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Bipolar supply from switching wall wart?

Discussion in 'General Electronics Discussion' started by oneoldude, Jun 16, 2013.

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  1. oneoldude

    oneoldude

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    Jun 16, 2013
    I am getting a couple tiny analog audio power amp kits from ebay. The kits use the TDA2030A chip and require a bipolar supply. Suggested is +- 12V DC of a couple amps. Probably not more than 3A. I want to use the amps in the shop to test and measure speakers. The little guys will be easy to move around.

    I have access to a bunch of identical inline switching power supplies that are 12V @ around 4A. I was wondering if I could use two of them with the outputs configured in a bipolar manner with some supply caps to settle everything down. I am concerned about the grounds through the switcher to the bipolar side especially since the switcher line plugs are three prong. If they were old time analog jobbies I would cut them open and use the transformers. But I know nothing about switchers. I drew up a schematic but don't have a way to post it.

    BTW, has anyone used a Tripath or equivalent amp to measure speakers? I have one of those little boards as well but the output grounds are not connected to chassis ground because each channel is internally in bridged configuration. I would use that (it too needs a bipolar 12V supply) but I fear the lack of proper ground could damage my sound card or my computer or give me false FR measurements. Any thoughts?

    Man am I tired of lugging around a big stereo amp when all I need is 2 or three moderately clean watts of audio power to measure a speaker.

    Any ideas are surely welcome.

    Thanks
     
    Last edited: Jun 16, 2013
  2. gootee

    gootee

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    Jun 16, 2013
    One test that you could do, with the supplies plugged in but turned off is measure the resistance from the 0v of one supply to the 12v of another supply..

    If the result is a low resistance, then it appears they can NOT be used as a bipolar -12-0-12 supply.

    If the result is not low resistance, then they still might not be able to be used that way.

    Another test, which would be a little more definitive, would be to connect the 12v of one supply to the 0v of the other supply, using a resistor of at least 1K Ohms. Then turn on the supplies. Measure the voltage across the resistor. If it's 12V, you're probably out of luck.
     
  3. oneoldude

    oneoldude

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    Jun 16, 2013
    I like your second test. It goes right to it.

    Could check resistance between ground on ac plug and 0 out. I if R = 0 I'm done.

    Then again, I might be able to find some wart that does not use a grounded AC plug. But their plugs are probably keyed to common so the same problem arises. Hmm.

    I checked the net and could not find any reference to this question. That surprises me because there are so many switched supplies laying around everywhere that end up in the trash. It would be great if they could be used for something like making a bipolar supply.

    Thanks
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,490
    2,832
    Jan 21, 2010
    Quite often the small switchmode power supplies are completely floating (many have 2 pin mains plugs).

    These can be connected as you wish.

    If the power supply has an earthed plug then it pays to check.
     
  5. oneoldude

    oneoldude

    14
    0
    Jun 16, 2013
    Well I went to my source today and they have a bunch of supplies all right. And they are all identical. But the voltage is too high and the current capability is too low. I guess I will unwind one of my toroids.

    Thanks guys.
     
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