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Can a common 9V power supply be used as a -9V supply?

  1. Mar 13, 2009 #1
    So if you have something like this:

    can you use it as a -9V supply?

    My circuit requires a +9V and a -9V supply, and since im having issues finding fixed +/-9V supplies, i was wondering if i could just use a 9V supply for the +9V, and another 9V supply as the negative?

    And then could the same be done with a 5V power supply like this:

    to get a -5V supply?
  2. jcsd
  3. Mar 13, 2009 #2


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    Staff: Mentor

    Yes, those are transformer-isolated power supplies, so you can stack them to make split supplies.
  4. Mar 13, 2009 #3
    It is perfectly safe to use one as a minus supply if the unit does not have a 3-pin power plug. I have found that some units have a 3-pin power plug with the ground connected to the outside shell of the dc power connector. One problem you will have (besides isolating the shell of the neg supply plug) is that unless you switch the common AC power, you will be turning on the plus power before or after you turn on the minus power. This assymetric power application can sometimes damage circuits (like when the minus supply provides bias to bases on discrete transistors). The 9-V unit you are looking at has a wimpy (100 mA) current limit. You can probably find 1 amp plug-in units for a few $ more.
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