What kind of sorcery is this? Transformerless power supply

[guidoilieff]

I found this state of the art thing on the street. It has a lm317k and a 317L. The front panel is labeled with a "12 - 24v" and "0 - 12v" switch. There are two bananna jacks and a male DC plug connector and a potentiometer to regulate the 317.

The main cable that plugs to the wall is connected to two fuses but there is no transformer.


How is this supposed to work? Why two 317? Can I use it to test my pedals? Thanks!

[digi2t]

Four diodes bridge. Insert AC. Diodes convert it to DC. Regulators reduce the DC to the required voltage. One regulator for the 12-24, the other for the 0-12.


Oh, oh, oh, it's magic.... you knoooooow. Never believe it's not so.

I would be hesitant to power a pedal with it though. A) Looks home made, so on the safety side, dubious. B) Anything with a capacity to "accidentally" blow through 9vdc is not something I would use on a test bench. Probably because I would be the first one to "accidentally" forget that I set it to 15v to test something else, and then hooked up a 9v circuit. 

[Ice-9]

If that is how it is wired then there is no mains isolation at all and could prove lethal. By all means draw out the circuit to confirm but I would just strip it down and save any useful parts for other uses.

[GibsonGM]

Transformerless = no isolation, just like Mick said.   

It is standard practice when working with low volt stuff like we do, to use some form of transformer....barring some very strange and catastrophic circumstances, no matter what you do, the line voltage won't "jump through" and bite you.  The current delivered by a transformer is also limited.    When converting something like an old radio that is mains-powered for 'modern use', you use a 1:1 isolation transformer for just this reason - to avoid ever having a hot chassis.

That puppy could be dangerous.  They get points for creativity, though, ha ha.   Found "on the street" - probably not FDA approved     

[wavley]

If that is how it is wired then there is no mains isolation at all and could prove lethal. By all means draw out the circuit to confirm but I would just strip it down and save any useful parts for other uses.

Yep, either get an isolation transformer or strip it, please don't use it.

There are vacuum tubes that run straight off the mains voltage for the heaters (which can be 117 volts, or wired in series to make 117 volts)  My shop refuses to work on them without installing an iso transformer, fuse, and grounded chassis.  If the circuit designer was too cheap to use a transformer, then the circuit is too cheap to die over.

[guidoilieff]

Four diodes bridge. Insert AC. Diodes convert it to DC. Regulators reduce the DC to the required voltage. One regulator for the 12-24, the other for the 0-12.


Oh, oh, oh, it's magic.... you knoooooow. Never believe it's not so.

I would be hesitant to power a pedal with it though. A) Looks home made, so on the safety side, dubious. B) Anything with a capacity to "accidentally" blow through 9vdc is not something I would use on a test bench. Probably because I would be the first one to "accidentally" forget that I set it to 15v to test something else, and then hooked up a 9v circuit. 

So the mystic being that owned this apparatus had an external transformer for this thing. The big 317 regulates from 220v to 12v?... I guess I'll just scrap the parts and make a new one.

Thanks for the reply

[digi2t]

Smart move. Bad enough I thought it was on 120v.

220V?

[GibsonGM]

So the mystic being that owned this apparatus had an external transformer for this thing. The big 317 regulates from 220v to 12v?... I guess I'll just scrap the parts and make a new one.

They COULD have had a 1:1 transformer external to that, but it doesn't look that way due to the power line having a plug on it, etc....maybe it plugged into something else that had an outlet, who knows.    It was part of someone's "bench", probably, that they had some purpose for, and it probably got the job done for them.  But I don't see it being useful for the things we generally do here, no...maybe someone told them it was dangerous and they tossed it!! 

[wavley]

So the mystic being that owned this apparatus had an external transformer for this thing. The big 317 regulates from 220v to 12v?... I guess I'll just scrap the parts and make a new one.

They COULD have had a 1:1 transformer external to that, but it doesn't look that way due to the power line having a plug on it, etc....maybe it plugged into something else that had an outlet, who knows.    It was part of someone's "bench", probably, that they had some purpose for, and it probably got the job done for them.  But I don't see it being useful for the things we generally do here, no...maybe someone told them it was dangerous and they tossed it!!

I can see where it would be possible that somebody was powering this off a variac and that's why it has a regular plug, that's plausible and safe in that situation.  It's not something I personally would do because I would never put a plug that *could* be plugged into the wall on something that wasn't supposed to be plugged into the wall.  Little 24v (or whatever non-tube voltage) transformers are too easily pirated out of something common and broken if I were just throwing something together out of spare parts.

[Rob Strand]

Another possibility is it ran from a lower voltage, not 220V AC, and someone has used a mains plug for the LV connection.   

Re-purposing mains plugs is dangerous.  It kills *other* people.  People who find stuff.

If you draw-up the ckt it will be obvious what the range of input voltages were.

[EBK]

The device in question is clearly a trap of some sort. 

It doesn't kill, but instead has more of a "stun" effect, causing potential victims to stop and endlessly question its inner workings and purpose.  Its appearance is designed to leave it unnoticed by most animals, and quite ingeniously, its ambiguously dangerous looking innards are designed to actually keep it away from electrical outlets so it has a greater chance of being in an open area, more noticeable to potential victims.

[Transmogrifox]

300MHz charge pump

[Rob Strand]

The device in question is clearly a trap of some sort.

Set by Psycho Neighbor:  "I know how to stop that guy testing stompboxes at 2am."

[highwater]

The device in question is clearly a trap of some sort.

Ah, like Nerd Sniping.

[guidoilieff]

EVERYBODY! I FOUND THE ANSWER!


It was somebody's project for a technical school. Judging by the components its and old artifact made in the 90's A.D.  The name of the school was etched on the copper side.



Sorry if I didn't get your answers before and thanks for reposting them. The best theory so far is that you plugged that thing into something else. Possibly into a degenerate experiment of the professor.




aaaaand sorry for my english

[imJonWain]

ahh technical HS electronics projects <3.  My teacher had an ancient neon sign transformer that I swear made almost 10" arcs.


trace it anyway!

[armdnrdy]

Oh, oh, oh, it's magic.... you knoooooow. Never believe it's not so.

Pilot   

[guidoilieff]

ahh technical HS electronics projects <3.  My teacher had an ancient neon sign transformer that I swear made almost 10" arcs.


trace it anyway!

I already disassembled it and made a power supply, but I can juxtapose the top and bottom layer

[guidoilieff]

I know this doesn't help at all, but that's the most I can do now.