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From: tmpstr on 21 Jul 2010 15:18
> Alternately, use a transformer with a turns ratio greater than 320:12,
> and control the duty cycle down.

If it makes it a wee bit easier to get a right transformer, i am ok to
add one more battery and make the input 24V.

> For this you may find it easiest to wind your own.  Cores are available
> for just this sort of thing.

I believe winding a transformer is some kind of black magic involving
lots of trial-n-error not to mention getting the correct core. Are
there any caveats here ?
If it comes to that, i am ok with winding my own transformer as well;
only that i may tie myself in coils & "learning" would be much more
than i had bargained for :-)

> Get isolated 320VDC, drive your AC with an H-bridge, and you shouldn't
> need a transformer on the HV side -- just output filtering.

This is what my original plan is. Get the 320VDC, then using PWM+H
bridge on it get the 230V and then filter it.
and the most important question i have is how to get the 320VDC out of
12VDC(or even 24VDC)
let me have a look at the ST design i hit upon...

Thanks
From: Tim Wescott on 21 Jul 2010 15:42
On 07/21/2010 12:18 PM, tmpstr wrote:
>> Alternately, use a transformer with a turns ratio greater than 320:12,
>> and control the duty cycle down.
>
> If it makes it a wee bit easier to get a right transformer, i am ok to
> add one more battery and make the input 24V.
>
>> For this you may find it easiest to wind your own. Cores are available
>> for just this sort of thing.
>
> I believe winding a transformer is some kind of black magic involving
> lots of trial-n-error not to mention getting the correct core. Are
> there any caveats here ?
> If it comes to that, i am ok with winding my own transformer as well;
> only that i may tie myself in coils& "learning" would be much more
> than i had bargained for :-)

On the plus side, transformers for switchers are fairly small, with not
too many turns (at least compared to the monster you'd need at 60Hz).

So the experiments won't take as much time.

And finding the smallest damn core that'll just barely work to keep
costs down when you make 100K units is a _lot_ harder than finding a
core that's plenty big enough to work for one stinking project.

Come to think of it -- you may even be able to harvest the transformer
out of a PC power supply of suitable wattage, and adapt it to your use.

>> Get isolated 320VDC, drive your AC with an H-bridge, and you shouldn't
>> need a transformer on the HV side -- just output filtering.
>
> This is what my original plan is. Get the 320VDC, then using PWM+H
> bridge on it get the 230V and then filter it.
> and the most important question i have is how to get the 320VDC out of
> 12VDC(or even 24VDC)
> let me have a look at the ST design i hit upon...
>
> Thanks


--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
From: John Larkin on 21 Jul 2010 16:35
On Wed, 21 Jul 2010 12:19:28 -0500, "George Jefferson"
<phreon111(a)gmail.com> wrote:

>>>If you use wall-warts or they may saturate and not be as effective but by
>>>paralleling them you'll reduce the current and hence the total flux
>>>through
>>>each individual core.
>>>
>>
>> Paralleling the primaries does not reduce flux density. Reducing
>> excitation voltage, or increasing frequency, does.
>>
>> All others things being constant, reducing the load current taken from
>> a transformer secondary *increases* flux density in the core.
>>
>
>
>Haha, you really have no knowledge of electronics do you? When are you going
>to learn the difference between current and voltage?
>

I took two semisters of Electrical Machinery in college, which
included a lot of transformer theory and practice. I got A's [1]. I've
designed transformers, too, from 60 Hz to over a GHz. How about you?

How's this for a pick-and-place machine?

ftp://jjlarkin.lmi.net/PP5.JPG

That transformer weighs more than one of my employees.

John

[1] My average on tests was about 60. But the class average was around
15, and the prof graded on the curve. I did learn a lot about
transformers.



From: Bill Bowden on 22 Jul 2010 00:41
On Jul 21, 1:35 pm, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Wed, 21 Jul 2010 12:19:28 -0500, "George Jefferson"
>
>
>
> <phreon...(a)gmail.com> wrote:
> >>>If you use wall-warts or they may saturate and not be as effective but by
> >>>paralleling them you'll reduce the current and hence the total flux
> >>>through
> >>>each individual core.
>
> >> Paralleling the primaries does not reduce flux density. Reducing
> >> excitation voltage, or increasing frequency, does.
>
> >> All others things being constant, reducing the load current taken from
> >> a transformer secondary *increases* flux density in the core.
>
> >Haha, you really have no knowledge of electronics do you? When are you going
> >to learn the difference between current and voltage?
>
> I took two semisters of Electrical Machinery in college, which
> included a lot of transformer theory and practice. I got A's [1]. I've
> designed transformers, too, from 60 Hz to over a GHz. How about you?
>
> How's this for a pick-and-place machine?
>
> ftp://jjlarkin.lmi.net/PP5.JPG
>
> That transformer weighs more than one of my employees.
>
> John
>
> [1] My average on tests was about 60. But the class average was around
> 15, and the prof graded on the curve. I did learn a lot about
> transformers.

I went to military school where they graded on a curve and always
failed the 2 lowest people out of maybe 25 each week. They called it
the "Tube list" because they were using vacuum tube computers to grade
the tests. If you failed 3 weeks in a row you were "tubed out" and
looking for a new job. One of my friends was on the brink of tubing
out and asked for assistance, so I sat next to him and let him copy my
answers to the test. He got a better grade than I did that week.

-Bill
From: Jasen Betts on 23 Jul 2010 07:14
On 2010-07-21, tmpstr <mahesh_nkr06(a)yahoo.co.in> wrote:
>
>> e.g., if I was going to do it:
>> 12VDC->PWM->Step Up-> Filter->sinewave
>>
>> It shouldn't be hard to find a 230V:12V transformer and hence you just run
>> it in reverse(although you might get a higher voltage since it won't be
>> exact).
>
> Did you mean PWM -> Filter -> Step Up ?
> this would allow me to use a 50Hz transformer (quite bulky for 50 Hz,
> 100 Watts. may need a trolley ;-) ).

here's a 60VA transformer at 1.5KG

http://jaycar.co.nz/productView.asp?ID=MM2014&CATID=19&form=CAT&SUBCATID=887

100VA should be less than twice that, but you may need more than 100VA
it depends on the power factor of the TV.



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