MooseFET said:
I assume you are willing to waste 128mW. I think you may have
jumped from this to the idea that it is better to waste the
power. I disagree as I will go into below.
I disagree with this I will explain after the modified drawing.
----------
Vsupply ---------+----------+-----! Down !
| | ! shifting !---IL
! \ ! circuit !
! / ! !
! \ ! !
! ! ! !
! +-----! !
! ! ----------
| )
| Rsec1)
_|_ )
/_\ |
| )
| Rsec2)
| )
| |
| !
| |
| |)
--!!-- | |)Inductor
! ! | |)
+-/\/\-+ | |
! ! +----------+
Iset-/\/\+-!+\ ! |--+
! >-+---------||
IL--!-/ |--+
|
GND
The real inductor blocks the bulk of the AC from getting to the
control windings.
The reason I backed away from a high impedance drive was
the realisation that the bottom end of that inductor is
always at a low impedance point, either 0V or Vsupply.
This means that, irrespective of the PWM frequency, the
inductor has to have a high enough value such that it
presents a high impedance to the 100Hz ripple voltage.
That's 'high' relative to the source impedance of the
ripple voltage.
I did a crude attempt at measurement of the source
impedance, measuring the change in the pk-pk volts
as a result of changes in resistive loading of the
control winding.
At 0.5A, 1A, and 1.5A DC control current, the output
impedance of the ripple was around 1.5 to 1.6 ohms.
The inductor's impedance has to be much higher than
1.5 ohms. The waveshape of the ripple is not a sinewave
it is saturation difference spikes at 100Hz so the
inductance need not be as high as first thought, but
it will probably need to be in the mH region, (at 2A
dc polarising current).
BTW: The source resistance seen by those two primaries is
the 590 ohm load and I am attracted by the coincidence
that 590 ohm transformed by two 240:18V transformers
in parallel would present 1.65 ohms on the secondaries.
That's probably yet another bizarre TW-ism though.
BTW2: The voltage across each secondary is of the order
of 55V pk-pk, and the resultant difference ripple is
around 2.5V pk-pk maximum. So they are not doing too bad
a job of looking at each other whilst one of them is
going into a region of low relative permeability.
I loose more power in the sense resistor but it makes the control
circuit much easier. By removing the capacitor on the windings
we end up controlling the actual current in the control windings
and not the voltage on them. This mostly takes the pole from the
inductance of the windings out of the servo loop. The PWM shows
a very high impedance to the control windings.
I had thought that the transformers effectively shorted
each other out so the impedance looking back into the
control winding would be mostly resistive. Worth a
quick check then.
The input impedance was measured at 50Hz and 5KHz
with a signal generator and scope. Calcs suggest the
control input looks like 1.4 ohms and 92uH in series.
1.4 ohms is near the sum of the secondary and transformed
primary resistances (1.3R), and I suspect that 92uH is
something like two leakage inductances in series.
I assume you have a feedback from the temperature of the heater
to the Iset. This will have a large phase lag. The mass will
integrate and the distance from the heater to the thermistor may
add a transport delay. The last thing you will want is another
couple of poles in the control circuit of the saturable reactor.
That's getting far too complicated for me atm.
