12 to 24 volt conversion.. MORE

[Paul Kraemer]

How about a surplus transformer with two filament windings? The otherwise 
empty 115v winding could have a buffer cap across it to tame the transformer 
ringing. Plus low power 115vac would be available for ???
A base winding, although the preferred way to go, is not eessential. Just 
cross connect the bases to the opposite collector multivibrator fashion 
through a couple small resistors. Or, if you really want the base windings, 
they can often be squeezed in on top of the other windings. For out of the 
box thinking, use a power fet to switch the transformer. Something like a 
555 timer chip could possibly drive the fet. Don't forget the back diode 
across the switching fet.
Paul K0UYA
----- Original Message ----- 
From: "J. Forster" <jfor at QUIK.COM>
To: <BOATANCHORS at LISTS.TEMPE.GOV>
Sent: Thursday, October 08, 2009 11:54 AM
Subject: 12 to 24 volt conversion.. MORE


> The problem of winding a transformer might be solved using a kludge of
> three filament transformers.
>
> First off, you'd need a pair of transformers capable of handling the
> needed output power, with the 115 VAC windings connected back-to-back.
> Thansformers roughly 115 VAC input and 24 VAC CT @ 10 A would be about
> right. One 24 VAC winding would be used to the collectors of the inverter
> transistors,m with the CT to +12 VDC. The other 24 VAC winding would be
> used w/ a FW half bridge. This isolated 12 - 15 VDC would be stacked on
> top of the battery.
>
> The trick is to use a smaller filament transformer, something like 115 VAC
> to 6 VAC @ 1A as a feedback winding to ythe transistor bases. All three
> 115 VAC windings would be connected in parallel.
>
> BTW, it'd likely be more efficient to build a stacking PS like this,
> rather than inverting the full power for the load.
>
> FWIW,
> -John
>
> ==================
>
>
>
>> There is quite a difference between 150 W  and 750 W class inverters. The
>> former are pretty straight forward and can use cheap parts like 2N3055s,
>> perhaps paralleled. The 750 W units will draw 70+ Amps @ 12 VDC and are a
>> lot harder. Also, they will flatten a typical car battery in under an
>> hour.
>>
>> As to magnetics, for silicon steel common in transformers, the rule of
>> thumb is 400 W/lb at 60 Hz. so a half pound core will do. More at higher
>> frequencies. It'd be easier to find an EI core in this size, rather than 
>> a
>> torroid.
>>
>> For running mil radios, I doubt any regulation would be needed.
>>
>> FWIW,
>> -John
>>
>> =============
>>
>>
>> [snip]
>>> Designing a DC-DC converter to deliver 10-30 amps at 24V is not an
>>> exercise
>>> for non engineers.  Hefty transistors and cores are needed, as well as
>>> sophisticated regulator circuits.  Sure, you can copy something, but
>>> even
>>> that requires some pretty good test equipment and, of course, buying the
>>> components that are modern enough to work well, like good FETs.  I have
>>> a
>>> little training and a lot of experience, some of which involved big
>>> transistors turning red hot, and I wouldn't try doing all the magnetics
>>> necessary to do this thing right! [snip]
>>
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