linear power supplies "outlawed"?

I did a Google search for "linear psu site:cpc.farnell.com" I didn't see any such warning on the half a dozen or so I picked at random.

I haven't bought a linear PSU for years. I always buy Switched-mode ones. (and I do buy quite a few for the projects I build, although often second-hand).

Wait until the green lobby find out about rf interference and mains bourn interference from switchers and they'll be insisting we use linear psu's !

Andrew

On Sunday 05 January 2014 17:42 Andrew Gabriel wrote in uk.d-i-y:

Not understanding SMPSUs - do they provide complete magnetic isolation like a simple transformer does?

TRhe beuauty of a simple linera PSU for home build projects is it's hard to get it wrong, provided you keep the mains wiring nicely separated from the ELV side.

In message , Andrew Mawson writes

If the green lobby were at all interested in RF and mains-born interference, they would have started complaining about it many years ago.

Yes, assuming you mean ones with external power output

They're quick, easy & simple. SMPSUs arent.

NT

On Sunday 05 January 2014 19:21 snipped-for-privacy@care2.com wrote in uk.d-i-y:

OK...

I noticed lots of Christmas tree lights still come with conventional transformers. Not much else though.

Only a transformer gives proper isolation from the mains for safety.

They don't kick out all the rf crud that switch modes do. I think also the main inefficiency is in the transformer itself. The switch mode supplies use a high frequency to aid efficiency, but in my view they do nasty things to stuff they are used with, like crash computers etc. I really think the main reason most manufacturers went to smpsus was weight in transit from the far east!

Brian

SMPSs have a transformer. It's just smaller than is traditional as they're running at a frequency a lot higher than 50Hz. The rest of the circuitry in one is then the oscillator and a some feedback control (hence being regulated by default compared to the old

There is only one caveat to this. SMPSs generally use opto-isolators to provide the feedback from the LV side to the HV frequency generator side. This and the transformer means they are totally isolated from the mains. However, some of the very cheap ones skip the opto-isolators[1] and use FETs or TRIACs or something[2] instead which ruins the mains isolation, especially should they break down.

On Monday 06 January 2014 09:24 Scott M wrote in uk.d-i-y:

That's the bit that worries me - I would not like to have any PSU with any sort of electrical coupling between the ELV and LV(mains) side, bar perhaps an earth connection (that's rare now, but used to be not unheard of to bond to earth to the -ve rail in ye olden days).

I don't think you pay by weight (within limits) for containerised surface shipping, you pay by the container, which you stuff with as much as will fit... Saw on the telly (so it must be true) that a 20' container from the Far East to Europe costs about £1500. Think how many wall warts you could get in a 20' container, shipping costs are minimal...

The reason is that you can make a SMPSU that'll work from 60 V to 250 V AC or DC. So one single power unit will work anywhere in the world. All you need to do is provide a selection of clip-on country specific adapters.

I do wonder how "green", cradle to grave, a complex SMPSU is compared to a simple linear supply. The SMPSU might be more effcient in use but will that energy saving be more than the extra energy it took to make?

Big linear supplies are less efficient to manufacture as they require more materials. Only the transformer and case/heatsink are recyclable. They also mess up the 50Hz supply waveform. Linear wallwarts for low power have better power factors as they are inherently more resistive and inductive (lousy regulation)even when feeding a capacitative load. Low power SMPSUs are now produced in such volume that the processes are ultra efficient, leading to less to recycle. Just look at the selling prices.

Yup, when we moved back here from the US, we hired a container, and its ultimate weight did not matter, it was just the container that we paid to ship.

If a 'normal' main transformer fails, it is possible to get a short between the mains and LV windings. Of course there are some designs which pretty well prevent this happening - but not likely to be used in a cheap wall wart.

I haven't seen any regulated ones which use anything other than opto- couplers for last ~20 years (and I have taken apart and modified quite a lot to convert them from constant voltage to constant current).

The 12V lighting transformers don't have any active regulation, and only check that the input power is within an acceptable range (which is why they have a minimum load), and rely of the much better regulation of the HF transformer to maintain output voltage over their design range. Philosophically, these are a different design, being plain HF transformer, rather than being the variable energy pump design of a regulated SMPSU.

One other thing - there's often a tiny capacitor between the mains inlet (if unearthed) and the secondary. I'm not sure what it's for, but my guess is that with the secondary floating, there's a good chance the whole thing could be capacitively coupled to the primary oscilating at 40kHz (and harminics) at ~160V (half the primary voltage swing), and acting like a LW radio jammer. The capacitor would "short out" the 40kHz (and even more so, the harmonics), which capacitively leaks through from the primary, and prevent the output acting like a jamming antenna.

On Monday 06 January 2014 09:43 Dave Liquorice wrote in uk.d-i-y:

wow - cannot believe it is so cheap! I guess you add local transport (lorry) to that too.

So 2.8p per unit. Blimey. No wonder ebay can sell stuff so cheap!