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Abruptly detaching a power source from a coil produces a back-emf: this is a well known fact.

What I would like to achieve, in my experimentation, is to convey ALL the collapsing magnetic field to the secondary side of a transformer, shorting the primary side immediately before the magnetic flux starts to collapse.

This requires a very fast switching system, probably through mosfets.

As a power source, I have a 12V/10A battery and the transformer is a big MOT (Microwave Oven Transformer): at the moment the power reaches the primary coil passing through a SPDT relay and, when it switches, the power is disconnected and the primary coil shorted.

Unfortunately the relay is not so fast to achieve the desired result so a small arc can be observed at the internal relay contacts, showing that the magnetic field has collapsed on both ends of the transformer.

Is there a way to overcome the problem?

A circuit diagram will be appreciated a lot!

Edited on 25/03/2015

The primary feeding/detaching source/shorting the primary/re-attaching source/feeding sequence (in a countinuous loop) of the primary side has to be accomplished thousand time per second...

I have to collect the spikes of the back-emf (due to the collapsing magnetic field) ONLY on the secondary side!

Like in a water pump, the water (electric field) must flow in one direction only; when the magnetic field collapses (using the above analogy) acts like a water flow under pressure which tries to exits from any hole in the piping system: I want to seal the entrance (primay side of the transformer) when the pressure is at its maximum level and convey all the flux to the exit (secondary side of the transformer); no way for the water (electric flux) to flow back to the entrance.

So, in essence, I need an electronic "valve" rapid enough to avoid the back-emf on the primary side.

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  • \$\begingroup\$ some accurately timed very fast mostfets might come near, but what are you really trying to achieve? \$\endgroup\$ – PlasmaHH Mar 23 '15 at 22:43
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Pl. connect a fast acting fuse (capacity depends on the flux level in transformer you desire to experiment with) )in line with battery and transforme.connect a fast acting FREE wheeling diode across the transformer primary with cathode to positive (reverse biased) and do the testing. V T Ingole

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  • \$\begingroup\$ Thanks Vijay for your fast answer. I'm not sure to have understood well your advice...a fuse? GBARRY is suggesting the same...in my little knowledge in electronics I know that a fuse is a very thin filament which burns once the rated current is exceeded: in which way do I have do play with? I edited my question above, please take a look of it and, if you already well understood that and given the correct answer, please provide a schematic and a simple list of components to let me understand your solution. Thanks in advance. \$\endgroup\$ – Devesh Mar 25 '15 at 9:49
  • \$\begingroup\$ This advice, for me, is the nearest solution to the problem. Thanks! \$\endgroup\$ – Devesh Mar 27 '15 at 12:31
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You're using the battery to charge the transformer? Well, a quick and dirty way could be to put a fuse in series with the battery. Then just short the transformer. For a little better solution, construct a current limiter and put it in place of the fuse. Actually, I'd still leave the fuse in.

Edited, base on the addition of:

"The primary feeding/detaching source/shorting the primary/re-attaching source/feeding sequence (in a countinuous loop) of the primary side has to be accomplished thousand time per second..."

Using a fuse, you only get ONE cycle; then you have to replace the fuse. This is fine if all you want to do is measure the spike on the secondary. Your edited proposal shows it must be continuous...now it looks a lot like a switching supply. @Vijay may have been thinking of a "resettable fuse", but those are not going to work any any reasonable repetition rate.

The "fast acting diode" or even @PlasmaHH's use of MOSFETs can do a pretty good job, but your assertion that you must capture ALL of the stored energy suggests that they won't quite be good enough. You request sounds like physics research where you are interested in absolute measurements, rather than an electronic design of a device where you are able to make some trade-offs in order to get performance that is "good enough".

Edited again...I thought about this some more.

If you short the primary, it will attempt to absorb all the energy from the collapsing field. It will be dissipated in the (low) resistance of the primary. If you want to capture all the energy into the secondary, you really want to leave the primary open during this time. This should be a lot easier to arrange.

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  • \$\begingroup\$ Thanks GBARRY for your fast answer. I'm not sure to have understood well your advice...a fuse? VIJAY is suggesting the same...in my little knowledge in electronics I know that a fuse is a very thin filament which burns once the rated current is exceeded: in which way do I have do play with? I edited my question above, please take a look of it and, if you already well understood that and given the correct answer, please provide a schematic and a simple list of components to let me understand your solution. Thanks in advance. \$\endgroup\$ – Devesh Mar 25 '15 at 9:47
  • \$\begingroup\$ Still a little unclear as to the ultimate purpose of this, but I've edited my answer to respond to your edits (now it has to run at 1000 Hz) \$\endgroup\$ – gbarry Mar 26 '15 at 8:40
  • \$\begingroup\$ Gbarry, thanks once more! My edit achieved the hoped result: to make my question clearer... You said "Still a little unclear as to the ultimate purpose of this" and PlasmaHH said "but what are you really trying to achieve?"... The answer is simple and I thought to have explained it in my question: to collect as many spikes as possible, without losses/leakages. Once we've at our disposal all the energy harvested then we can use it for our purposes. \$\endgroup\$ – Devesh Mar 27 '15 at 12:27
  • \$\begingroup\$ You can see it as a "performance gain" or, from another point of view, as a way to produce a coefficient of performance (COP) better than previously. Gbarry, you said also "Using a fuse, you only get ONE cycle", that's why I got surprised about your first advice and edit my first question... \$\endgroup\$ – Devesh Mar 27 '15 at 12:28
  • \$\begingroup\$ In response to your assertion "If you short the primary, it will attempt to absorb all the energy from the collapsing field" IMHO it isn't true if there is a channel (the core) which redistribute the collapsing magnetic field along its ferromagnetic path, expecially if there is, at the output of the secondary side, a load that will absorb the entire flux. \$\endgroup\$ – Devesh Mar 27 '15 at 12:28

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