Tablet PC maximum volts

Discussion in 'General Electronics Chat' started by Man_in_UK, Jul 31, 2013.

  1. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    I am having problems in powering a Sumvision Cyclone Voyager 10.1 tablet PC. Space restrictions require the 5v PSU cable to be extended by 3 meters.
    This extention is causing a slight voltage drop due to resistance and making the tablet not fully charge. I have decided to build a PSU to overcome this problem

    Can anyone suggest what the maximum voltage would be into a device such as this?

    I cannot find any data sheet that would cover such information but I cannot help in thinking that there must be a regulator inside it. Would you risk
    6 or 8 volt source or do I play safe with 5.5.
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    3m of copper doesn't have all that much drop! Are you sure there is no digital communications between the charger and the tablet to establish the charge current?
     
  3. wayneh

    Expert

    Sep 9, 2010
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    I think you're solving the wrong problem. The cable is either flawed or your interpretation is not right.

    A mV voltage drop would not make the difference. I speculate the extension may not have data conductors, just power, and therefore your device will not charge because it doesn't see what it needs on the data lines.
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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    The max length of cable for the usb standard is 5 metres.
    This is set by the conductor data requirements, not the power conductors.

    So 3m should be fine.

    As others have already said, make sure your extension is fully wired.
     
  5. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    When I first made the extension cable it was a male to female lead to extend the two pin plug that came on the PSU. At the time I did the maths to find that the volt drop should be nothing to worry about, but when it came to use the extended PSU lead it did not work.

    A quick fix was to take out the mid point plug and socket, solder the extension wire directly to the PSU wire and trim the wire back as much as possible. It will power the tablet but not very well.

    The company who are fitting the tablet got round this problem by building their own 5v dc-dc converter in miniature and mounting it close to the tablet. It does appear to be working correctly but .... I would like to move this new dc converter farther away and adding a 3 meter cable is causing the same problem again. The tablet will power up but the battery indicator is showing that it is not charging. The company fitting the tablets are adamant that my 3 meter cable is the cause.
     
  6. BobTPH

    Active Member

    Jun 5, 2013
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    Are you talking about a USB type cable, or does this tablet, unlike all others I have seen, use a barrel type 2-wire power connector, which you seem to be implying.

    Bob
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    So is your system not usb?

    Please tell us the detail of the connections etc.
     
  8. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Last edited: Jul 31, 2013
  9. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    This is not a USB type, just a plain old 2 pin. Positive pin and negative sleeve.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    What wire did you use for your cable? Perhaps your cable conductors really are too thin.
     
  11. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    The wire used is twice as thick as the standard wire that comes on the PSU. Going any bigger will not help.

    I would like to know if anyone has got any ideas as to what power input circuit a tablet PC would use. If it has a built in regulator I may get away with an extra volt from the PSU without much trouble.
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    That is not what my links in post #8 suggest, have a really good look at the jack on the end of the psu lead.
     
  13. wayneh

    Expert

    Sep 9, 2010
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    It's time for pictures. Something about your extension wiring is screwed up. Maybe more eyeballs will help diagnose it. Be sure to include the jacks in the pictures. We shouldn't be made to guess and do your research for you.
     
  14. BobTPH

    Active Member

    Jun 5, 2013
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    I suppose it is possible that the added inductance of the longer wires are messing up the circuits inside the tablet. If it is feeding a switching regulator right away, there would be a voltage drop whenever the switch was turned on, and the additional inductance would make this larger.

    Bob
     
  15. Man_in_UK

    Thread Starter Senior Member

    May 13, 2008
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    I cant do pictures just now but I do have more information.

    The standard charger that comes with the unit is 5v at 2amps.

    I have cut the cable off this charger to be left with 90cm of two core cable. I can't say exactly what gauage the wire is, but it is not very thick. Terminated with a 2.5 x 0.8 jack plug.

    I connected this lead via a 3 meter extension of 6 amp 24/0.2 two core onto a 5v 5amp supply.

    Sure enough, when the tablet is connected I get a 0.5 volt drop at the connection point. It does appear that the current demands of these things is enough to be effected by a few ohms of extra cable.

    I think my only solution now is to force one of these things open and see what sort of input cuircuit is in use.
     
  16. BobTPH

    Active Member

    Jun 5, 2013
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    3m of 24 ga wire is about 0.25 Ohms. At 2 A that will indeed drop 0.5V. Use heavier wire and replace all of the wire to eliminate the drop in the original wiring.

    Bob
     
  17. Abovethelaw

    New Member

    Aug 2, 2013
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    This is a common problem. Oftentimes, system designers will bump up the output voltage to, say, 5.2V or 5.4V to account for the drop in the cable. Without this, you can easily drop to 4.5V before hitting the tablet connector and not give enough headroom to the single cell battery charger. You should be safe with 5.5V. I definitely wouldn't go to 8V.
     
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