Transformers

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

Reply to
Rodney
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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

Reply to
Rodney

--
What you\'re talking about isn\'t called a "transformer", it\'s called
an "AC to DC converter".  It has a transformer _in_ it, which is
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Reply to
John Fields

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 manufacturer as a percentage = 100 * (V_no_load - V_full_load) / V_full_load. Larger transformers have better load regulation than small 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 diode drops) under no load.

Reply to
Andrew Holme

no

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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.

Reply to
Ralph Mowery

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:

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Enjoy.

Reply to
Kitchen Man

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 the original AC adapter. Thanks,

Rod

Reply to
Rodney

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 the original AC adapter. Thanks,

Rod

Reply to
Rodney

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

Reply to
Michael Black

--
Here are some excellent data sheets with load curves.  Takes the
mystery out of it.
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Reply to
John Fields

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