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Basic question about AC Adapters

Discussion in 'General Electronics Discussion' started by RhinoCan, May 1, 2011.

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  1. RhinoCan

    RhinoCan

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    May 1, 2011
    I apologize in advance if this question doesn't belong in edaboard at all. If it doesn't please tell me a better place to ask it.

    Can anyone tell me how to determine how many volts and milliamps an AC Adapter needs to provide if you have the device that needs the AC Adapter but no specs for the adapter itself?

    I am NOT an electronics hobbyist, just someone trying to help a friend. My friend has a pair of computer speakers but has lost the AC adapter for it. He would like to buy an AC adapter for the speakers but doesn't know exactly how many volts/milliamps it should provide. The information is not marked on the speakers and he long ago lost any documentation that came with it which might say what the specs of the AC adapter should be. I've googled the speakers in the hopes of finding a manual online or even tech support from the manufacturer but came up empty. These speakers are pretty old and date back to his first computer; I'm guessing the company is no longer operating. I believe the company (or at least the make of the speakers) is Quick Shot or something similar; I've forgotten the name since I was at his place a few days ago.

    I'm hoping there is a way to determine the proper AC Adapter, perhaps by using my very basic analog multimeter and touching the contacts to the jack where the AC Adapter gets plugged in. Or maybe a way to get "close enough" via trial and error using other AC Adapters. I have a variety of old AC adapters from dead devices and am hoping that one of them will work with my friend's speakers.

    I have nearly no knowledge of electronics at all so please keep the technical jargon to a minimum in your reply ;-)
     
  2. davenn

    davenn Moderator

    13,802
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    Sep 5, 2009
    greetings, welcome

    firstly this isnt the edaboard forum, its the electronics point forum. I also belong to the edaboard forum. ;)
    you are in the right place :)

    ok we need to establish if the adaptor puts out AC or DC to the amplified speakers.
    The only way you are going to do that is open up the speaker that the power plugs into. Once having done that take a good sharp photo of the circuit board (component side) so we can see if there is a bridge rectifier there. If there is, then you need an adaptor that has an AC output. If not, then a DC one.
    Lets get that bit sorted for a start and go from there :)

    Dave
     
  3. RhinoCan

    RhinoCan

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    May 1, 2011
    Thanks for your reply, Dave.

    We may not need to get a picture to figure this out. While the AC adapter jack did not have any volts or milliamps listed, I remember that it did say "DC" just below it. That should be a reliable indicator that it is DC, right? If so, what do I do next?

    By the way, sorry for the reference to Edaboard. I posted there first but wasn't sure if it was an appropriate place so just copied and pasted my post there to THIS forum. I forgot to remove the Edaboard reference.
     
  4. davenn

    davenn Moderator

    13,802
    1,941
    Sep 5, 2009
    no probs :) both are excellent electronics forums

    OK on the DC, hopefully it also indicates the polarity of the connector... if positive is centre pin ?

    most speakers would be either 9 or 12VDC at 0.5 - 1 Amp. connecting a 1A one doesnt matter as the unit will only draw the currect it needs. the voltage is the critical factor.
    I would try the 9V one and see if it works. It may work but sound cruddy/distorted if its not getting enough voltage.

    see how ya go

    Dave
     
  5. RhinoCan

    RhinoCan

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    May 1, 2011
    So, just to be clear, I should look at the speakers, see if there is a symbol showing whether the DC is the center pin? I'm not sure how to draw that symbol but its like a letter C with a dot in the center if I'm thinking of the right thing. Can you point me to drawings of the different possible variants of that symbol and what each variant means and then tell me which variant I'm looking for? I should probably report on exactly what the symbol is first before attempting to connect any AC adapters, right?

    What risk, if any, is there to the speakers - and me! - if I connect the wrong voltage to the speakers, such as giving it 9V when it actually needs 6V or 12V? Also, the worst thing that can happen to the speakers is that they get fried, right? No chance of fire, explosion or electrocution for me or damage to Doug's computer, right?
     
  6. davenn

    davenn Moderator

    13,802
    1,941
    Sep 5, 2009
    yes thats right looks like this .... just note the + and - may be swapped. + is usually the centre but not always....

    [​IMG]

    if its 6V and you put 9V into it it will probably survive, as it would if its 9V and you put 12V into it. No explosions, puter damage electric shocks etc
    as I said in a previous post, if you put 9V in and it really needs 12V it may just not sound clear, maybe the power on LED if it has one may not be usual brightness.
    chances are there may be a voltage regulator chip in there anyway.
    the unit is basically just a ~ 5W stereo amplifier

    If you have a 6V plugpack try that first and work your way up if you need to.

    Dave
     

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  7. RhinoCan

    RhinoCan

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    May 1, 2011
    I went over to my friend's place this afternoon and determined that the polarity on his speaker was the opposite of what is shown in your diagram, Dave.

    To make a longish story short, I tried the first of the spare AC Adapters I had lying around, which was a 9V 300 mA one, and it worked fine. Problem solved!!

    Thanks a lot for all your help!
     
  8. davenn

    davenn Moderator

    13,802
    1,941
    Sep 5, 2009
    excellent nothing like a bit of success :)

    yup as I said they can sometimes be wired opposite, some manufacturers are a real pain in the
    butt

    Dave
     
  9. nbw

    nbw

    48
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    May 8, 2011
    Do you have a DMM? That's a real easy way to tell what your adaptors are putting out :)
     
  10. RhinoCan

    RhinoCan

    7
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    May 1, 2011
    I'm guessing a "DMM" is a digital multimeter? If so, the answer is no. I have a very cheap analog multimeter. I'm not sure if that is "close enough" for your purposes.

    In any case, I wasn't actually trying to determine the output of an AC adapter. I was trying to find out what AC adapter to plug in to the speakers since I had no information on how many volts and milliamps it should be getting.

    It turned out that a 9V 300mA adapter worked very well on my friend's speakers and they are now working very well.
     
  11. nbw

    nbw

    48
    2
    May 8, 2011
    Yep - DMM is that (good for folks like me whose hands are old and tired) :) Pleased you got the speakers going!
     
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