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Thread: How Do I Test a AC Power Adapter?

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    Default How Do I Test a AC Power Adapter?

    I'm tryin to test an AC power adapter for my modem (Billion 7404VGO) so that if it is the PSU they will just send me a new one and save both me and the supplier a lot of hassle.

    I have a multimeter but I'm not sure what the best was is to go about testing it. Can someone please tell me? Thanks



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    what are you testing for?, output? put meter to dc volt say 20 and test output lead, with unit plugged in an turned on, a short? test both leads with meter set to ohms, disregard the earth and make sure its not plugged in

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    measure the resistance of the 2 ac pins. should have very low resistance. if yes, your primary winding is ok.

    then measure the resistance of the output plug. between the + and - terminals. again you should have a very low reading. if yes, your secondary winding is ok.

    then connect the power supply to a power point, and check the output voltage with multimeter set at volts dc.

    make sure power point (gpo) is working by connecting something else like a lamp to verify.


    if you have a faulty power supply usually the thermal fuse blows on the secondary winding, you can tell this has happend if you have high resistance reading on the second test mentioned,



    hope this helps

    cheers

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    I'm testing just to see if it is working like it should. e.g. if it will run my modem the way it is.

    What would be a low resistance?
    I set the ohms to 200 and got a reading of 128

    For the second test I set the ohms to 2000 and got a reading of 1361. I put one terminal from the multimeter on the inside of the plug and the other on the outside. Is that right?

    How do I check the output voltage for the third one? Do I do the same as the second? I don't want to blow myself up as I have done before lol.

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    Quote Originally Posted by Buttsack View Post
    I'm testing just to see if it is working like it should. e.g. if it will run my modem the way it is.

    What would be a low resistance?
    I set the ohms to 200 and got a reading of 128
    seems high, should get about 10 -15 ohms
    For the second test I set the ohms to 2000 and got a reading of 1361.
    seems high again, should be under 10-20 ohms
    I put one terminal from the multimeter on the inside of the plug and the other on the outside. Is that right? yes

    How do I check the output voltage for the third one? Do I do the same as the second? yes, with the meter set to voltsI don't want to blow myself up as I have done before lol.
    im assuming its a transformer type power supply. if its a switch mode power supply all you can do is check for output voltage. if no output voltage , is faulty and return it

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    For the third test I get 0.35 when it is set to 20 and I get the same reading when it isn't plugged in. So does this mean that the AC adapter is stuffed and I should get them to send me a new one out?

    Sorry mate just re-read your post. Thanks heaps for your help I understand it a lot more now. I'll be getting them to send a new one out. Thanks

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    OK, i get the resistance test on the primary and secondary connections of the supply, but you will only get a resistance measurement on the secondary of the supply if it is AC output, a DC ouput will have a diode in series and a cap in parallel so ohms measurment will not work(Switch mode won't give you a read at all).
    If the output is AC (and some modems use an AC supply eg Bitblitzer) than measuring DC will not give you a read anyhow!
    What we need to know is if on the case of the supply it will give output in voltage and either a symbol which is hyphens over dots (DC) or the ~ symbol which is AC, then you can tell what resistance and voltage type of reading you will get.

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    Simple test:

    Plug it into the mains, switch your multimeter to AC volts (20 range), put one probe into the end of the low voltage plug, the other on the outside of the barrel and see if the meter reads about halfway or more.

    If it doesn't, switch the meter to DC volts (20 range) and try the same test.

    If there's still nothing - it's poked.

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    Even simpler test:
    Get a 12V light bulb from you car (ideally 12W but 5W will do) and hold it with a bit of wire on the plug. It should glow nice and bright.
    Unlike all the recommendations above you are testing it properly under load.
    That is what really counts.
    My Billion 6404VP uses a very basic unregulated 12V 1A DC supply that you can get anwhere. The polarity is written on the supply and the modem (barrel outside minus)
    If I hold a multimeter over the plug without load, I read 15.8V if that helps.

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    I agree with you on the load test, but not any car bulb will do, ie , if the supply is 12V 1 Amp, and you use a 21 Watt (common) globe you will be trying to draw more current than the supply was designed for (about 1.75 Amps), so make sure the load test reflects the operating conditions !
    Common sense isn't !!!

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