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DC transformer power supply question, 9V dc too much ?

Discussion in 'Troubleshooting and Repair' started by ElecTedII, Sep 20, 2012.

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  1. ElecTedII


    Sep 20, 2012
    Hi ALL!
    I have this Omron Blood Pressure machine that I use on a daley basis. I've had it for awhile, the cheap plastic leed has twisted itself off, right at the base of the plug in AC to DC low voltage Xfer.
    I just repaired it, maybe for the last time, because I'm down to hairs to solder on to.
    The question is, as with many of you, I have plenty of AC-to-low-voltage-DC power supplies, hanging around doing nothing!
    The Omron power supply puts of 6V DC, 500 ma.
    The power supply I have with a good leed puts out 9VDC, at 500 ma. The qustions is, will that kill the BP machine? Or will it ONLY use what it needs, and the extra 3VDC won't matter ?
    Many thanks, ElecTed II
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I wouldn't advise it.

    If I couldn't find a 6V 500mA power supply, I'd go for any power supply rated for 6V and more than 500mA first, then maybe I'd try a 5V power supply rated at 500mA or more.

    Do you know if the current power supply is a switchmode supply? If it is, it normally light, and has a wide input voltage range (like 90 to 220V). If it's heavy and specifies a single input voltage (say 120V) then it has a transformer in it (and it is -- at best -- a linear power supply).

    The above is important. If it has a transformer, it may not be well regulated. If it's a switchmode supply it likely is.

    It is almost always going to be OK to replace a power supply containing a mains transformer with a switchmode one, but going the other way may cause problems in some cases.

    edit: It's the extra current it won't use unless it needs it, not the voltage
    Last edited: Sep 20, 2012
  3. ElecTedII


    Sep 20, 2012
    Got a pic of Xfer

    Hi **Steve**,
    Thanks for the reply.
    Don't quite know whata switchmode Xper is. So I decided to take a pic of the Omron Xfer. You know....a picture worth .....

    Many Thanks, ElecTedII

  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Note the input voltage. It's a single voltage, not a range. This power supply has a transformer in it.

    If you have a multimeter, can you measure the output voltage? (I want to see how close it is to 6V. That will tell me more.
  5. duke37


    Jan 9, 2011
    12W in, 3W out. Almost certainly a lossy linear supply.
  6. ElecTedII


    Sep 20, 2012
    DCV out

    I've got 6.37 coming out.
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    OK, that means it is at least minimally regulated.

    You probably need something between 5.5 and 6.5 volts at 500mA or more.

    Before you plug it in, make sure the voltage is not more than 7V.

    This should ensure you won't break anything.

    Oh, and make sure the polarity is correct.
  8. CocaCola


    Apr 7, 2012
    FYI you can get these wall warts on Ebay for a few bucks, or you can go to Radio Shack and get robbed at about $30...
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