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Originally posted by simonbaker
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Continuing the train of thought... Yes I agree equal flux at the moment of "TX turn-off", but the flux is all due to coil 1. When MOSFET gate opens (TX turn-off), coil 1 has lots of current, coil 2 does not.
Well coil 1 can only have a maximum current of V/R where V = supply - diode drops etc and R=coil wiring + mosfet ON resistance. However at TX turn off the coil attempts to maintain that current and it can only do that by greatly increasing the voltage.Like water flowing out of a tap ... if the tap is very suddenly shut off then there will be a pressure spike that is maximum at the tap due to the inertia of the flowing water back up the pipe ... and voltage is analogous to pressure. At this point in time coil 2 only has the induced TX current which is limited by the amount that can flow into the damping resistor ... ie not much. With perfect coupling between the coils and since the coils are 1:1 the induced voltage can only = v_supply so the current will be v_supply/r_damping. Yes the flux is due to coil 1 but coil 2 'sees' it equally.
The currents cannot change instantaneously, so my thought is that the flux contributed by each coil is quite different and you need to solve for the current in each coil to see how the total flux changes during the period after TX turn-off.
The current in a coil in response to an applied voltage cannot change instantly .. I agree .... however just at the point of TX off we are talking about two coils connected in series with a flux of field Strength H cutting them. Now the critical thing about flux is that every part ( eg electrons etc ) of the coils 'sees' the flux whereas only the connected part of the coil sees a voltage pulse ( initially ). So if the flux is influencing every electron in the coil ( quantum experts shut eyes now ) then each electron is accelerated by the collapsing flux ie the 'instantaneous' current is proportional to the rate of flux collapse in BOTH coils since both coils see the same flux.
Imagine the coil as string of beads where each bead is an electron. When I apply a voltage to the first 'bead' it either moves away or is attracted depending on the polarity however when it moves it has to pull or push all the other beads in the coil ... ie the bigger the coil ... the longer the line of beads to push or pull.
However with a flux .... each bead is pushed or pulled individually so the 'inertia' if you like is not a whole string of beads as in the voltage case but only 1 bead.
So can you have an instantaneous current ? .... at the timescales that we are working in ... the answer is Yes. ( Quantum experts open eyes now ).
That is the flux that the target will see. I believe that at the moment of TX turn-off, most of the very large current in coil 1 is absorbed by capacitance since it cannot instantaneously flow through coil 2. At least I think it cannot instantaneously flow through coil 2 -- unless somehow the flux from coil 1 allows it to, but that doesn't feel right.
You can see from the experimental results that the series connected coils develop a symmetrical voltage across the damping resistor .... intuitively the current must be equal in both coils. I know you don't feel right about the flux ... however remember that the flux was generated by coil 1 it does not 'belong' to coil 1. The diodes in the circuit act as switches and the circuit 'changes' at the TX point turn off.
In fact, as the current in coil 1 decays, the current in coil 2, I assume, is growing. I would think this might cause the total dphi/dt initially to be diminished because the the total ampere-turns of the two coils are sort of staying constant - their rates-of-change are canceling. Is it possible this reduces the usefulness of the "center-tapped" design?
I still haven't spiced this so I'm just speculating -- the proof is in your experiments which look good.
I have spiced the result and it agrees with the experimental. I thought of this walking home from work and only built it up last night at 9pm so I have not done a schematic even.
Your latest circuit sounds interesting but I don't understand the scope traces. Can you explain each trace in detail?
Pic 1 is the flyback pulses measured at the input to the diff amp section. The differential coil throws out symmetrical but opposite polarity pulses... just like a centre tap transformer really.
Pic 2 is a close up of the pulse base showing mild underdamping but demonstrating the symmetry of the signal.
Pic 3 is just showing the +ve flyback timing in relation to the gate drive.
Next Pic shows the target response.
The coil is a 25 cm dia monocoil bifilar wound with 16 effective turns 0.7 mm dia magnet wire.
Regards,
-SB
Well coil 1 can only have a maximum current of V/R where V = supply - diode drops etc and R=coil wiring + mosfet ON resistance. However at TX turn off the coil attempts to maintain that current and it can only do that by greatly increasing the voltage.Like water flowing out of a tap ... if the tap is very suddenly shut off then there will be a pressure spike that is maximum at the tap due to the inertia of the flowing water back up the pipe ... and voltage is analogous to pressure. At this point in time coil 2 only has the induced TX current which is limited by the amount that can flow into the damping resistor ... ie not much. With perfect coupling between the coils and since the coils are 1:1 the induced voltage can only = v_supply so the current will be v_supply/r_damping. Yes the flux is due to coil 1 but coil 2 'sees' it equally.
The currents cannot change instantaneously, so my thought is that the flux contributed by each coil is quite different and you need to solve for the current in each coil to see how the total flux changes during the period after TX turn-off.
The current in a coil in response to an applied voltage cannot change instantly .. I agree .... however just at the point of TX off we are talking about two coils connected in series with a flux of field Strength H cutting them. Now the critical thing about flux is that every part ( eg electrons etc ) of the coils 'sees' the flux whereas only the connected part of the coil sees a voltage pulse ( initially ). So if the flux is influencing every electron in the coil ( quantum experts shut eyes now ) then each electron is accelerated by the collapsing flux ie the 'instantaneous' current is proportional to the rate of flux collapse in BOTH coils since both coils see the same flux.
Imagine the coil as string of beads where each bead is an electron. When I apply a voltage to the first 'bead' it either moves away or is attracted depending on the polarity however when it moves it has to pull or push all the other beads in the coil ... ie the bigger the coil ... the longer the line of beads to push or pull.
However with a flux .... each bead is pushed or pulled individually so the 'inertia' if you like is not a whole string of beads as in the voltage case but only 1 bead.
So can you have an instantaneous current ? .... at the timescales that we are working in ... the answer is Yes. ( Quantum experts open eyes now ).
That is the flux that the target will see. I believe that at the moment of TX turn-off, most of the very large current in coil 1 is absorbed by capacitance since it cannot instantaneously flow through coil 2. At least I think it cannot instantaneously flow through coil 2 -- unless somehow the flux from coil 1 allows it to, but that doesn't feel right.
You can see from the experimental results that the series connected coils develop a symmetrical voltage across the damping resistor .... intuitively the current must be equal in both coils. I know you don't feel right about the flux ... however remember that the flux was generated by coil 1 it does not 'belong' to coil 1. The diodes in the circuit act as switches and the circuit 'changes' at the TX point turn off.
In fact, as the current in coil 1 decays, the current in coil 2, I assume, is growing. I would think this might cause the total dphi/dt initially to be diminished because the the total ampere-turns of the two coils are sort of staying constant - their rates-of-change are canceling. Is it possible this reduces the usefulness of the "center-tapped" design?
I still haven't spiced this so I'm just speculating -- the proof is in your experiments which look good.
I have spiced the result and it agrees with the experimental. I thought of this walking home from work and only built it up last night at 9pm so I have not done a schematic even.
Your latest circuit sounds interesting but I don't understand the scope traces. Can you explain each trace in detail?
Pic 1 is the flyback pulses measured at the input to the diff amp section. The differential coil throws out symmetrical but opposite polarity pulses... just like a centre tap transformer really.
Pic 2 is a close up of the pulse base showing mild underdamping but demonstrating the symmetry of the signal.
Pic 3 is just showing the +ve flyback timing in relation to the gate drive.
Next Pic shows the target response.
The coil is a 25 cm dia monocoil bifilar wound with 16 effective turns 0.7 mm dia magnet wire.
Regards,
-SB
.... I dont see any point to improve beyond this or there will be no pulse left .. 
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