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Wall adapter voltage issue

Discussion in 'General Electronics Discussion' started by bigone5500, Aug 24, 2014.

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  1. bigone5500

    bigone5500

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    Apr 9, 2014
    I found an old a/c wall adapter I could use on my arduino that has 12vdc, 1000mA rating on the case. When I plug it up and take a reading it shows 15.87 volts. Is this normal? I'm thinking that if I put it into service, then the voltage will drop to the rated 12vdc...but I don't know enough about it to be sure.
     
  2. shumifan50

    shumifan50

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    Jan 16, 2014
    put a load on it and check the voltage. Use a 12V light bulb.
     
  3. bigone5500

    bigone5500

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    Apr 9, 2014
    Will do, thanks.
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    That is normal for the old type of adapter that uses a heavy transformer that steps down the AC mains voltage directly. These adapters don't have any regulation, so the output voltage varies quite a lot depending on the load current.

    The marked output voltage applies when the maximum rated current is being drawn from the adapter. Even then, it is only approximate, and variations in the AC mains voltage will affect the output voltage. When there is no load on the adapter, the output voltage can be 50% higher than the marked voltage - sometimes even more!

    These adapters also have ripple on the output voltage, at twice the AC mains frequency. This won't be a problem for an Arduino though.

    Modern switching adapters are fully regulated and are better if you want an accurate output voltage. They generate noise, and this makes them unsuitable for some types of loads - AM radios, for example - but they are a better option. They are also much lighter, and often cheaper, than metal transformer types.
     
  5. bigone5500

    bigone5500

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    Apr 9, 2014
    So this will not be an issue for use on an arduino? Even if I am only pulling a couple hundred mA?
     
  6. bigone5500

    bigone5500

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    Apr 9, 2014
    I applied a load to the adapter. The load used is a small bulb rated at 14.4vdc at .12A. The reading before load is 15.8vdc and while load applied is 14.3 volts at .125A.
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Not likely to be a problem. Which model Arduino are you using? Can you post a link to the technical information for it?
     
  8. bigone5500

    bigone5500

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    Apr 9, 2014
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    By "technical information" I meant a schematic diagram.
     
  10. bigone5500

    bigone5500

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    Sorry...I don't know where to find this...
     
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK. It's very unlikely there will be any problem.

    If the board uses the 12V rail for something other than generating the logic rail (using a regulator), that other function could be affected by the voltage being too high. But I don't think it would.

    The other concern is the regulator that drops the 12V rail down to the logic supply voltage. This regulator will have to drop more voltage than normal. If it's a linear regulator, that could cause it to get hotter than it should. Power dissipation (i.e. heating) in a linear regulator is equal to the voltage across it multiplied by the current through it. So if you're only drawing a few hundred mA from it, and it has some kind of heatsinking (even just a large copper area on the board), it should be OK. If possible,

    try to find the regulator on the board and see how hot it gets after say 10 minutes of operation. If you can hold your finger on it indefinitely, there's no problem.
     
  12. bigone5500

    bigone5500

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    Ok, thanks Kris. I will take a photo and post it with my phone. Maybe you can tell something by that. tmp_32249-20140824_162723-1676455737.jpg
     
    Last edited: Aug 24, 2014
  13. bigone5500

    bigone5500

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    Apr 9, 2014
    These two are of the adapter. It has a 2200uf 25v cap. Could I change the cap and reduce the voltage? Or is that controlled by the rectifiers? The xfmr output is 12.19vac. The diodes are RL201, datasheet.

    tmp_2536-20140824_163425676455737.jpg tmp_2536-20140824_163439-1020284031.jpg
     
  14. davenn

    davenn Moderator

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    Hi Jared,
    no, changing the cap or diodes wont make any difference

    As Kris said earlier, this is an unregulated PSU. They always read a few volts higher till they have a load applied to them
    if you REALLY want to make sure the board always receives 12V then use a regulated 12V plugpack, they are readily available

    cheers
    Dave
     
    Last edited: Aug 24, 2014
    KrisBlueNZ likes this.
  15. bigone5500

    bigone5500

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    Apr 9, 2014
    Thanks dave. That brings me to a final question regarding this. I have a small computer psu that I turned into a testing supply that contains only a pigtail with a molex connector for IDE hard drives. I switch it on with a toggle switch. It outputs 12v at 1.5A max. Would it be a bad idea to power the arduino with this?

    Thanks for the help.
     
  16. davenn

    davenn Moderator

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    shouldn't be any problems there
    just confirm with a multimeter that the 12V rail is indeed 12V ± 0.5V

    cheers
    Dave
     
  17. bigone5500

    bigone5500

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    Apr 9, 2014
    The output is 11.51v. Nice!
     
  18. davenn

    davenn Moderator

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    you should be good to go :)

    if you look at that pic you did in post #12 you will see a 5V regulator just to the right of the power input socket
    There may well be some parts of the board that require 12V and other parts that need 5V

    cheers
    Dave
     
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