Transformers - are they safe?

Hi
My octogenarian mother has become fearful that the small black transformers which are supplying electricity to various things around her flat might burst into flames.
She has a good reason for her fears, one of them some time ago became very hot indeed to touch. She has had it fixed by an electrician she knows and it is now always at a mildly warm temperature.
However after that experience she now has taken to unplugging these transformers all the time which actually involves plugging and unplugging quite a lot just to turn things on or off.
Conversely in my little house I just leave these transformers plugged in and "on" all the time. I do turn off the things they are supplying but I don't bother turning off at the wall very much.
Is my mother right or am I right?
Perhaps the truth lies somewhere in the middle?
--
Patrick
Brighton, UK
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Yes over the years we've had the odd few do a nasty and catch light but fortunately when people were around!...
--
Tony Sayer


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patrick j was thinking very hard :

The modern ones are supposed to be fitted with a thermal fuse. If they should over heat, the fuse melts and that is the end of the wall wart (plug in transformer) for further use. They are designed to be plugged in and on 24/7, but there were a few problems with the early ones in the bad old days.
I would be more concerned as to how the overheating one was 'fixed', as all modern ones are sealed units.
--

Regards,
Harry (M1BYT) (L)
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A carefully used junior hacksaw opened one for me. In a previous thread about this, people suggested using a small cutting disc as used in Dremel (and other) miniature electric drills.
Sylvain.

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Harry Bloomfield wrote:

The old traditional 'wall wart' (transformer, some diodes and a capacitor) seems increasingly rare, with many devices this days being supplied with some form of SMPS (which has a much small but high-frequency transformer).
These always run much cooler : in practice does this make them safer?
regards, Andrew
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Pretty well every electronic device has a low voltage power supply - whether internal or external like a wall wart. So if those need to be unplugged for safety, so do alarm clock radios and video recorders, etc. Most cookers etc too.
--
*Am I ambivalent? Well, yes and no.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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wrote:

As the quest for minimising electricity usage is so popular perhaps there should be some regulation preventing the use of transformers instead of switch mode power supplies. Obviously a single wall wart is not going to make any difference, but when the vast number of these things is considered their total combined effect may be considerable.
The inductive load imposed by transformers has a very bad effect on the efficiency of the supply. I seem to recall from school that if all the loads placed on generating stations were inductive rather than resistive then four times as many generating stations would be required.
This consideration was sufficiently important for factories with numerous motors to install capacitor rooms where large capacitors were wired across the supply to neutralise the inductive effect and present a resistive load. The power factor of the factory would be an important element in calculating the firms electricity bill.
Roger R
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Many of these things are not supplying any current but left plugged in. And a decent transformer will use very little more quiescent current than a SMPS.

And SMPS mess up the waveform. Try looking at mains these days on a scope.

Most factories would use three phase motors.

--
*(on a baby-size shirt) "Party -- my crib -- two a.m

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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wrote:

However small the current drawn by the wall warts they still present an inductive load, even when the appliance itself is turned off. Multiply by millions. By contrast the millions of alternative SMPS won't add up to a large inductive load even though they may have poor effect on the waveform in your home.
I don't think the considerations of inductive loading in factories is changed by utilising three phase. The three phase load is still inductive with negative effects on the power factor. The use of three phase, apart from enabling smaller sized motors, enables the load to be applied equally to all three phases, rather than just loading one phase for which the supply company would have to find other users to balance out the phases or end up with a highly undersirable large neutral return current.
Roger R
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Roger R wrote:

smps produce far worse mains current waveform than small wall warts. You've got it backwards there. Lagging load is correctable relatively simply, but the kind of current waveform smpses produce isnt, so the generators have to supply for it.
NT
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snipped-for-privacy@care2.com wrote:

But the current drawn by a /small/ SMPS will be too small to mess up the waveform, especially when off-load. Hence appliances of under 50 W consumption are exempted from the harmonic current emission limits of EN 61000-3-2. Larger SMPSs require power factor correction these days.
The magnetising current waveforms of typical small mains transformers isn't particularly sinusoidal either.
--
Andy

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There's another important reason. A three phase motor delivers constant (rather than 100Hz) torque. That makes the machine smooth running with the benefits that brings.
Jon
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Dave Plowman (News) presented the following explanation :

Which can and do cause power factor problems. Whether the correction is applied locally at the motor, or as an overall single correction for the entire factory depends upon the type of use the motors are subjected to.
If all the motors normally run and the factory has a predictable power factor, then correction can be done in one go at the mains. If motor use is irregular and not predictable it needs to be done at each individual motor.
Ballasted fluorescent lights present the same problem and sometimes will have a built in PF correction capacitor.
SMPSU's are no more efficient off load than are transformer type PSU's, but on load they are much more efficient.
--

Regards,
Harry (M1BYT) (L)
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On Mon, 01 Jan 2007 16:54:07 GMT, Harry Bloomfield
|Dave Plowman (News) presented the following explanation : |>> This consideration was sufficiently important for factories with |>> numerous motors to install capacitor rooms where large capacitors were |>> wired across the supply to neutralise the inductive effect and present a |>> resistive load. | |> Most factories would use three phase motors. | |Which can and do cause power factor problems. Whether the correction is |applied locally at the motor, or as an overall single correction for |the entire factory depends upon the type of use the motors are |subjected to.
Our department had a *huge* three phase synchronous motor which did the power factor correction for the whole factory.
--
Dave Fawthrop <dave hyphenologist co uk> Google Groups is IME the *worst*
method of accessing usenet. GG subscribers would be well advised get a
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Harry Bloomfield wrote:

Off-load the efficiency is surely always zero (zero power out divided be finite power in). Thus it's only meaningful to quote a standby power consumption (power input with no load).

That's not necessarily the case at all.
--
Andy

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Andy Wade wrote:

What..not more efficient than zero? :-)
Agreed SMPS are not used for efficiency, because they will not match a good transformer, which is up there at the 9-98% sort of areas..they are used because they are CHEAP.
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Another sweeping statement? SMPS will be smaller and lighter for the same current output. Have the ability to work properly on a wide range of input voltages, which a transformer supply can't.
As regards being cheaper, how do you price such things? In all practical costings they're more expensive.
--
*If we weren't meant to eat animals, why are they made of meat?

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) wrote:

No, they are not.
That's why they are used. At some point - about 20-50W..the iron costs and the copper costs of a decent 50Hz transformer outweigh the minimal costs of a couple of chips and some high voltage transistors and a small ferrite transformer running at 10Khz. The point at which you DON'T see a wall wart, and you DO see an SMPS.
Apart from maybe HIFI units, you will be pushed to find any equipment using an iron cored 50/60Hz transformer at over 50W..OTOH you will be hard pushed to find anything NOT using one under that power level.
The exception being the National Grid of course ;-) Where efficiency really matters, and lifetime is long enough to render massive transformers the optimal solution over many years of amortization.
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Mobile phone chargers?
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Andy Hall wrote:

Chargers for practically anything in fact and adaptors supplied with a lot of low power kit - just checked the collection and even below ~20W SMPS warts are more common than iron.
--
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