I'm out of wound mains transformers. Where did my life go wrong?

[Whales]

"Oh!", he thought.  "I have a box full of those under my bed!".

Whales crouches down and pulls on a large plastic box.  It slides out from underneath his bed on poorly-round plastic rollers, kicking up a small storm of dust and cutting trenches into his floorboards.  He peers inside.

In one end of the box are dozens of mains power cables.  Everything imaginable is in there, from the humble jug-plug to a figure-8 mains connector cut through the centre to fit a laptop power brick.  At the other end is a small cardboard box, once the home of several reams of A4 paper, now the residence of Whales' worldly collection of mains transformers. 

He dives in to the cardboard box, sifting through the unwanted switch-mode supplies and dust bunnies.  Deftly flicking the pieces of useless quarry to the floor beside him, he works his way down to the bottom of the box.  Clunk.  Clunk.  Clunk.

The box is now empty.

"Oh", he murmurs.  After a brief pause he piles everything back into the box and tries again.  The same result.

A few moments pass as Whales realises what has befallen him.  He starts pulling everything out from under his bed. 

It can't be true.  There has to be one somewhere.  Large black ABS boxes of joy.  Heavy lumps of love.  So wide they inhabit multiple outlet spaces on a powerboard, so beautiful when they don't land on your foot.

---

Meanwhile at the other end of Sydney, a silver-bearded man sits calmly in his Altium mansion.  Although he has no fire nor central heating, his room stays warm.

Neatly laid out in front of him is an array of power-boards with a hundred wall-wart transformers plugged in.  Each has its output cord cut and supplies electricity to nothing, but somehow power to their owner.  Gently they hum together, singing a melody that Whales could no longer hear.

--

Children: hoard your wall warts and use them sparingly.  If you plan well then your supply of supplies can last you well into retirement. 

Don't be like Whales: destitute and searching the streets for discarded transformers.  Be smart and save your well-wound kin.

[Gyro]

You're not wrong.   Transformer wall-warts are getting really difficult to find, even on ebay. I still tend to reach for one if I'm doing any sort of low noise analog project.

[bktemp]

For devices that run 24/7 for many years I always choose a transformer + linear regulator instead of a SMPS. If correctly designed, it will run for >25years without any problems.
Most switchmode power supplies fail after 2-10 years because of bad capacitors. Even good quality capacitors have a much shorter life in a SMPS than in a linear regulated power supply because of the much higher peak currents.
Almost all modern consumer stuff seems to use a SMPS wall-wart bought from some other manufacturer. That simplifies designs, because the designers do not need to care about mains with its stringent insulation requirements.

[Kilrah]

For devices that run 24/7 for many years I always choose a transformer + linear regulator instead of a SMPS. If correctly designed, it will run for >25years without any problems.

You'll have lost way more than the value of replacement caps in heat though.

[bktemp]

For devices that run 24/7 for many years I always choose a transformer + linear regulator instead of a SMPS. If correctly designed, it will run for >25years without any problems.

You'll have lost way more than the value of replacement caps in heat though.

If the device is not easily accessible or you can't tolerate its downtime, you don't care if it wastes 2W more than a SMPS.

[Kilrah]

Maybe in those cases yes. Although I wouldn't trust an SMPS less than a transformer one myself, since I've had transformer ones that failed (rectifier, thermal protection) and no comparable SMPS that did under normal conditions thus far.

[Ian.M]

If a 50Hz or 60Hz transformer is under-built with insufficient copper, it runs hot when loaded, and with insufficient iron it saturates at peak flux, rapidly increasing its quiescent dissipation with even slight input over-voltage.   This tends to result in cheap & nasty, unventilated linear wallwarts running hot, often to the point they cook their primary insulation, and either blow the embedded thermal fuse or develop a shorted turn leading to total failure. 

Reasonable quality, affordable AC output wallwarts are still fairly easy to find new.  The cheap & nasty switchers cant drive them out of their market niche.  Therefore if you need a low noise supply, consider AC input with the bridge rectifier and reservoir caps as part of your application circuit.  That also gives you more options if you need a negative rail.

[CatalinaWOW]

Check your local thrift stores (or whatever you call them in your country. The places that resell donated surplus goods.  For charity or profit.)  Most have them.  Some charge their weight in gold, others have good prices.

[Andreas]

The box is now empty.

There has to be one somewhere.  Large black ABS boxes of joy.  Heavy lumps of love. 

Try these (as long as you can still get them):

http://de.rs-online.com/web/p/steckernetzteile/7300174/

Ok pricing has gone up by a factor of 3 since they were freely available.
(now its restricted for industrial use).

with best regards

Andreas

[johansen]

If a 50Hz or 60Hz transformer is under-built with insufficient copper, it runs hot when loaded, and with insufficient iron it saturates at peak flux, rapidly increasing its quiescent dissipation with even slight input over-voltage.   This tends to result in cheap & nasty, unventilated linear wallwarts running hot, often to the point they cook their primary insulation, and either blow the embedded thermal fuse or develop a shorted turn leading to total failure. 

Reasonable quality, affordable AC output wallwarts are still fairly easy to find new.  The cheap & nasty switchers cant drive them out of their market niche.  Therefore if you need a low noise supply, consider AC input with the bridge rectifier and reservoir caps as part of your application circuit.  That also gives you more options if you need a negative rail.

Unfortunatly, saturating the transformer core is required in order to "regulate away" the primary side copper loss, if you want to get something even close to reasonable for both power density and voltage regulation, which usually approaches 50%.

for a wall wart transformer, finished weight of 184 grams, core cross section ratio 1.6:1, so not ideal but you have to fit it in the box somehow.., you would be looking at, for reasonable efficiency:

.65 watts of iron loss (5 watts per kilogram core material), but at 20va per kilogram excition requirements, you will be losing an additional .3 watts in the primary just to excite the core to 1.3T, which is about the start of saturation for cheap E cores. So, .95 watts of no load losses.

and at 2.7 watts output, you would have 20% voltage drop due to copper losses alone, not including leakage inductance.

significant improvements can be made to efficiency if the primary takes up 70% of the core, just to compensate for no load exciton copper losses, but my spreadsheet doesn't have that as a variable yet, so I can't change that variable (yet, lol)
in the example i gave, the 2.7 watt output is achieved with a primary loss of 1.24 watts, secondary at .24 watts loss, for a total efficiency of 55%.

my spreadsheet doesn't include leakage inductance, nor some other effects that are not usually modeled for back of napkin transformer calculations so its reliability breaks down at very small transformers, such as wallwarts.   

[Siwastaja]

For devices that run 24/7 for many years I always choose a transformer + linear regulator instead of a SMPS. If correctly designed, it will run for >25years without any problems.

Correctly designed SMPS runs long as well, possibly the same 25 years. (Standard design flow includes analyzing ripple current and heating in capacitors.)

On the other hand, I have seen many linear supplies that have failed in less than 5-10 years. So, they are not always "correctly designed". Not even close! Some have melted the plastic closure.

Yes, the odds are that a random mains transformer + linear reg supply lasts longer than a random SMPS on a somewhat higher probability, but as your case is about a scenario where high reliability is important, it cannot be based on luck, but it must be based on correctly analyzed design - SMPS or not. Higher reliability is definitely easier (for the designer) to achieve in linear design if cost, size, weight and efficiency are unimportant - fewer things to analyze than in SMPS. Too often, mains transformers with linear regulators are undersized to save on copper and iron in transformer (weight & price) and aluminium on heatsinking (size & price); exactly the same case as skimping on electrolytic caps in SMPS designs (low-quality parts or going over the specs, or both).

Anyway, both transformer based and SMPS require proper engineering analysis for thermal behavior. Both fail for thermal reasons.

[SeanB]

Still have a box of small 2VA 12VAC transformers, used to have a box of 998, but a lot were used over time, either as transformers or as ballast mass. Got a lot of old industrial plug in controls as well, where I have a choice of 18VAC secondary, 110VAC or 220VAC primary, 11VAC secondart 110/220 primary, and even a few 48VAC primary ones. Whole lot of useful 5A DPDT relays, with 24 VDC coils (sigh) and a good number with 5A SPCO 12V coils.

As Siwastaja says above, you can have a reliable linear or SMPS unit, you just have to do a few things, like pay attention to thermal design, have good passive cooling and use good quality components with generous derating. Siemens Telex power supplies would run for around 2 decades between services, the most common fault being the small capacitors ( orange Siemens electrolytics) drying up. Later ones used Phillips capacitors there, and had a good lifetime. Even with failed caps they would run, but a power dip would leave then in a startup mode until they warmed up enough to carry on, till they finally did not start ( left it like that for a week waiting for the callout to be completed, the tech arrived with a complete used machine, and simply changed the diode ID board and plugged the replacement in. He did not want to carry the old one away, so I had a good time stripping it to parts to see how it worked) and you changed the few small cooked caps.

BU208D transistors almost never failed, the TV industry did a good job in finally designing a SMPS that would run for decades without issue. Old CRT TV sets were designed for reliability, even when they were replaced by the flat screen monitors the old ones are still around and working in many applications. There is a SMPS design which is highly stressed, the most common cascade failures being caused in the most part by failed solder joints, capacitors that dry out or transformers that fail from voltage stress. The first TV sets with linear supplies were horribly unreliable, often with a failure rate that meant at least a service call a year to replace failed parts.