Electronic transformer

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I am looking the most economical and lightweight way for a 12v dc power supply and as a thought comes to my mind to use an electronic transformer used for halogen lamps supplying 12vac 60W,then rectifying it for the further.I dont have any of them available in the house and i dont want to buy one only for the test because something in the "idea" stinks a little bit as the only clever in the planet.

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Diodes have been used to convert ac to dc since selenium was invented. But even diodes have their problems. 0.7 volt drop at 5 amps is 3.5 watts wasted. A shotkey with .3V drop reduces this to 1.5W wasted, but mosfets with lo on resistance wired as a 'synchronous rectifier' (don't ask me what the schematic looks like) is wizzy hi tech stuff, and ever lower power dissipation. Now get rid of the halogens and use leds.

Imagecraft compiler user

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The generic "electronic transformer" for halogens is usually based on an app note from ST microelectronics.
http://www.st.com/st-web-ui/stat...

They are not too useful for a general power supply as the output is not rectified or regulated. You can add some high speed diodes to rectify if you wish.

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The transformer you describe ought to do it. The "12V" number is RMS and the peak is 1.4 times that, or about 16.8V. Rectify full-wave or half-wave (full-wave takes 4 diodes, dissipates a bit more power in the diodes, but takes power symmetrically from the power line. Up to a few watts, this should be just fine. With less than a full 60W load, the output may be a bit higher than 12V RMS.

I would almost bet that a 12V 0.5A or 1A "wall wart" would be cheaper. And you ought to be able to get those at your local shop.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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I want to use it as a dc power supply,not for lighting purposes.Also cheap is the ballast type 12v transformer compared with a traditional transformer.
The goal is to make a 12/5A dc even unregulated with the most possible lowest price less than 10€ and the use of an electronic transformer does not look impossible.

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geoelec wrote:
The goal is to make a 12/5A dc even unregulated with the most possible lowest price less than 10€ and the use of an electronic transformer does not look impossible.

You might consider going to a local Hamfest. One year at the Dayton Hamfest I picked up several 12 VDC, 5 Ampere switching power supplies for a couple dollars U.S. ---> each.

Of course, I am assuming that this "Electronic Transformer" is actually a switching power supply that is specifically tweaked for Halogen lighting.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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If it is a switching supply transformer, then all bets are off. If it is a 50/60Hz transformer, then there is a possibility.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ka7ehk wrote:
If it is a switching supply transformer, then all bets are off. If it is a 50/60Hz transformer, then there is a possibility.

Jim

Please clarify, Jim. I'm not sure what you are conveying.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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I would not use one of those electronic transformers for anything. Not even for 12V halogen lighting. In my experience they tend to be cheap and nasty. Just my opinion.

John

Four legs good, two legs bad, three legs stable.

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Yes, they are cheap and nasty. I'm not sure how they perform on part load. They are designed specifically for running a fixed load lamp and the architecture is somewhat different to your usual flyback style switchmode.

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microcarl,

The Dayton Hamfest is a fair distance and a few oceans apart from geoelec's stated hometown of Thessaloniki, Greece. Also, I suspect they don't Hamfests, but rather Gyrofests.

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Lambfests?

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My point is this:

For proper operation, a transformer primary has to have a certain "impedance" (that is, "reactance"). If the impedance is too low, there will be too much primary current even when there is no load (vast simplification, but correct).

A switch mode supply operates, depending on the design, somewhere in the range of 10KHz to a few MHz). The transformers for these supplies are wound with no more turns than is necessary to get the needed impedance at the operating frequency.

At power line frequencies, which are on the order of 50Hz, it takes many hundreds of times larger inductance to have the primary current in the proper range.

Thus, a switch mode transformer does not have enough primary inductance to work at power line frequencies. Been there, done that, smelled the delightful aroma of very hot insulation!

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net