# Transformers

Discussion in 'Electronic Basics' started by Rodney, Aug 13, 2005.

1. ### RodneyGuest

Hi,

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.

Rod

2. ### Andrew HolmeGuest

Are you talking about the AC voltage across the transformer secondary, or
the DC output after rectification and smoothing?

The former depends on "Load Regulation" which is quoted by the transformer
transfomers.

The AC output voltage is specified in volts RMS. The rectified and smoothed
DC output will be almost a factor sqrt(2) higher (peak voltage less the

3. ### RodneyGuest

I'm talking about the DC output. I was thinking of a wall transformer in
particular. The load would be a device rated at 10 volts but requiring
little current. The transformer would be rated at 10 volts DC output but
having a fairly large current capacity at the output. I've measured the no
load output of some and it was too high until I put a load on it to bring it
down to the rated output voltage.

Rod

5. ### Ralph MoweryGuest

The small unregulated DC supplies will have a higher unloaded voltage. The
low voltage AC comming out of the transformer in it is rectified to DC and
maybe a small filter capacitor in it or the device it powers will charge to
a voltage about 1.4 times the AC voltage. As the load is increased the
capacitor can not charge to the peak value and hold it. If the supply was
regulated this would not hapen.

6. ### Kitchen ManGuest

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.

Now, on to your question. I have also noticed the same phenomenon,
and I have wondered the same thing about what would happen if the
rating of the supply was grossly in excess of the load attached. The
following article is a very good reference for the question:

http://www.glitchbuster.com/wallwart.htm

Enjoy.

7. ### RodneyGuest

Any "wall adapters" that I've dismantled didn't have capacitors, just
rectifiers, but thanks for the information. But my original question was
would this higher voltage, in some cases, damage a load that was too small a
current drain to
bring the voltage down to the proper level. If you had a 10 volt DC device
which did not draw
enough current to bring the supply level down to the 10 volt rating and the
DC supply was sitting at 14 volts it could be possible to damage DC device.
That would be in the case that the AC/DC adapter had a high current
capability. I think it would require a sufficient load to bring the voltage
down. So if someone wanted to buy an AC adapter for their video game,
for example, they would buy one with the right voltage rating and it might
not matter if the current rating were higher but then if the current rating
were too high and the adapter thus had a high wattage it could be
theoretically
possible to damage the video game, or whatever it may be, if the video game
did not draw enough current to bring the AC adapter down to the rated
voltage. In
this case you would be supplying a 10 volt device with about 14 volts. This
is mostly theoretical since I think most things have internal voltage
regulators but personally, to be on the safe side, I think I would buy a
replacement adapter with a current rating that was not a great deal higher
than that of
Thanks,

Rod

8. ### RodneyGuest

Any "wall adapters" that I've dismantled didn't have capacitors, just
rectifiers, but thanks for the information. But my original question was
would this higher voltage, in some cases, damage a load that was too small a
current drain to
bring the voltage down to the proper level. If you had a 10 volt DC device
which did not draw
enough current to bring the supply level down to the 10 volt rating and the
DC supply was sitting at 14 volts it could be possible to damage DC device.
That would be in the case that the AC/DC adapter had a high current
capability. I think it would require a sufficient load to bring the voltage
down. So if someone wanted to buy an AC adapter for their video game,
for example, they would buy one with the right voltage rating and it might
not matter if the current rating were higher but then if the current rating
were too high and the adapter thus had a high wattage it could be
theoretically
possible to damage the video game, or whatever it may be, if the video game
did not draw enough current to bring the AC adapter down to the rated
voltage. In
this case you would be supplying a 10 volt device with about 14 volts. This
is mostly theoretical since I think most things have internal voltage
regulators but personally, to be on the safe side, I think I would buy a
replacement adapter with a current rating that was not a great deal higher
than that of
Thanks,

Rod

9. ### Michael BlackGuest

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