Silly question about voltage and current

[MarkE]

Why is this important, because we can use the pos energy to pulse a coil, now the diode works bidirectional it passes the pos bemf from the negatively pulsed coil into another coil, pulsing this coil with pos energy its bemf will be neg and will pass through the same diode, now your coils can ring back and forth.

Even if you fix the MOSFET orientation you are in trouble.  That is a good way to ruin a perfectly good MOSFET.

[Dave45]

Even if you fix the MOSFET orientation you are in trouble.  That is a good way to ruin a perfectly good MOSFET.

Ok fix the mosfet orientation - the mosfet is not what we are discussing here. This is how a discussion turns into miles of senseless posts.

We are talking about the bemf of a negatively pulsed coil and its polarity not the mosfet orientation, so its wrong, I told you I took it from a schematic that was handy.

[MarkE]

Ok fix the mosfet orientation - the mosfet is not what we are discussing here. This is how a discussion turns into miles of senseless posts.

We are talking about the bemf of a negatively pulsed coil and its polarity not the mosfet orientation, so its wrong, I told you I took it from a schematic that was handy.

Dave, all coils have real resistance.  All diodes have resistance and forward voltage drop.  Therefore, anything that you do to connect a coil through a diode dissipates energy that you put into the coil to build up its magnetic field. 

Forty years ago when transistors were expensive so called bi-level circuits were popular for things like solenoids and stepper motor drives.  A drive voltage would be applied to a coil for a certain period of time to build up the current, and then a lower sustaining voltage would be applied that would ideally match the I*R drop of the coil in order to hold the current.  The power drawn from that second, lower voltage supply is the ongoing loss in the magnetics.  Since the mid 1980's we have had wonderful switching drivers that deliver the same effect by periodically connecting the coil to a power supply and then allowing the current to recirculate either through the coil or back through the power supply.  These devices are carefully designed to minimize losses.  And there are always losses.  The bottom line is that any combination of coils and switches you can think of, it is almost certain that someone else has already used in some form or another.  Resistance, eddy current losses, and hysteresis losses aren't going away.  They eat at the energy stored in any non-superconducting magnetics ever built.

[MileHigh]

Dave:

My request is that if you are going to put up a diagram, then make it complete.  Assuming that it is being driven by an external voltage source, what are the polarities of that voltage source?  You put arrows up for current, are you talking about electron current or conventional current?  You say "positive this" or "negative that" with reference to something in your simplified schematic, but you put no polarity indications on the components you are talking about.   It would take you five more minutes to mark up your schematic segment, so why not do it?  Why should the readers of the thread have to do mental gymnastics trying to figure out what you really mean?

Just take the example of a "positively pulsed coil."  You can just as easily call that a "negatively pulsed coil."  If you don't define which of the two terminals of the coil is your voltage reference, then your statements about the polarity of the pulsing of the coil are meaningless.

Why is this important, because we can use the pos energy to pulse a coil, now the diode works bidirectional it passes the pos bemf from the negatively pulsed coil into another coil, pulsing this coil with pos energy its bemf will be neg and will pass through the same diode, now your coils can ring back and forth.

Yeah, sure, that's really clear.  I can do mental gymnastics trying to figure out what you are saying, or you can do a proper diagram or sequence of diagrams to effectively communicate what you are trying to get across.  About the only thing that I can say is that relative to schematic snippets I have seen, there will be no coils ringing back and forth.  Why don't you draw out a timing diagram along with a properly done schematic diagram to explain what you are trying to get across.

MileHigh

[Farmhand]

Dave, if you don't draw things correctly then it is going to throw the conversation off.

Well this is all a bit odd, I have evidence to show that when a coil is snubbed by a freewheel diode the current in the coil stops immediately as compared to when the coil is allowed to discharge into a higher voltage.

Dave you are trying to use electron movement "current" for the coil charging then applying conventional current to the coil collapse. The coil is charged with positive potential and discharges current in the same direction of current flow when the coil is switched off, very simple the discharge of a coil is not Back emf is is forward emf. Understand that and you'll get somewhere.

Here these scope shots show the voltage and current through the coils when the coil is allowed to discharge through a higher voltage load and also what happens when the coil is snubbed by a free wheeling diode. As we can see the snubbed coil's current ceases immediately with no extra current ( the energy is wasted) likely burned off in the diode and coil, where the coil discharging to a higher voltage takes time to discharge.

Not talking in absolutes but to me the theory that the current recirculates in the snubbed coil falls flat on it's face in reality.

The top two shots are from a snubbed coil with a free wheeling diode (current stops immediately on switch off), and the bottom two the coil discharges into a load at double the supply voltage and current continues after switch off. Yellow are current traces across a CSR and blue are drain voltage traces.

The bottom of the "on" times looks angled because the scope grounds were connected to the positive rail at a discharge capacitor, I can assure you the switching was sharp as anyone would want. I grounded the scope there for ease of connection so I could measure the current and voltage together and keep the grounds in the same place.



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