geodav said:
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?
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
rating.
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.
Chris