[AMRadio] Re: Pw Supply

Gary Schafer garyschafer at comcast.net
Thu Dec 7 18:17:09 EST 2006

See below..

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Geoff/W5OMR
> Sent: Thursday, December 07, 2006 4:48 PM
> To: Discussion of AM Radio in the Amateur Service
> Subject: Re: [AMRadio] Re: Pw Supply
> Gary Schafer wrote:
> > I am not so sure the correct answer was given and there was no
> explanation
> > of why two separate transformers would not work other than the "primary
> > current seemed higher".
> >
> > It seems this would make a much better topic to argue about than some
> other
> > recent ones.
> You didn't see Jim's post?  That's not what he said.
> > We needed 60 volts CT at 200 amperes with 208 vac primary. The secondary
> isolation from the primary had to withstand 200,000 volts DC. The core was
> a large 'C' core, and the whole thing sat in a oil tank. The layered
> layers of mylar insulation resulted in considerable leakage inductance
> within the transformer. Before we built the beast we tried two 30 volt @
> 100 ampere standard transformers to power up a big electro-magnet. It
> worked but the primary I was 2X what it should have been. Maybe the
> Tripplett clamp on ammeter was miss-behaving from the every half cycle
> current draw, but I also recall blowing circuit breakers, and darkening
> portions of the building. The boss was concerned to say the least. After
> building the 60v CT transformer prototype, everything worked fine, and had
> expected primary current. So was it core saturation from high DC current
> one way only, a power factor issue, or something else? I really don't
> know, and since that was 1981, my recall could be put in question. After
> all , over 50 now, so CRS is a fact of being an OM!  ;-)
> >
> >   I had to make a 0-200amp DC linear series regulator for that supply. A
> large water cooled heatsink and 32 TO-3 2N6258's in parallel.  What a
> monster that was!
> I don't have a good reason as to why, either.  I can't explain to you
> why the primary current of two power supplies doubles, when the output
> of the two supplies are in series.
> Meaning, I draw 3 amps from the power supply, when running 1500v @ 300mA
> on the final, and the modulator biased off at 150mA (0v bias @ 1500v),
> but if I turn on the second supply, and bias the modulator back to 150mA
> (with 2700v instead of 1500v) the primary current on the two supplies
> jumps from 3 to around 8.5 or 9amps.
> 'Splain that one?
> --
> 73 = Best Regards,
> -Geoff/W5OMR

Maybe I picked the wrong subject to get an argument going. :>)
Except for Geoff no one else seems up for it. :>)

Anyway, I did a little reading about two transformers with secondary's in
series and it seems that when you do that and use a full wave rectifier,
that each transformer acts like a half wave circuit and there is a DC
current in the winding (because it is pulsating DC) which tends to saturate
the core of the transformer. So it seems that one of Jim's assumptions was

Now if you put two transformer secondary's in series and use a BRIDGE
rectifier you don't have the core saturation problem because there is no DC
in the individual cores. Current flows both ways in each transformer over
the full cycle.

Geoff, on you problem with current increasing from what I can glean from
your post you double the voltage and set the modulator current to the same
as it was with lower voltage. That = more watts so more current in the
primary. Different problem than transformers in series.  

Gary  K4FMX

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