Re: Transformers
- From: et472@xxxxxxxxxxxxxxxxxxx (Michael Black)
- Date: 14 Aug 2005 02:15:18 GMT
Kitchen Man (nannerbac@xxxxxxxxx) writes:
> On Fri, 12 Aug 2005 21:19:45 -0700, "Rodney" <rdavies@xxxxxxxxxxxx>
> wrote:
>
>>I've noticed that a transformer with, say, a voltage rating of 10 volts can
>>sit at a considerable higher voltage than 10 volts if it is not under load.
>>Once a current is being drawn from it the voltage will drop to that
>>specified. I was thinking that a high wattage transformer, even though it
>>may be specified at 10 volts could sit at a much higher voltage if, even
>>under load, too small a current were being drawn from it. That is, if the
>>load were too small. The higher voltage could possible damage the load
>>circuit in some cases, I think. Please correct me if I'm wrong.
>
> Others have noted that you are using incorrect terminology - you are
> referencing an unregulated plug-in DC power supply. A lot of people
> call this a wall-wart, 'cause it tends to look like your wall is
> growing a wart when you plug one in. I think it's a silly name, but
> to call it a transformer is simply wrong.
>
Actually you'll see the same thing with cheap and small transformers,
be they in AC adaptors or standalone transformers.
The manufacturers want cheap (and small), so they design for it. They
use a thin wire, and that wire has higher resistance. Load it down,
and of course you'll get a lower voltage, the same thing as when
you put a load on a high impedance audio source. Think voltage divider.
The manufacturers know this, and design for a closed system. They
know the load and the needed voltage at that load, and so design
the transformer accordingly. It works fine.
It only falls apart if someone starts using it for some other purpose.
Take a transformer out of a clock radio, a nice small transformer and
at first it seems like a decent voltage. But start using it for much,
and you see that the voltage may be too low for your use.
As discussed here recently (or in one of the newsgroups in the hierarchy)
it isn't always a sales gimmick to tell the consumer to use the
manufacturer's AC adaptor. They may have good reason for you to
use it, because then they know it will supply what is needed. Pick
another adaptor that seems to be right, and it's anybody's guess
whether or not things will work as expected.
Get into decent transformers, and specifically more universal
transformers, and the problem goes away to a large extent. A higher
current transformer, or even one where small size and cost is
not a factor, and the windings will use thicker wire to handle
the current. That thicker wire will have lower resistance, and
so the load will have to be greater to see the droop under load.
I can pull a nice 6.3v filament transformer out, and it will be
only a small percentage over 6.3v with no load. It is expected
to supply some amps of power, and hence the design can't have
a higher output impedance. Thus it is designed to supply the
needed voltage in the first place, rather than at a certain load.
If you load it enough, then of course there will be droop. But
it will be a much higher load.
Michael
.
- References:
- Transformers
- From: Rodney
- Re: Transformers
- From: Kitchen Man
- Transformers
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