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Old 10th January 2008, 02:56 PM   #1
engels is offline engels  Israel
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Default Switching voltage converter for HV supply

I wonder if there's any chance to adapt a switching 220->110V voltage converter for a tube HV supply?
I was about to spend some ~ $20 on such thing and preform tests but maybe I'm on a wrong way already?

What I wanted to try is something tiny labelled 1600W and intended for heaters and other power-consuming devices.
I don't want no voltage doublers - just some rectified 130V for preamps and maybe compactron power tubes (130V plates & screens).

I was testing a 12V switching transformer in heaters' supply some time ago. It was labelled 10A and almost melt under 1.5A load. A "regular" 4A transformer worked fine with the same load so I understand you may use a 0.1 multiplier to get approxiate current capability spec. for switching trannies.

But that's the only thing I know about them.

Are they lethal like auto transformers?
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Old 10th January 2008, 05:01 PM   #2
dhaen is offline dhaen  Europe
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The main thing is that you choose a solution that has isolation from the mains.
Even so, there will be some electronics on the primary side (the mains input side), and that presents some problems to engineer or modify safely by DIY.
This kind of project is probably best left for experienced engineers.
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Old 10th January 2008, 05:04 PM   #3
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by dhaen
This kind of project is probably best left for experienced engineers.
And even they get nervous around stuff connected directly to the mains.
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Old 10th January 2008, 05:26 PM   #4
kevinkr is offline kevinkr  United States
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Default Re: Switching voltage converter for HV supply

Quote:
Originally posted by engels
I wonder if there's any chance to adapt a switching 220->110V voltage converter for a tube HV supply?
I was about to spend some ~ $20 on such thing and preform tests but maybe I'm on a wrong way already?

What I wanted to try is something tiny labelled 1600W and intended for heaters and other power-consuming devices.
I don't want no voltage doublers - just some rectified 130V for preamps and maybe compactron power tubes (130V plates & screens).
<snip>
I've run into these when I was living in Italy. They're basically a chopper and work properly only with resistive loads like heaters and hairdryers. They provide no isolation from the line and will likely fry anything else connected to them. (Based on my experience)

Rectifying the output of one of these will not get you the 130V you seek (the pk voltage is still over 300Vpk with one of these just the duty cycle is changed, some may be nothing more than a 1/2 wave rectifier, others are scr/triac based iirc) - and a conventional transformer would really be much safer.

Switching transformers need to operate at their design frequency - hopefully you did not try to run the 10A example you cite above from your 220V 50Hz mains.
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Old 10th January 2008, 09:03 PM   #5
engels is offline engels  Israel
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Default Re: Re: Switching voltage converter for HV supply

If there's no isolation from mains I'd rather stick a tube in a socket.

Quote:
Originally posted by kevinkr

Switching transformers need to operate at their design frequency - hopefully you did not try to run the 10A example you cite above from your 220V 50Hz mains.
Why not? It was a switching transformer intended to work from 230V mains. Actually I was running it from my variac but it's basically the same as mains.
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Old 10th January 2008, 09:30 PM   #6
kevinkr is offline kevinkr  United States
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SMPS switching transformers do NOT operate directly from the mains - the mains are first rectified to dc and then converted to high frequency pulses at typically 100kHz or more. No wonder that transformer burned up - it was NOT designed to operate at 50Hz!!! (Some early smps transformers operated at low as 20kHz or so, but their winding inductance would be insufficient for operation at mains frequencies in any case.)


Frankly it is a wonder to me that the thing did not catch on fire.. Inductance in such a transformer is measured in uH, not the H typical of a mains operated transformer. The excitation current would have been close to the level of current that flows just due to the winding dcr.

Actually pretty hazardous...
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Old 10th January 2008, 10:38 PM   #7
engels is offline engels  Israel
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Default what?

Quote:
Originally posted by kevinkr
SMPS switching transformers do NOT operate directly from the mains
I think there's some misunderstanding. I'm talking about plain stupid adapters sold at supermarket stores that ordinary folks buy and stick in a socket to get their 12 or 110VAC.

Click the image to open in full size.

something like this looks as if it is intended to work from mains - or am I dreaming?
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Old 11th January 2008, 05:37 PM   #8
kevinkr is offline kevinkr  United States
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I see what the confusion is, you aren't talking about switch mode power supplies, but the cheap converters you show in the picture.
Those units are typically based on scr/triac based choppers (or sometimes just a big diode) and can only be used with resistive loads as I stated in a previous post. They also provide no isolation from the line, what you really need is a transformer. Look around for an "industrial control transformer" they come in all sorts of configurations - I'm currently using one with dual 440V primaries and a 220V secondary configured as a 120V to 240V step up. The best thing about these is they provide full isolation from the line and are usually inexpensive. I got mine on eBay, but I assume shipping to Israel would be prohibitive.

When I was a kid there used to be a lot of scrap metal dealers between Tel Aviv and Jaffa that had tons of surplus electronics as well - have you been there and are they all gone now? Tel Aviv had a rather active electronics hobbyist scene in those days as well, lots of shops full of mostly tube based kits.. (I was there in the years just following the six day war.)
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