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Old 7th July 2012, 01:57 PM   #1
emosms is offline emosms  Bulgaria
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Default 12v AC - 300v regulated DC switched power supply

Hi,
Plan to build a unit with a double triode preamp.
Unit to be supplied with 12v AC - It would provide me:
- app. 6v AC or mby 6,3v regulated DC for the valve heater
- 9v DC regulated for the other stuff
I could easily get over 300v AC using reverse 9v mini transformer,
but I plan an extremely slim unit (1 inch height inside the box)
I would use switched power supply if it is not complicated.
---
Expected consumption for the 300v - 0,5w. Lets say 0,6/0,7.
1w would be abundant
---
Any suggestions for schematic? Where to start from?
Best Regards
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Old 7th July 2012, 03:16 PM   #2
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Try doing a search on the humble TL494 to see how many times this has already been covered. The TL497 can be used too.
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Old 7th July 2012, 03:27 PM   #3
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Typically you would design the TL494 circuit to give a regulated 6.3V DC output and just calculate the turns required to give you the HT. As the HT current is fairly low and fairly stable it will be fairly stable due to the stabilisation of the LT supply.

At 1.5W the core required would be very small. You could get away with a ferrite toroidal ring. (As indeed car audio amps invariably do).

Do note that the commercial designs often run at VHF so will use much smaller cores than you will be using with a TL494.

Last edited by KatieandDad; 7th July 2012 at 03:30 PM.
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Old 7th July 2012, 03:38 PM   #4
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Have a look at page 3 of this lot. The top circuit on page 3 gives you the general idea.

TL494 circuit examples - page 3

All you need to do is modify the secondaries on T1 and alter R8 in the feedback circuit. Obviously the secondary circuits will need to have the components selected to suit your application.

At 1.5W you probably don't need U2 and Q1 and Q2. You would need to look at the datasheet to see ifthe TL494 will drive the transformer directly.

If you a newbie to transformer design, you will need to do some reading to find out how to calculate the number of primary and secondary turns that you will need.

You might be better starting with an RM6 pot core of known parameters for your first project.

http://uk.rs-online.com/web/p/transf...cores/0180115/

Last edited by KatieandDad; 7th July 2012 at 03:51 PM.
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Old 7th July 2012, 03:51 PM   #5
DUG is offline DUG  Canada
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I would not feed a 9V:xxxV transformer with 12V

If you use a 18:120 and a 24:120 you would get aprox 180Vp and 135Vp from each when driven with 12V.

Stack the secondaries and use one full wave bridge.

Put a current limiting resistor in the secondary to limit initial turn on current.

This would total to 315V. Loose a few volts for diode drop and add some R-C filtering and you should have a 300V supply.

If you wanted a cleaner 300v then use two 18:120V (or one 18:240V) and add a linear regulator on the 360V.

TI has a DC-DC controller IC (TPS40210) that would work very well but the design and component selection would be a little more complicated. (and you would need a PCB designed) http://www.ti.com/lit/ds/symlink/tps40210.pdf

hope that helps
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Old 7th July 2012, 03:58 PM   #6
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Mains transformers are not designed to be used in reverse. They do work but are not very efficient, and of course they are bigger and more expensive than their ferrite counterparts.

The OP stated that he has a height restriction of 25mm, that's pretty much going to preclude any mains transformers. You can get 3W toroidal transformers but they aint as cheap.
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Old 7th July 2012, 08:29 PM   #7
DUG is offline DUG  Canada
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Hammond 229E24 24:230V 2va, use 2, .65" high plus pin height

should be able to work with something like that (if you can find them)

KatieandDad are correct that a toroid type would be an easier fit and in the long run easier to find
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Old 8th July 2012, 02:20 PM   #8
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As you are going to have to build a "chopping" circuit anyway, why not do it properly.

YES, it is possible to build a 50/60Hz oscillator with a simple NE555, but why not employ the bespoke chips and get it right first time.
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Old 8th July 2012, 03:13 PM   #9
DUG is offline DUG  Canada
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emosms started with 12VAC.

why go to DC and then chop it to 50/60 Hz

just run the 12VAC to the transformers
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Old 8th July 2012, 03:59 PM   #10
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OK I misread the original thread - yes he does have 12V AC to start with.
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