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Mains adapter (SPN 4079A) and voltage with no load

Discussion in 'Electronic Components' started by Joe Smith, May 15, 2005.

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  1. Joe Smith

    Joe Smith Guest

    I have a mains adaptor (marked as "model SPN 4079A") which is made in
    the UK and which is marked ... "OUTPUT 12.5v 400mA".

    I plugged it in the mains here in the UK and measured the voltage at
    the end with the round concentric applicance plug. It was about 17.3
    volts!

    This seems rather a too high. I am worried this will damage a unit I
    have got which needs 12 volts and which I was going to use this
    adaptor for.

    Could this high 17.3 volt reading be entirely due to the lack of load
    on the adapter or is 17.3 volts too high even for that?

    Unfortunately I forget where this SPN 4079A adaptor came from.
    Google is not my friend either! :-(
     
  2. If the appliance requires a regulated adaptor, then it may well damage
    the appliance. What is the appliance? Someone might be able to say
    if it needs a regulated supply or if it includes it's own internal
    regulator, or doesn't care about having an unregulated supply.
    Yes, it's an unregulated adaptor. It probably approximates rather more
    to a 17.3V power supply in series with a 12ohm resistor (which would
    drop the voltage to 12.5V at 400mA), but that's a rather simplistic
    model of an unregulated PSU.
     
  3. Very likely with no load and not regulated.
    You need to load it with a resistor so it draws the same sort of current
    as the unit you wish to run it with. And check again. If it's a very low
    current device, it may well still be too high. But a regulated 400mA 12
    volt wall wart costs less than a tenner from Maplin, etc.
     
  4. alitonto

    alitonto Guest

    Once I was in a similar situation.
    If you can, connet the item to a 12 v car battery and measure the
    current draw.( the battery will have 12 to 13.5 v and provide whatever
    current is needed)
    Then if it is about 400ma then it would be ok to use the adaptor
    because anything drawing about that current will cause the voltage to
    drop to about 12-13 volts.( as per the specs of the adaptor).
    Ali Tonto
     
  5. mc

    mc Guest

    What you are seeing is quite normal for an unregulated adapter. Use it with
    a load of 300 to 400 mA and you'll get the right voltage.
     
  6. Yes, but most devices have fluctuating amperage demands, thus max 400mA and
    when idle less.Only a pocket light will absorb a constant current.That's why
    regulated power supplies were invented.
     
  7. I had an adjustable power supply, from 1.5 to 12 V, which in 1.5 V outputed
    7.2 V!Definitely get a regulated power supply they are not that expensive.I
    got an 6 V for a radio for less than 12 euro, or you can construct one, if
    you have got a soldering iron and can do a little soldering.email me for
    schematics.Devices like cell phones must be having their own regulation,
    because their power supply has only rectifiers.
     
  8. That's why circuitry that is very tolerant of wide voltage changes was
    invented. Especially considering that a battery operated radio might
    start out at 9V and have to operate clear down to 7 or 6V before it
    quits. It's just the nature of batteries.
     
  9. Robert Baer

    Robert Baer Guest

    Not true!
    *All* unregulated "wallwarts" have an output voltage higher than the
    "spec" even at full load.
    Measure some and find out for yourself.
     
  10. And just exactly _what_ axiom do you base your unequivocal statement on?
     
  11. mc

    mc Guest

    *All* unregulated "wallwarts" have an output voltage higher than the
    All generalizations are false, including that one. I've found some that run
    low.
     
  12. mc

    mc Guest

    *All* unregulated "wallwarts" have an output voltage higher than the
    .... However, there is a kernel of truth to what you say... the rated voltage
    tends to be treated as a minimum, so they're probably more likely to be high
    than low. But the ratings are very inexact.
     
  13. Robert Baer

    Robert Baer Guest

    Fact, supported by more measurements than you would want to do.
     
  14. Robert Baer

    Robert Baer Guest

    Plwase specify brand, make and model and/or where i could buy one of
    those wierdos.
     
  15. Robert Baer

    Robert Baer Guest

    One of the reasons they are so high, appears to be that they aer
    "designed" for 110VAC input, and power here in the US seems to be
    semi-universal 123VAC.
     
  16. mc

    mc Guest

    But Japan, as I understand it, is still 100V. I wonder if they are intended
    to work OK there even though nominally designed for America.
     
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