5v Power Supply outputing 5.43v

Discussion in 'General Electronics Chat' started by CrAcKeZ, Feb 4, 2015.

  1. CrAcKeZ

    Thread Starter New Member

    Feb 4, 2015
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    Hi there,

    I'm new to this and honestly I don't know if you can help me. I could talk all day about me and my hobbies/projects but I'll stick to this one as I'm wondering if it can be fixed easily.

    I've ordered a power supply of 5v 4A to use it as a microusb charger(for any type of phone or tablet), the power supply has a round jack and I have an adapter to tranform it into a microusb. I got this Chinese power supply but when I measure it on a multimeter the reading(without load) was 5.42-5.43v(it isn't 100% stable but this is normal, it might be my multimeter, it's not a perfect one although I have a better one this is pretty accurate too)

    So can anyone help me about this? I'm good at soldering things but I don't know how can I fix this so it will output 5.0v or at least something that doesn't go as high as 5.43!

    I can upload as many pictures as you like(from every possible angle) ..just ask me(quality wise although not an expert on circuits... it's one of the worst I've seen - the actual output voltage on the board is not used(although it outputs the same voltage), and I don't see to be powered on(AC) from the right parts of the board too)

    [​IMG]

    [​IMG]

    [​IMG]
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Welcome to AAC!
    Can you explain "the actual output voltage on the board is not used(although it outputs the same voltage)"?
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,009
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    The output voltage is likely due to the normal (poor) tolerance of the output voltage due to normal component variations.
    It will drop slightly under load and probably will be about 5V with a 4A load.
    If you want to drop the voltage under light loads then you could add a 5A Schottky diode in series with the output.
    Unless you have a schematic there's likely no way to know how to lower the voltage otherwise.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I don't know of any components that are fine at 5 and explode at 5.42-5.43v. Most good phones will have an in-line diode and current control to monitor charge. The lithium batteries measure about 4.6 v completely charged so something saves them from the 5 volt USB adaptors.
     
    Roderick Young likes this.
  5. WBahn

    Moderator

    Mar 31, 2012
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    The output is within 10% and this is a pretty common spec. If you need something tighter, then you probably shouldn't be using a wall wart or you should be using something with a bit higher output voltage and then using a regulator to hold your 5V pretty tightly.
     
  6. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    There's your problem. ;)
     
  7. wayneh

    Expert

    Sep 9, 2010
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    I agree. A voltage measured without load and without confidence in the accuracy of the meter is not a reason to dig into a circuit board. Don't forget that the leads alone might drop enough voltage under load to require a few tenths extra volts at the supply to avoid being low at the device.

    I would be SO much happier to see 5.4V than, say 4.9V.
     
    absf likes this.
  8. CrAcKeZ

    Thread Starter New Member

    Feb 4, 2015
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    In the board theres a -v and a +v which for the output(which is not used ..cutted wires - check second picture at bottom)

    -------------------------------

    I hear somewhere that if voltage is so high it might reduce the life of the components in a phone(which seems about right, I mean I've got some other 5v power chargers outputting 5.0v, 5.43 seems much to me even if there's no load)

    No schematics sorry

    I didn't say it's not accurate(the multimeter), just not as good as the one I've got in shop. I'll test with that too and get back to you - still I don't think it will be much of a difference

    -------

    On the other hand I prefer to use 18.5v to a laptop that needs 19v rather than 19.5v (I'm a computer guy, my daily job)
     
  9. Evil Lurker

    Member

    Aug 25, 2011
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    I'm gonna go out on a limb and guess that little SOT-23 on the secondary is an SMD version of a TL431 and that either R25 or possibly R17 (cam't see that one though) are the feedback resistors that sets the output voltage. Basically if you want to tweak the output voltage (and if you are brave) you can jumper a high value pot across the lower resistor (the one connected to ground) of the shunt regulator's voltage divider reference and see what sort of effect that has on the output.
     
  10. SoftwareGuy

    New Member

    Oct 29, 2013
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  11. wayneh

    Expert

    Sep 9, 2010
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    My prediction: If you alter this charger to reduce the no-load voltage, your phone or tablet will cut out and reject charging from it because the voltage will sag below spec under load.
     
    Last edited: Feb 5, 2015
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Before you start changing components, load the supply with 10 ohms and see if the output voltage pulls into tighter regulation. Low cost supplies rarely have the tweaks needed to regulate down to zero load.

    ak
     
    KLillie likes this.
  13. CrAcKeZ

    Thread Starter New Member

    Feb 4, 2015
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    Thank you all guys, I'll try to load it a bit and measure the voltage (hope it's just a notch above 5v and not 5.40+) - sorry for the delay, daily job sucks my time :D

    I want to be sure so I won't damage my phone :p (probably not and the voltage will go down when in load but it's a risk I don't want to take, better safe than sorry)
     
  14. CrAcKeZ

    Thread Starter New Member

    Feb 4, 2015
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    Tried to edit my previous post but didn't see an edit option, maybe because I'm new here dunno. Anyway, after bringing my good multimeter measuring the power supply was 5.44v.

    I had a resistor laying around me for a different project that's 5w 40Ω(not ideal :p) so I put it on and measure it again, 5.41v with 0.135A draw(0.73035w), I need to draw more power to see if it falls near 5v because 5.41v is still high in my opinion.

    Now that I think of I might have some resistors hidden from some failed projects(never started or was too big for my usage), I'll try and get back :D
     
  15. CrAcKeZ

    Thread Starter New Member

    Feb 4, 2015
    15
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    I manage to find a 6Ω resistor ..pretty good load, so I've connect it and measure it final volts 5.26v and current with this 6Ω(50W it's big :p) resistor? 0.821A or a total of ~4.32watt

    Do you think it's still safe to use it in a phone/tablet? modern phones use at least 1A and some 2A so voltage might drop a bit more(maybe) ..if more power is needed(speculations)

    (Now I get an edit button alright :D )
     
  16. wayneh

    Expert

    Sep 9, 2010
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    Did you include all connection wire you plan to use in the final application? You may drop a few tenths more. If not, add a schottky diode to drop ~0.3-0.5V. Or you could use a low ohms resistor, for example 0.2Ω to drop 0.2V at 1A charging current.
     
  17. CrAcKeZ

    Thread Starter New Member

    Feb 4, 2015
    15
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    I just put the resistor nothing else, but I'm not planning on having the resistor in a permanent way

    About the diode do you mean something like this?

    [​IMG]

    Although not an expert on this my understanding is that this can be done with two ways but the resistor way although cheaper it will waste more energy(also increase heat as a result of that)
     
  18. wayneh

    Expert

    Sep 9, 2010
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    Yes, the diode would be placed as shown.

    Whether a resistor or diode, if it drops voltage it will dissipate the energy as heat. The advantage of the diode is that the ∆V is nearly constant, whereas the ∆V across the resistor depends on the current passing through it.
     
  19. DC_Kid

    Distinguished Member

    Feb 25, 2008
    638
    9
    what does the V read when you load it down say to 2 or 3 amps ?? my guess is you'll see the voltage roll down as the current goes up.
     
  20. CrAcKeZ

    Thread Starter New Member

    Feb 4, 2015
    15
    0
    most phones will draw somewhere between 500mA to 2A so in that range I need to be sure it's safe(voltage-wise), if I draw 3+ Amps it will probably be in a safe range.

    Other solution is to test a phone and see how it reacts ...any phone to spare? :p hhahahaa
     
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