# Back EMF From Only A Small Inductor?

#### russwr

Joined Aug 29, 2017
68
How much back EMF volts can I get from positive 35v DC on+ off, at 6 amps DC , to inductor 1.75 millihenry to 10 ohms resistance to ground? I need = > 100volt over the resistance. Fast type diodes inverse parallel. The Formula depends on speed of a switch type as solid state transistor, (from it's data sheet part number), and amount of inductance. I have neon bulbs type variable voltmeter 110 to 470v. Is there a simple diagram to test this as actually flashing? E = L X di /dt
Or maybe the main line has resistor only, and charges coil L on the side, same time. Then coil L reverses back and sends high volts through resistor to ground in same original direction as positive power supply.

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#### LowQCab

Joined Nov 6, 2012
4,305
It sounds like You want to get ~1000-Watts out, from a ~210-Watt input .........
I don't think it's going to work that way.
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#### Papabravo

Joined Feb 24, 2006
21,314
I have no idea what you are trying to accomplish, but it sounds like trying to catch lightning in a bottle. Back EMF is transitory and not something you can capture and use. How about producing a schematic diagram instead of the offered word salad, which is as useless as a screen door on a submarine.

#### BobTPH

Joined Jun 5, 2013
9,284
Back EMF is produced by motors, not inductors.

What you are referring to, I presume, is flyback voltage.

#### Irving

Joined Jan 30, 2016
4,065
Back EMF is produced by motors, not inductors.

What you are referring to, I presume, is flyback voltage.
To be fair Bob, V = -Ldi/dt whether its a motor, a solenoid or any other form of inductor. Its still Back EMF as it opposes the change in current through the inductor.

#### BobTPH

Joined Jun 5, 2013
9,284
I disagree, the back EMF in a motor comes from the spinning motor acting as a generator, not from voltage reversal of an inductor.

Before posting that , I did a google search on back EMF, and every definition found had to do with spinning motors.

Counter EMF is sometimes used for inductors.

#### Irving

Joined Jan 30, 2016
4,065
I beg to differ... one of dozens of examples...

" If there is a changing current, the magnetic field surrounding a conductor changes in intensity. Lenz’s law tells us that a changing current causes an induced emf in such a direction as to produce a current opposed to the original change. If the current increases, a back emf opposes the increase. If the current decreases, the back emf (now a “forward” emf) tends to prevent the decrease. The effect is analogous to inertia in mechanical systems. It can be greatly increased by making the conductor in the form of a coil. The addition of an iron core ordinarily increases the effect still more. "

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#### BobTPH

Joined Jun 5, 2013
9,284
I don’t see the term “back EMF” in that.

#### Ian0

Joined Aug 7, 2020
10,276
How much back EMF volts can I get from positive 35v DC on+ off, at 6 amps DC , to inductor 1.75 millihenry to 10 ohms resistance to ground?
A little bit more than the Vceo voltage of the transistor that you are switching it with.

#### russwr

Joined Aug 29, 2017
68
A little bit more than the Vceo voltage of the transistor that you are switching it with.
Thanks!

#### Irving

Joined Jan 30, 2016
4,065
@BobTPH You need glasses more than I do! Go look again...

@Ian0

#### MisterBill2

Joined Jan 23, 2018
19,461
Based on post #1 it seems like the TS is wanting to produce a high power inductive kickback voltage. Where the fast diodes in parallel fit is not clear. So at the very least a coherent description of the circuit described is needed.
The reality is that all of the energy in any inductive kickback spike comes from what is stored in the magnetic field that is collapsing when the current stops. And if the magnetic material is saturated then there is no way to get any more energy. And as the spike gets taller it also gets narrower.

#### BobTPH

Joined Jun 5, 2013
9,284
@BobTPH You need glasses more than I do! Go look again...

@Ian0
No, you do. I found several defs for “back emf” that were general like that, but “back EMF” always refers to motors. I even found a dictionary that gave both definitions:

Back emf vs back EMF

#### russwr

Joined Aug 29, 2017
68
Based on post #1 it seems like the TS is wanting to produce a high power inductive kickback voltage. Where the fast diodes in parallel fit is not clear. So at the very least a coherent description of the circuit described is needed.
The reality is that all of the energy in any inductive kickback spike comes from what is stored in the magnetic field that is collapsing when the current stops. And if the magnetic material is saturated then there is no way to get any more energy. And as the spike gets taller it also gets narrower.
Extra Watts energy not desired. Only higher voltage spike desired. In a different way, power supply could also charge both load and inductor same time as two separate lines, The outer diode directs the higher volts through the load in same direction as power supply. I asked for a sample circuit with a transistor. A hand switch is too slow for milliseconds timing. The formula requires fast switching. (push switch with NE555 monostable pulse output fires transistor/MOSFET for X milliseconds that allows resistor load to operate from power battery. When off, the inductor produces higher 100 volts through same load, but not at the exact same time. I never saw a back EMF coil make high volts.

#### Irving

Joined Jan 30, 2016
4,065
No, you do. I found several defs for “back emf” that were general like that, but “back EMF” always refers to motors. I even found a dictionary that gave both definitions:

Back emf vs back EMF
Well if you use a US dictionary....

The Oxford English Dictionary says they are synonyms, and that has always been my understanding and general usage - even at Uni, the great Prof Eric Laithwaite, who taught me Electrical Engineering, didn't differentiate, and he should know!

#### LesJones

Joined Jan 8, 2017
4,227
If the inducter is in parallel with a 10 ohm resistor and the initial current through the inducter is 6 amps then the inducter will try to maintain the current of 6 amps through it. So the initial voltage across the resistor will be 60 volts.

Les.

#### MisterBill2

Joined Jan 23, 2018
19,461
It seems to me that the TS is asking about a manually operated "boost" switcher supply. See my previous post for why it will not work very well.

#### MrAl

Joined Jun 17, 2014
11,725
How much back EMF volts can I get from positive 35v DC on+ off, at 6 amps DC , to inductor 1.75 millihenry to 10 ohms resistance to ground? I need = > 100volt over the resistance. Fast type diodes inverse parallel. The Formula depends on speed of a switch type as solid state transistor, (from it's data sheet part number), and amount of inductance. I have neon bulbs type variable voltmeter 110 to 470v. Is there a simple diagram to test this as actually flashing? E = L X di /dt
Or maybe the main line has resistor only, and charges coil L on the side, same time. Then coil L reverses back and sends high volts through resistor to ground in same original direction as positive power supply.
Hi,

If I understand you right, you are going to charge up an inductor of 1.75mH with a DC source that takes the current up to 6 amps, then discharge it into a 10 Ohm resistor.
For this the overall circuit function changes from E=L*di/dt because there is also resistance in the circuit, although that is still how the inductor itself behaves.

The formula for the circuit would be:
iL=-i0*e^(-t*R/L)
where
iL is the decreasing current through R over time,
i0 is the initial inductor current (6 amps),
R is the resistor value (10),
L is the inductor value (1.75mH),
t is time in seconds.
You can ignore the minus sign and use:
iL=i0*e^(-t*R/L)

Using the values you provided this comes out to:
iL=6*e^(-5714.285714285715*t)
What this tells us is that after one time constant (175us) the current will be down to about 2.2 amps, and after 5 time constants (875us) the current will be less than 0.05 amps.

It's also interesting that the voltage will start out at 6*10=60 volts and gradually decrease over time in a similar manner to how the current decreases. Ohms Law: E=i*R, with 'i' decreasing over time.

The current (and also the voltage across the resistor) will be almost zero in less than 1 millisecond, is this something you want to happen?

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#### MrAl

Joined Jun 17, 2014
11,725
Hello again,

About the definition of Back EMF, Back emf, back emf, etc.

I think it depends on the context. If you are dealing with a motor then it is the counter EMF caused by the rotation of the rotor, and if dealing with an inductor it is the counter EMF caused by the changing current and thus the changing magnetic field.

I've read and heard the use of Back EMF in both situations myself. The alternative would be to restrict the use of Back EMF to motor applications and use instead Counter EMF for an inductor. I'm not sure it would make sense to have to do that. A lot of references use Back EMF for both, if you disagree then you should look into why these references do that.

#### MisterBill2

Joined Jan 23, 2018
19,461
We could also choose to call the spike generated when the current is halted an "Inductive spike", and choose to not argue about easy stuff any more.

AND, I amazed looking at a catalog of clothing used to protect individuals from "EMF", which in that publication is used to describe those deadly "Electro-Magnetic-Fields" that are poisoning us constantly. That whole fraudulent business is a testimony as to why we all should have a lot more science education in the very early grades of school.