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ac to dc adapter help

Discussion in 'Electronic Basics' started by geodav, Oct 12, 2003.

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  1. geodav

    geodav Guest

    I want to convert ac to 6 volts dc for use on a portable radio. It is
    recommend that I use one that is rated at 1000mA. I have a converter
    that is rated at 6 volts but only 500mA.
    What difference does it make if I use the one I have. Will it harm the
    equipment? What does the mA rating mean anyway?
  2. A supply is rated for some maximum load current while still providing
    the rated voltage within some tolerance. If you use an under
    (current) rated converter, you can expect the voltage to sag below
    rating and excess heating during peak current demand. For an audio
    amplifier, this occurs during the loud parts of the program. I doubt
    that the voltage sagging below 6 volts will do any harm to the radio,
    but the audio quality will suffer. As long as you check that the
    supply is not too hot, and don't crank the volume up, you might get by
    with this converter. If you have a volt meter to monitor the supply
    voltage (during operation), and can be sure it does not spend a
    significant fraction of the time forced below 6 volts, then it is
    doing pretty well.
  3. I'd also like to add to John's post - you run a very real risk of destroying
    the adapter. Not only do you risk blowing out the transformer windings,
    you could also exceed the rated current of the diodes in the rectifier

    I would say if you have any choice at all - don't do it.

  4. SHAUN

    SHAUN Guest

    so, Russell can you explain how mA relate to voltage sag?

  5. One thing to remember is that some wall bricks have fuses in them, which
    will render them inoperable if you exceed their specs... You can't fix them,
    you have to throw them away.

    I'd just buy another one. They are fairly cheap. Here is one for $4 (plus
    shipping, of course.)

    (I've destroyed at least one big 1.5A brick by drawing too much current.)

    Bob Monsen
  6. Steve

    Steve Guest

    mA (milliamps) is a measure of current. It seems you don't yet know
    how ohms law works? Suggest you see the other thread in this
    newsgroup with the subject "I need to learn this once and for all..."
    for some good explanations of volts, amps and ohms.

    A good analogy is to think of an electric circuit as a garden hose.
    The difference in pressure between the ends is akin to voltage. The
    amount of water flowing through the hose is akin to current. The
    diameter of the hose is akin to resistance.

    Say you have a DC supply of 10 volts at 1 ampere. This supply runs at
    full spec when attached to a 10 ohm load.

    ohms law is 'volts = current * resistance'
    so it can be seen that 10 volts = 1 ampere * 10 ohms.

    Imagine that suddenly the resistance drops (hose gets wider.) The
    same voltage (10V) feeding a smaller resistance (say 5 ohms) wants to
    provide more current;

    10 volts = X ampere * 5 ohms - rearranging you get;
    X ampere = 10 volts / 5 ohms. ie; X = 2 amps!

    The load wants to draw 2 amps from the 10 volt supply. However, if
    your supply can't handle 2 amps, or is limited to a maximum of 1 amp,
    then physics demands that the voltage drops;

    X volts = 1 amp * 5 ohms
    hence, putting 1 amp through 5 ohms results in 5 volts.

    So, one of two things will happen. You DC supply will TRY to provide
    more current, and it may destroy itself trying. Or, the supply
    voltage will drop due to the limited current that is allowed to flow
    from the output.

    hope this helps!

  7. Steve

    Steve Guest

    google groups are stuffing up, not sure if my previous post made it,
    so here's the abridged version;

    see the other thread in this group "I need to learn this once and for
    Search google for ohms law and electronics basics.

    basically, if the supply is limited to providing 1 amp at 10 volts for
    example, if you try to source more current, one of two things will

    1; the supply s#@ts itself trying to give you more current.
    2; the supply gives you the current, but the voltage drops as a

    ohms law will explain this nicely, but only if you take the effort to
    understand current, voltage and resistance and how they interrelate.

  8. SHAUN

    SHAUN Guest

    mA (milliamps) is a measure of current. It seems you don't yet know
    how ohms law works? Suggest you see the other thread in this
    newsgroup with the subject "I need to learn this once and for all..."
    for some good explanations of volts, amps and ohms.

    Thank you Steve, that analogy really helped!

  9. Chris

    Chris Guest

    You probably don't want to do this for several reasons. First, as was
    mentioned elsewhere, you might exceed the forward current rating of
    the diodes (unlikely in this case, because almost all the low voltage
    wall warts use the 1N400X for lower current (below 1A). I suppose you
    could also be exceeding the ripple current rating for the cap, leading
    to overheating.

    But the main reason you want to avoid going overcurrent with a wall
    wart is that the transformer is designed for that current. The mass
    of iron, as well as the wire gauge of the wire on the primary and
    secondary, are specified with the rated current in mind. You may have
    noticed that transformers which are rated for higher VA (volts * amps)
    are heavier, and have thicker gauge wire.

    When you exceed the rated current on a transformer, several bad things
    happen. First, the bulk resistance of the wire in the transformer
    leads to a greater voltage drop across the wire, meaning lower output
    voltage and more heating. The second problem is that, during the peak
    periods of the AC waveform, you saturate the transformer. That means
    voltage sag and more heating (and major output clipping and
    distortion, as was mentioned earlier). The net effect of this will be
    that, when you crank up the volume on that favorite Caruso arpeggio,
    your wall wart transformer will overheat and open up. Maybe not right
    away, but it will happen. Wall warts that are made to be UL/CSA or
    other safety agency registered are guaranteed not to start a fire
    under overload conditions, but that doesn't mean they'll survive a
    constant overload. Also, when the wall wart pops, there may be a
    momentary voltage spike transient that may damage your equipment.

    A good rule of thumb is that if you're replacing a wall wart, get one
    of the same voltage with either the same or slightly greater current
    rating. For your job, you can get a 6V one with 1 to 1.5A current

    By the way, make sure not only that the wall wart has the same type
    plug, but that the plug has the same polarity as the one you're
    replacing. Otherwise, you may be hooking up the power backwards,
    which is almost guaranteed to fry your amp.

    Good luck.
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