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Author Topic: How much power does an unloaded transformer consume?  (Read 25386 times)

Don Kingsley

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Don Kingsley  asked the Naked Scientists:
   
How much power is used when only the primary of a transformer is connected; for instance a cell phone charger is plugged into the line, but the phone is not attached?

Thanks

What do you think?
« Last Edit: 15/10/2011 17:01:02 by _system »


 

Offline Geezer

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How much power does an unloaded transformer consume?
« Reply #1 on: 15/10/2011 17:26:43 »
If the transformer was "perfect", there wouldn't be any at all. However, no transformer is perfect, so there will be some loss. The amount depends, to some extent, on the quality of the transformer.

One way to find out is to put your hand on the device and see how much heat it produces when there is no load on it. If it feels "hot", it is wasting a lot of energy. If it is barely warm to the touch, it's probably only wasting a couple of watts, which is probably about as good as it gets.

To get a sense of the amount, think about how hot a forty watt incandescent light bulb gets (if you still have any!) Nearly all of the power consumed is producing heat. A light bulb has quite a large surface area, but forty watts still makes it almost unbearable to touch. Forty watts coming off something a lot smaller (like a soldering iron or a halogen light bulb) will give you a really nasty burn.

EDIT: BTW, I just checked a couple of cell phone chargers that were plugged into the wall, but not connected to cell phones. I could not detect any heat from them at all, so they cannot be wasting much energy.

I suspect they don't actually have a transformer connected directly across the incoming supply. They are probably switch-mode power supplies. If I can find an old one I'll rip it apart and try to find out, or maybe someone else has already done that.

In any event, if you can't detect any heat coming from your charging device, it's not wasting energy, so don't worry too much if you happen to leave it plugged in sometimes when it's not in use. 
« Last Edit: 15/10/2011 20:25:18 by Geezer »
 

Offline Soul Surfer

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How much power does an unloaded transformer consume?
« Reply #2 on: 16/10/2011 00:05:31 »
To be technical, for a transformer coupled device the loss depends on what is called the leakage inductance of the primary coils of the transformer.  This is essentially the amount of magnetic flux in the coil that is not connected into the secondary.
 

Offline syhprum

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How much power does an unloaded transformer consume?
« Reply #3 on: 16/10/2011 06:04:36 »
A simple way to check if a charger is switched mode or
 transformer is to put it close to an AM radio, switched mode devices generate a considerable amount of interference.
The standard way to check transformer efficiency on large transformers is first to check the power consumption with the regular input voltage connected and the output open circuit which gives you the core loss.
Then to check the power consumption with reduced input voltage sufficient to give the specified output current with the output short circuited that gives you the copper loss .
The sum of these two will give the total losses when the transformer is operating at full power
« Last Edit: 17/10/2011 10:26:57 by syhprum »
 

Offline Geezer

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How much power does an unloaded transformer consume?
« Reply #4 on: 16/10/2011 06:12:40 »
A simple way to check if a charger is switched mode or
 transformer is to put it close to an AM radio, switched mode devises generate a considerable amount of interference.

Yes, but it's more fun to bust one open  :D (They don't make it easy you know.)
 

Offline Geezer

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How much power does an unloaded transformer consume?
« Reply #5 on: 16/10/2011 06:14:37 »
To be technical, for a transformer coupled device the loss depends on what is called the leakage inductance of the primary coils of the transformer.  This is essentially the amount of magnetic flux in the coil that is not connected into the secondary.

SS, does that result in the eddy current losses, or are they additional losses?
 

Johann Mahne

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How much power does an unloaded transformer consume?
« Reply #6 on: 16/10/2011 07:58:29 »
Quote
To be technical, for a transformer coupled device the loss depends on what is called the leakage inductance of the primary coils of the transformer.  This is essentially the amount of magnetic flux in the coil that is not connected into the secondary.
The leakage inductance is the effect seen when loading the transformer. It's really the stiffness of the transformer and is normally quoted by the manufactures as % impedance.
The question was about an unloaded transformer, that's just connected to the mains. This is the current caused by the magnetizing current. The magnetizing current is essential to produce your secondary voltage, and it is unaffected by the normal load range.It's normally kept to a minimum.
 Most transformers don't have that much magnetizing current, but UPS transformers and cvt's may well have more.

 Re geezer: Switch mode supplies do often connect across the main indirectly, through a rectifier and capacitor. For example my own flyback switch mode battery charger does this. It has about 300 v dc across the primary, but only generates charge in the flyback mode ( 8 amps dc ). It's really light and small as all SM supplies are. It does draw more current than normal even if the load is not coupled because in a flyback supply you need to store energy in the core to be released into the load.
 

Offline Geezer

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How much power does an unloaded transformer consume?
« Reply #7 on: 16/10/2011 08:23:12 »
Switch mode supplies do often connect across the main indirectly, through a rectifier and capacitor.

That's not really indirect. It's really quite direct  :)

Switch mode supplies produce a high voltage DC supply by rectifying the mains supply, then "chopping" it at high frequency to produce an isolated voltage very efficiently. This drastically reduces the size of the transformers required to produce the isolation, but it's really only possible because of some very fast semiconductor technologies.
 

Offline SeanB

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How much power does an unloaded transformer consume?
« Reply #8 on: 16/10/2011 12:04:48 »
Even simpler is to look at the mass. If it is light and is made after 2009 it will be switching, and if heavy it is magnetic.

Magnetic transformers use around 5W in no load, mostly due to them being wound to run the core at or in saturation, to reduce the volume of iron needed. As well the copper is the thinnest they can wind, to reduce the mass of copper, though the losses are going to be much higher. In a small transformer like used for a cell charger, you will find that the losses are actually higher than the power delivered to the load, as the design compromises made are all to reduce cost and volume, all of which increase loss.

In a transformer used to provide significant power the design is for low loss in both the core and the windings, as they are going to spend a lot of time under light load, and the losses will be a lot of power to dissipate. Thus they run a very thick winding, with a massive core.
 

Offline CZARCAR

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How much power does an unloaded transformer consume?
« Reply #9 on: 16/10/2011 15:41:48 »
just plugged into a KILL A WATT & it read 0 on both a cellfone charger & a 9v dc charger.BUT the 9v charger read a 3 when it was reading VOLTAMPS INSTEAD OF WATTS...............whats a VOLTAMP?
 

Offline Bored chemist

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How much power does an unloaded transformer consume?
« Reply #10 on: 16/10/2011 16:39:41 »
It's complicated
http://en.wikipedia.org/wiki/Volt-ampere
In this case it's probably the current being fed into a capacitive load. Since most of that stored energy is then returned to the mains the overall power transfer is zero, but there is still a current.
 

Offline syhprum

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How much power does an unloaded transformer consume?
« Reply #11 on: 16/10/2011 20:48:12 »
The main advantage of a switched mode PSU is that it can cope with a varying input voltage of 250v to 110v.
A transformer type PSU could be designed to cope with such a range but would have to generate twice the required output voltage when operating from a 250v input and incoperate an internal voltage reduction circuit an expensive and lossy system.
« Last Edit: 16/10/2011 20:50:47 by syhprum »
 

Offline Geezer

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How much power does an unloaded transformer consume?
« Reply #12 on: 16/10/2011 21:01:53 »
The main advantage of a switched mode PSU is that it can cope with a varying input voltage of 250v to 110v.
A transformer type PSU could be designed to cope with such a range but would have to generate twice the required output voltage when operating from a 250v input and incoperate an internal voltage reduction circuit an expensive and lossy system.

That's certainly one advantage, but I think the main advantage is that it's cheap. The price of transformers hasn't changed much in a long time, whereas the price of the semiconductors required to make a switcher have fallen significantly. The weight saving is also a big advantage to the manufacturer/distributor, as well as to the user.
 

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How much power does an unloaded transformer consume?
« Reply #12 on: 16/10/2011 21:01:53 »

 

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