Re: LF: A transductor for power regulation?

[Andy Talbot <andy.g4jnt@gmail.com>]

The easiest way to think of a transformer  , I believe, is to accept it has "infinite"  (oar at least arbitrarily high) inductive reactance, and that part of it can be ignored.   So an open circuit on the secondary is transformed to an open circuit on the secondary;  ditto a short circuit.   And any impedance by N squared.   A practical "very high" Xl is ensured by using high Ue ferrite, and using enough turns that your ferrite is not degraded by saturation.

As soon as that "infinite" inductance begins to drop, it appears in parallel with the pri / sec  and contributes to the transformed load impedance.  Which lowers it with shunt inductive reactance.

Even worse is when the ferrite is driven into saturation, and the AC component drives it around the B-H hysteresis curve.  This dissipates energy in the core, and dissipated energy appears [electrically] to be added resistance.   Whether it appears in parallel or in series is moot;  it is there, and dissipating.

A tapped inductor, or two coupled inductors may have an incidental impedance transforming action , but if the inductance, or Xl, is significant and enters the matching  or load impedances - it is NOT a transformer

It is a pity that transformers and inductors both have the same circuit symbols, and similar wound construction in some cases.

So trying to control a transformer with DC saturation is very definitely not on.  But controlling a saturable inductor very definitely is acceptable.  If that inductor happens to be part of a matching network, or be tapped, or have two windings - you have to study , calculate and think what it is going to actually do

Andy  G4JNT

On 19 April 2017 at 09:41, DK7FC <selberdenken@posteo.de> wrote:

Hi Andy,

Yes yes, i agree, and that's what i don't fully understand yet. Last night i played a bit with the transductor that is laying arround here in a cupboard. It didn't take much time to build up a 1:1 transformer. I took my small MF PA and a 50 Ohm dummy load. I watched the sine wave across the R with a scope. About 50W were passed through the transformer. Then i started to run a DC current into the saturating coil. The amplitude on the dummy load decreased but the PA consumed even more current! Not a good working point.
But then, how is this transformer working? It looks like it can only work in a series resonant circuit, as you say. So it is not really a transfromer.
Actually i built something similar a few months ago, when i wanted to increase the resonance frequency of a HV mains transformer (antenna C = 480 pF switche in parallel to the HV winding). The idea was to saturate the two outher legs of the tripple leg transformer which has its windings on the center leg...

73, Stefan


Am 19.04.2017 08:59, schrieb Andy Talbot:

Stefan 

your idea of saturating the transformer is, I think, flawed.    If you reduce mu-r of the core by saturating with DC, the primary winding will have vastly reduced inductance and appear as a near short circuit to the transmitter - not what you want

The idea of saturable reactors applies to resonant inductors, where you use the DC to change the inductance.    So it could be applied to a class D transmitter with a tank arrangement, to retune the tank circuit to adjust for match, or control amplitude.

Andy  G4JNT

On 18 April 2017 at 17:18, DK7FC <selberdenken@posteo.de> wrote:

Hi all,

I'm just thinking about transductors and find them them quite fascinating again. I read a bit Wikipedia and thought about the use as a steerable transformer.
Imagine you have a hard switching class D PA, H bridge or so. The output is a constant voltage source and it is no problem to connect no load, as long as a proper type of low pass filter is used.
Then imagine a simple 1:1 ferrite transformer, 50 Ohm to 50 Ohm. The transformer has a 3rd winding for a DC current (to compensate the AC component transformed from the RF windings, 2 cores must be used, which are in parallel for the RF and anti-serial for DC).
Then i can saturate the transformer with the DC so the 皴 falls down to 1.
With this arrangement i could build a linear PA out of a switch mode PA, even a fast one, which avoids key-clicks, at least in QRSS-3 (you remember the mode) or OP32 or even EbNaut on LF?
Sounds like an interesting experiment at least. The question is how warm the transformer would become when the RF output current is permanently reduced to 50%.
Has someone ever tried that on LF/MF?

73, Stefan