reports 0.00A while the drive is idle and varies from 0.13A to 0.18 while copying to, or recording to, the hard drive.
** with the meter on the USB charger it reads 0.35A at idle and from 0.30A to 0.40A while writing.
Thus, the y cable/charger reduces the draw on the host quite a bit while active and completely while idle.

In my measurements, the USB charger had the higher voltage but never more than a few hundredths of a volt different from the host port. Unfortunately, with one meter I can only measure one source at a time so I can't say that the meter itself doesn't  cause significant variations, depending on where it is placed, but I don't believe that should happen.

 Only in one case did anything seem the least untoward with the host. On one computer the power-on LED lit if the setup was plugged into a port with the computer off and stayed lit if the setup was left in place when the computer was turned off. A little misdirecting but seeming no danger.

It may be, as you wrote elsewhere in this thread, that I was lucky not to damage something, but so far my evidence doesn't support any "bad things" happening. I was initially somewhat worried, before I started this thread, due to statements in some customer reviews about problems with several different powered hubs, but those problems could have different sources.

I've tried to logic it out but I just don't have an adequate knowledge to figure out circuit behavior in the unusual circumstances of two power sources attached to the same circuit. I suppose something like that is very common in country wide power grids but those are very complex and I doubt I would become enlightened by reading about them.

One thing that seems reasonable to me is that the host device powers the USB port, not the other way around. On a computer there are multiple voltages, and some much higher power requirements, so the USB charger might cause some difficulty but could not come close to running anything. On the video recorder interfaces, there must also be a power supply that provides for the multiple functions inside the box, so the USB power must be at least somewhat isolated from various other parts.

I don't challenge your knowledge but could you make it clearer? Do you have solid reasons to believe there is danger or is it more just a feeling?


 

Re: extra USB power

Reply #28 –
Quote from: AndyH-ha on
... I don't challenge your knowledge but could you make it clearer? Do you have solid reasons to believe there is danger or is it more just a feeling?

The most likely scenario for damage is similar to the one you noticed where the power LED lit up. The circuitry powered by the 5v supply in the computer may draw several amps. The external power supply may overheat or fail if trying to supply more current than it is rated for. In some cases there is a fuse or other protective device for the USB port in the computer to prevent damage if the external device tries to draw more then the USB-specified maximum. The fuse or device may also be damaged if more than the permitted current flows from the external power supply.
There are so many possible combinations of circuit topology that it's hard to say just what will happen. If I said "go ahead, it's perfectly safe" and you blew something up, you would not be happy. So if, as you say, you're not sure what a specific combination of USB devices will do, it's better to be safe than sorry. Personally, I'd use a modern powered hub and be done with it.
Regards,
   Don Hills
"People hear what they see." - Doris Day