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power loss in battery chargers...USB Answered

a transformer, being a pair of electromagnets, such that it reduces the voltage of household power to whatever voltage your battery powered device needs, will cost you somethin if its plugged into the wall, even if yoour phone, laptop, etc isn't plugged in.

but it seems that these 5v usb chargers are too small to contain a transformer. I dont think they get warm when they are idly plugged in.  I imagine they may just have a rectifier and zener diode. Is that the case?  Do they waste power when left in an outlet?


Here's are 2 pictures of a transformer with 1 end cut off of the core. In 1 pic, it is holding up a 4 lb plate while running on 110v AC. In the other pic, it is holding a 15 lb weight while running 110V DC. Although the coil burned out shortly after that, it shows that a transformer is electromagnetic.

transformer 4 lb plate.jpgTransformer 15lb weight.jpg

Should've changed "here's" to "here" Ah, well.

Actually, a transformer can have a single coil too.

A transformer has nothing whatsoever to do with electromagnets.

The little supplies you are talking about now draw less than 0.1W when they are not on load. Left on for a year, they'd draw less than a unit of electricity.

steveastrouk: "A transformer has nothing whatsoever to do with electromagnets".

What? Ammm. Yes they do.

electromagnetc coil is an electrical conductor, such as a wire, in the
shape of a coil (or spiral or helix). It is a device that when a
time-varying current (AC power) is passed through the coil winding will
induce a time-varying magnetic field along the axis of the coil. When we
reverse this and place a time-varying magnetic field along the axis of
the coil, it will induce a time-varying current through the coil

An electromagnet is nothing more than a electromagnetic
coil with a time-varying current (AC) passing through it. If the core of
the coil is made up of just air, it will be very weak. If we wind the
coil around iron or another magnetic material it concentrates the
magnetic field and increases the strength of the electromagnet.

The reverse is also true. The iron core will cause a time-varying magnetic field to induce a larger time-varying current (AC).

transformer is nothing more than two electromagnetic coils placed in
close proximity to each other. We place a time-varying current (AC) on
coil1 . It induces a time-varying magnetic field. When coil2 is placed
within the magnetic field, that field will induce a time-varying current
(AC) on the second coil. This is called mutual induction.

If the
two coils are wrapped around an iron core, and an AC power supply is
connected to coil1, a magnetic field is generated and passes through the
inside of the iron core. Since coil2 has been wrapped around the same
iron core, the magnetic field varies inside of coil2.

Bottom line is that a transformer is made of up two coils. One acting an an electromagnet to drive the second coil.

Ammm. No. An electromagnet is not an "electromagnetic coil", it is SPECIFICALLY an electromagnet coil driven with DC. A good electromagnet design is a crap transformer.

Bottom line is that a transformer is made of at least two coils, and runs on AC. An electromagnet has one and runs on DC. That doesn't make them "the same".

i disagree on transformers and electromagnets. What is the primary winding but an electromagnet? It's just a winding of wire around an iron core. it is essentially switched on and off 60x per second.

at 1 point i had a device similar to the kill-a-watt. some power supplies drew more power than others.

Try getting something to stick to a transformer, or even be rattled by it. A transformer HAS no flux leakage.

It is not a matter of "disagreement".

My point in describing a transformer as an electromagnet was that the primary coil is connected, and drawing power, regardless of whether there is a load on the secondary.

A transformer has very _little_ flux leakage. It can be detected by an audio pickup coil. Essentially the transformer is an electromagnet in a closed box. A cat in a closed box is still a cat.

The primary draws virtually no power when there is no load, at least for a decent transformer. Cheap transformers leak. Toroidal transformers pretty well do not.

A cat in a closed box is not a dog because you choose to think of it like that.

Steve Astro: "try getting something to stick to a transformer" ok. I openned the "box" of a small 12v wall wort. (cutting the core) The unmodified primary coil lifted a 4 lb steel plate. When i switched to dc, it lifted 15 lb. I'd say that this cat has claws. I was not surprised when i burned out the coil shortly after that. Explaining _why_, however, is something I would appreciate help with. I think this could be an " understanding transformer fundamentals" instructable. Care to collaborate?

I used an angle grinder with a cutoff blade to remove 1 side. A cross section of the core at that point resembed a "3" instead of the "8" of it's original configuration.


4 years ago

Ehh... side stepping something here...
Transformers really only work on AC current and USB chargers are DC. There might be some specific example where this isn't true, but for the most part, part of the point of using usb, is to mess with DC power. If you runn an AC current through a circuit designed to run DC current in it, you're going to be lucky if nothing melts.

I believe most just use a diode to rectify the current. Maybe a capacitor to filter out weird noise.

As far as wasting energy, it depends if there's a closed circuit that persists once the device that's being charged is unplugged, and that might depend somewhat on the individual charger's circuit design. Some cellphone chargers do have LEDs that remain lit, even when not connected to a device.

I don't think my cellphone charger maintains a closed circuit once the cellphone is unplugged, but someone else's might not do that.

Even if they maintain a closed circuit, we're talking about something that powers an LED. An LED is 200 milliamps (the maximum rating for an LED) * 5 volts (the maximum standard voltage rating for a USB line). This comes out to be about 1 watt. Over the course of an hour it'd probably be less the 100 watts, so less then a tenth of a kilowatt hour.

That said every bit helps, yeah?

you're missing the point. We're taking about switch mode mains USB chargers.

Even whena charger is disconnected from the load it will still draw current

Well, this conversation has inspired me to pull apart some stuff that wasnt broken. I also pulled apart an automotive ciglighter to usb adaptor. That one doesnt looks like it might be a tiny toroid transformer, or maybe a choke or filter

the cigarette lighter chargers use a simple linear regulator, unless you get A modern high power charger, the ones that can push2 amps. No ac there.

These USB style chargers use a high frequency switching circuit with tiny high frequency transformers and a rectifier circuit to change the AC volts from your home electrical outlet to 5 volts DC output for charging. An integrated circuit in the adapter controls current and voltage to the battery to prevent under and over charge. These adapters are extremely efficient (around 92%) and use very little current when charging a battery and basically no power when a load (battery, etc) is not connected.

hmmm. all i really know about high freq transformers is tesla coils. but 5v is rather the opposite end- low v. So is hi freq just inherently more efficient? does the IC shut down ppower to the primary when no load on secondary?

Yes, high frequency is inherently more efficient.

A well designed transformer draws no current on no load.

What causes high freq to be more efficient? What makes the difference between a well designed transformer and a poorly designed one?

The higher the frequency, the higher the rate of change of flux. For complex reasons, the volume of the transformer is related to the rate of change of flux, so higher frequency = lower volume/weight etc. A smaller transformer has proportionately shorter flux paths ("reluctance"), and is more efficient.

A well designed transformer is arranged so that it has very little leakage flux, and all the flux from the primary turns link all the turns of the secondary. If you use the iron of the transformer too hard, it gets warm from hysteresis losses = wasted energy. If you use too thin a wire, it gets warm from resistance losses. The "iron" losses and the "copper" losses are played against the cost of the transformer, and the desired regulation of the output. You'll see transformers have a "10%" regulation - If that was a 100VA transformer, at 100V, then open circuit on the secondary you'd measure 110V. If you load the transformer with 1A, the output volts would THEN be 100V.

Cheap , small transformers often run hot - you need a fixed number of turns in a transformer design to work, the turns have to be thin....copper losses go up.

I designed a transformer with a 9000VA rating @36V - that is STONE COLD when the secondary isn't connected.

i'd like to know more about reluctance. i think i have a fair understanding of impedance, or once did. Also, tho i had a feeling that the current would go through the roof when i cut through the iron core, i cant really explain _why_

conceptually, i could see that cutting 1 side off a single window transformer would leave a horseshoe electromagnet. And cutting 1 side off a double window transfrmr (which was the experiment i conducted) left a kind of double sided symetrical horseshoe mag with the windings on the center prong of an "E" core.

But, then, does current go up because so much mag flux leaks? And if the weight lifted by this mag was sufficiently flat, and contacted each of the 3 prongs completely, would the flux leakage be stopped?

There are a few reasons that may have contributed to frying the primary. 1)heat from the angle grinder

2)water used to cool the core during this abrasive cutting. I accidentally dropped the whole thing in water while trying to gingerly dip just the iron.

3) i used alcohol to drive the h2o out. maybe the enamel was somewhat soluble in 90% alcohol.

4) i applied power while it was still wet with alcohol. *

5) the 15 lb weight i had available is a kettlebell. The handle is curved, and so, it only touched the center prong of the "E" shaped core. I suspect flux leakage at the gaps of the outer prongs of the "E" contributed to excess current.

*re: safety, this was all done outside. if the coil had arced, the alcohol ignited, etc, there was little harm woulda come of it. Safety glasses worn. I've seen wires pop and send tiny bits o' hot metal around, and i didnt want 1 of those embedded in my eye.

Question: What was going on in terms of impedance, reluctance, flux leakage, current, etc? I appreciate any expert viewpoints on this.