Transformerless PSU using a capacitor

. A device like an automatic dawn to dusk light switch can be safe without an isolation transformer"

Sure, but there are different standards for construction and isolation to p revent shock.

In this country we have the UL, which is actually independent but carries a lot of weight. If they find that something was not UL approved and it burns your house down they got grounds for not paying the insurance claim.

What Phil said about toasters n shit is true. But you don't plug them into anything else, like your TV or stereo. plenty of times people have plugged their smartphone into my garage stereo, now if it was also plugged into a c harger that did not provide isolation then the whole system becomes "hot ch assis".

I remember hot chassis TVs. The had an antenna isolator and many of them, t hat was the only input they had. Some had video inputs that were isolated b y optos. Some people put in their own earphone jacks and in some cases took their lives into their own hands. Later, Zenith came out with a kit that h ad a transformer and it provdeded the isolation.

In this country you can't run isolated wire in the same conduit with non-in sulated wire. That means if you dig a trench to get good power out to your garage, like 220 for your compressor and welder, you cannot run phone wires , cable wires, intercom wires or even that 24 volt stuff along with it. You need a separate conduit for it. There is no way to tell if a tree root com es up and bends it and makes it short out, causing an insidious hazard. And folks, I am sure I am not the only one in the world who goes out to the ga rage for a smoke barefoot. There are good reasons for most of the NEC here. In fact the NEC are about the only laws that really make sense.

I agree that some things do not need to be isolated, as long as they don't plug into anything else. Now they got kitchen appliances controlled by smar tphone. Now if they are wifi that is one thing, but if they connect with RJ

-45 that is another.

."Still, in the trade, the resistive AC line cord was known as a "curtain burners" because people would often coil them up in a neat little ball instead of spreading them out as the owner's manual directed. "

Now there's something I didn't know.

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** An un-isolated phone charger would be a dangerous and highly illegal device.

I have done a Google search and find NO evidence that there are ANY on sale.

I explained the point to Mike Coon in this thread.

Up to YOU to prove they exist.

..... Phil

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was boldly written on it "transformerless AC radio" apparently the manufac turer was boasting. A guy opened it to find out what was inside and showed us this cap drop power supply so he warned the friend against possible elec tric shock when he touch any metal part. the friend replied there were no m etal parts so its safe. Not long before that he got zapped when he forgot t hat the antenna was made of metal.

** The antenna only needs a low value series cap to make it safe.

Not so long ago, lots or radios and TV were "hot chassis" designs with the antennas connected via small caps or isolating baluns.

Such sets normally have no video or audio connectors.

.... Phil

So a single failure in a cap would render the safety factor null. It wouldn't even affect the operation of the radio, so no one would know.

--

Rick C

sale.

IIRC, you "explained" that to some people a transformer is necessarily an iron-cored device working at mains frequency. Whereas I think if it quacks like a duck, it's a duck (so to speak). Not a very fundamental distinction, but thanks for the citation...

Mike.

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** It has gotta walk like a duck too, you can buy gadgets that make a realistic quacking sounud. Duck hunters use them. ** Just Google "transformerless PSU schem". Most of the hits are for isolated SMPS schems.

.... Phil

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nefficient state, by all means do so. But don't come crying to me if it bur ns your house down. Mainly, don't fall asleep with the thing running.

't many options even if you don't care about originality (I don't). That r adio in my example has 5 tubes, two IF XFRs, a gang tuner and a dynamic spe aker all crammed into a 4X10 chassis pan. The plastic cabinet covers this like a glove. There's simply no room to mount a 25W resistor or mitigate t he heat that it produces.

urners" because people would often coil them up in a neat little ball inste ad of spreading them out as the owner's manual directed.

adding fuses and polarized line cords to them, I instruct them on the dang ers of line connected metal chassis and never plug in the radio if one of t he plastic knobs falls off. I also instruct them to leave all old radios u nplugged when not in use and to not leave them running unattended.

The logical option with curtain burner sets is to build a separate psu to p lug them into. Use a plug that can't go into any of the usual mains adaptor s.

NT

All this talk of ponds and transformers reminds me...

Years ago a colleague sold me (for a trivial sum) a Bell & Howell "Slide Cube?" 35mm transparency projector (which I still have; model 981Q). It was made for the US market, so I also got a 500W autotransformer to power it from the UK mains.

This made the kit very unwieldy, so I set out to convert it for 240V. Swapping out the 500W lamp was easy, and getting a 240V fan motor not too tough (though fitting it into the housing was trickier). But the optical path included an automatic focus adjustment, and the electronics needed a low voltage supply. It turned out that this had been supplied by a secondary winding on the fan motor, and of course my replacement fan did not include this. So I had to shoe-horn a small mains transformer (no readily available switch-mode supplies back then) in as well. But it did work, and I may pluck up courage to see if it still does!

Beware motors masquerading as transformers!

A small postscript. A yachting magazine had an article about isolation of marina power supplies for boats, both for safety and for prevention of galvanic corrosion of underwater parts. But they mentioned the use of an "autotransformer". Since I owned one, I was sensitive to the fact that this would not provide isolation and could be dangerous, so I wrote to the magazine. I am happy to report that they published the correction!

Mike.

A small 'buck' configured transformer will reduce the filament voltage without crating a lot of waste heat.

--
Never piss off an Engineer! 

They don't get mad. 

They don't get even. 

They go for over unity! ;-)

te:

inefficient state, by all means do so. But don't come crying to me if it bu rns your house down. Mainly, don't fall asleep with the thing running.

en't many options even if you don't care about originality (I don't). That radio in my example has 5 tubes, two IF XFRs, a gang tuner and a dynamic s peaker all crammed into a 4X10 chassis pan. The plastic cabinet covers thi s like a glove. There's simply no room to mount a 25W resistor or mitigate the heat that it produces.

burners" because people would often coil them up in a neat little ball ins tead of spreading them out as the owner's manual directed.

er adding fuses and polarized line cords to them, I instruct them on the da ngers of line connected metal chassis and never plug in the radio if one of the plastic knobs falls off. I also instruct them to leave all old radios unplugged when not in use and to not leave them running unattended.

Two problems: 1) there's precious little room in the radio in question for even a small buck transformer and 2) these radios are series string and hav e no power transformer to buck...

I can understand a space problem, but you don't need a power transformer, to add a buck transformer. It is a simple autotransformer with the secondary wired anti phase, in series with the filaments. It is tiny, since the secondary only has to be the proper voltage, and it will handle the filament current. I've used them for decades, on both electronics and industrial machine tools.

--
Never piss off an Engineer! 

They don't get mad. 

They don't get even. 

They go for over unity! ;-)

:

rote:

d inefficient state, by all means do so. But don't come crying to me if it burns your house down. Mainly, don't fall asleep with the thing running.

aren't many options even if you don't care about originality (I don't). Th at radio in my example has 5 tubes, two IF XFRs, a gang tuner and a dynamic speaker all crammed into a 4X10 chassis pan. The plastic cabinet covers t his like a glove. There's simply no room to mount a 25W resistor or mitiga te the heat that it produces.

in burners" because people would often coil them up in a neat little ball i nstead of spreading them out as the owner's manual directed.

fter adding fuses and polarized line cords to them, I instruct them on the dangers of line connected metal chassis and never plug in the radio if one of the plastic knobs falls off. I also instruct them to leave all old radi os unplugged when not in use and to not leave them running unattended.

for even a small buck transformer and 2) these radios are series string and have no power transformer to buck...

Makes sense. The only time before this that I used a buck transformer was with another transformer. Should have given that a bit more thought. Than ks.

No problem. Most people have never even heard of boost/buck transformers. :)

--
Never piss off an Engineer! 

They don't get mad. 

They don't get even. 

They go for over unity! ;-)