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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 27th Nov 2019, 9:32 am   #21
kalee20
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Default Re: Back-EMF from series and parallel inductors?

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Originally Posted by Argus25 View Post
To clarify a little more, the term "back emf" I think was coined to describe "counter-emf".

Counter emf was to describe something being countered, in this case the voltage or emf source applied to an inductor (or as pointed out applied to a motor).

This is why "back emf" or a back emf supply is an inappropriate term for these types of impulse power supplies that rely on a collapsing field for a high voltage pulse.

Looking at the plethora of examples on the internet where collapsing fields generating a voltage peak have been called "back emf" its not a wonder there is so much confusion about what it means.
I'd be very much inclined to agree with Argus, use of 'back-EMF' is inappropriate for the nuisance voltage spike generated by a relay coil, or for the flyback voltage in a power supply or timebase transformer.

But, unfortunately, it's become established. And ultimately, it's what people understand by a word or phrase which matters, whether or not the words of the phrase are appropriate or not.
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Old 27th Nov 2019, 12:03 pm   #22
Argus25
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Default Re: Back-EMF from series and parallel inductors?

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I'd be very much inclined to agree with Argus, use of 'back-EMF' is inappropriate for the nuisance voltage spike generated by a relay coil, or for the flyback voltage in a power supply or timebase transformer.

But, unfortunately, it's become established. And ultimately, it's what people understand by a word or phrase which matters, whether or not the words of the phrase are appropriate or not.
I was thinking you would probably come up with some words of wisdom.

In the end of course it may well be just semantics, because V = - L(di)/dt is never violated.

It got me to thinking whether there could be a better way to name the flyback or impulse power supply.

If a pressure gradient is applied to an elastic fluid filled tube, initially there is a reactive pressure from the column of fluid, which like any mass , resists a change in motion. So you get a back pressure from it (like back emf). After a while the fluid current reaches a stable flow rate and the reactive pressure disappears, just as back emf does.

The equivalent of disconnecting the inductor, is cutting off the flow, say with a valve that isolates the pressure source abruptly from the fluid column.

When that happens the motion (kinetic energy of the fluid) generates a reverse or negative pressure. Like pulling on a mass moving away from you. Due to the system compliance (capacitance) it sets up a series of oscillations in the tubing. Much like the inductor when it had a current & magnetic field established, and you shut it off abruptly.

In plumbing of course this is called a "Water Hammer" effect.

So maybe these impulse power supplies could be called an "Electron Hammer" And you harvest the first half cycle for your high voltage.
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Old 27th Nov 2019, 5:35 pm   #23
Neutrino
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Default Re: Back-EMF from series and parallel inductors?

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Originally Posted by Martin G7MRV View Post
So what is the 'correct' terminology in this case?
emf

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Originally Posted by Argus25 View Post

It got me to thinking whether there could be a better way to name the flyback or impulse power supply.
Boost Converter

Most TVs have had booster/efficiency diodes for as long as I can remember (PY81, PY800, PY88 and PY500).
http://www.r-type.org/pdfs/py800.pdf

Numerous boost converter ICs are available:
https://uk.rs-online.com/web/c/semic...st-converters/

David
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Old 27th Nov 2019, 6:59 pm   #24
kalee20
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Default Re: Back-EMF from series and parallel inductors?

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Quote:
Originally Posted by Argus25 View Post
It got me to thinking whether there could be a better way to name the flyback or impulse power supply.
Boost Converter
Unfortunately 'boost converter' is already 'taken' as a name - it's a different connection of switch, inductor, diode to the 'flyback' converter (also known as 'buck-boost.'

Boost converter always gives an output voltage higher than input (and is inherently NOT short-circuit proof). Flyback can do everything.

They do both rely on the (ahem...) back-EMF of the inductor when the switch is turned off.
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Old 28th Nov 2019, 1:11 am   #25
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Default Re: Back-EMF from series and parallel inductors?

To my understanding, back emf is stated in Lenz's law. Lenz's law states that the current induced in a circuit due to a change or a motion in a magnetic field is so directed as to oppose the change in flux and to exert a mechanical force opposing the motion.
As I and a lot of blokes who worked on rail circuits with track circuits, the voltage generated can be nasty.
For a simple inductor, the voltage is ( in simplest form) = L x (di/dt). For a transformer ( before someone comes in saying it's different- it is) , thre's also the effects on the secondary etc.
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Old 30th Nov 2019, 7:18 pm   #26
Argus25
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Default Re: Back-EMF from series and parallel inductors?

Here is something that is very helpful in designing a flyback converter.

If the feedback which generates and sustains the oscillations originates from the transformer itself, it solves the problem of the inefficiencies that occur when the rate of change of current with time becomes non-linear and tapers off. Since the induced voltage is proportional to the rate of change of current, the feedback simply falls away as the rate of rise of current becomes non linear, and then as that happens the feedback voltage drops suddenly and abruptly cuts off the transistor.

So the circuit automatically adjusts itself to suit the inductance properties of the transformer and you don't have to worry about the transistor being turned on for too long by the driver circuit, plus it gets rid of a driver transistor. And on top of that, the circuit is intrinsically short circuit proof.

Flyback supplies are used in aircraft exciters, to get the voltage up enough to strike a GDT. Typical circuit attached.
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