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win8lee

USB charging whilst using USB

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hi all,

 

I've quickly realised the down side of charging using the USB port is that it's difficult to use it for USB devices at the same time. So I've been investigating using it for both.

 

Testing various USB hubs/adapters and male micro USB connectors cut off the cables, I've found certain male micro USB connectors (cut off a charging cable) tell the tablet not to put out 5v and other types of male micro USB connectors (cut of an OTG adapter) tell the tablet to put out 5v (and thus power a hub etc).

 

Does anyone know the difference between these 2 types of male connector? Does one type had the ID pin grounded within the connector?

 

The problem is, using 1 type with a hub allows be to charge the tablet and use the hub, but when I'm not charging, the hub is not powered

 

Using the other type, the hub is powered from the tablet, but I can't charge the tablet. If I simply apply 5v to the + and - of the tablet micro USB connector, it still doesn't accept charge. It only outputs the 5v.

 

So, how can I tell it to output 5v to power the hub (and any USB peripherals) whilst also accepting charge when it's available?

 

Thanks

 

 

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I think we all would like the answer to that.
From what I understand, USB OTG doesn't accept a charge when in host mode, as it's supposed to be providing power, not accepting it.

You can have adapters for OTG work in data-only mode where the USB device accepts external power, if you're plugging in something that's power-hungry and you don't want it to drain your battery.
I use one of these for plugging in my USB radio stick on my phone as they draw a bit of juice and it would drain my battery really bad.

 

Once the port's in OTG host mode though, it won't accept charge. Trying to charge it is reversing the flow of electricity and could damage the device.

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Interesting. I'm not entirely sure how that works.

E: If you click "Item Description" then you get a pretty good rundown of the device, and they show it working with the V975i running Win8.1 (same as V975w)
mcgrah, I think you've found the effing holy grail here :D 

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ok, so I've chopped up most of my collection of micro USB plugs and sockets to work out what's what!

 

You probably know this already;

 

to charge the tablet, the ID pin needs to be grounded. How is this done with a charging cable which only has 2 wires? It's already shorted in the micro USB connector.

 

ok, so now I test a OTG connector, which doesn't have the ID pin grounded, however it has 5 wires in the cable (the extra yellow is meant to be the ID wire). If I short this to ground, it should turn it into a charging cable.....nope, why? Because the yellow wire is not actually connected to the ID pin!

 

I have a selection of connectors and sockets coming for further testing, but I think the solution is simple;

 

The issue is -

using an OTG cable to a USB hub provides (ID pin not grounded) causes the tablet to output 5v to power the hub and USB devices.

using a charge cable to a USB hub (ID pin grounded) causes the tablet NOT to output 5v, so the hub and devices are not powered. Splicing in a 5v power source powers the hub and charges the tablet. This is what we want, but we also want the hub to be powered when the charger is disconnected.

 

This could be done with a simple relay or FET. When the charger is connected the ID pin is shorted to ground, when it's not connected, the ID pin is open circuit. Note the charger has to be around 2A to charge the tablet and power the hub/USB devices.

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At LEAST 2A. The stock charging brick is 2.5A labeled.
I've not been able to find an alternate charger of 2.5A or higher for USB output. Closest I got was one that claimed it could put out up to 4A on a "fast charge" port but it actually charges worse than the stock brick!

 

I'm always on the look for something that will give good clean power for charging. The original iPad chargers are a good example, but they don't put out quite enough power for this.

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A relay would be far too oversized. Unfortunately, a FET would best work in a reverse scenario. We would want the gate to conduct when not energized.

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Have you noticed this from Onda pages: Onda V975w Tablet build in Micro USB Port and support Charging and OTG function.But do not support Charing when use the USB OTG function.

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Koomikko, this is essentially true according to USB spec. You're not supposed to be able to charge and use OTG simultaneously.

What these are doing is basically tricking it into charging while using OTG.

 

The first link actually shows it working on the V795i/w.

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great, so when you unplug the charger, do all the USB accessories remain connected?

 

I've got one coming too!

 

I actually made one too, using a little switch instead of a relay to simplify things. The hard part was finding a micro usb cable with all 5 pins connected which is very rare!

 

I used a micro USB hub and swapped its micro USB connector for the 5 pin version, then connected a mini switch from ID to gnd. Wit the switch closed the tablet supplier 5v to the hub, with it open, the charger supplies 5v to the hub and tablet and charges it.

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great, so when you unplug the charger, do all the USB accessories remain connected?

 

I've got one coming too!

 

I actually made one too, using a little switch instead of a relay to simplify things. The hard part was finding a micro usb cable with all 5 pins connected which is very rare!

 

I used a micro USB hub and swapped its micro USB connector for the 5 pin version, then connected a mini switch from ID to gnd. Wit the switch closed the tablet supplier 5v to the hub, with it open, the charger supplies 5v to the hub and tablet and charges it.

 

Electrically, it's quite possible that the circuitry inside the tablet can handle switching like that. It's no different than switching between DC power and battery. Still, without an oscilloscope, it's hard to say if there will be a slight power disruption during switchover or not.

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ok, just picked up mine from DHL. I confirm it works like the one I made. It's a 4 port hub and you have to manually switch between charging+OTG and just OTG. There's a 3rd position for the switch which doesn't appear to do anything different.

So if you are charging and using OTG, when you disconnect the charger, the hub is powered down, you have to flick the switch, so the tablet supplies the 5v instead. It would have been nice if it automatically switched over.

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