# Non-changing current with C-EMF

Discussion in 'General Electronics Discussion' started by XRZ, Jul 30, 2014.

1. ### XRZ

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Jul 30, 2014
I have a circuit, that has an exterior magnetic field, that changes. Naturally, the conductor will produce an opposing EMF & an opposing current. My circuit requires, a constant current regardless of C-EMF, of course with the supply of more power. Now, how can this be solved in a general?
1) Fix the circuit's voltage to oppose counter-EMF.
2) Have a constant current source that will change and stabilize the current.

What are you recommendations, or solutions in such situations?

2. ### Gryd3

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Jun 25, 2014
Have you thought about providing shielding for the circuit to prevent it from picking up external magnetic fields?
What kind of circuit are you working on?

5,165
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Dec 18, 2013
Can we have a bit more detail please. A circuit diagram would be good, and as Gryd3 said what's it for.
Thanks

4. ### XRZ

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Jul 30, 2014
Ah, forgot to state that this is an experiment demonstrating how current in a conductor can be maintained in the presences of a changing magnetic flux.
This is a simple diagram schematic:

Therefore, we can't shield the magnetic field, and the methods of how the flux changes does not matters as well. The power can increase to accommodates more voltage for current, but at what cost? higher resistance?

5. ### Gryd3

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Jun 25, 2014
If I had tried this circuit, I'd use a constant current source. (They typically have feedback to determine the amount of current being used, and auto-adjust to keep it consistent)
You can help protect from sudden spikes by using filters in the form of inductors and capacitors.

May I ask what this experiment will be for? It sounds like a school project

6. ### XRZ

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Jul 30, 2014
A constant current source "changes" its voltage to maintain the same current right? So that the whole time current is the same? Only voltage is changing throughout the process? How long does it take to maintain current if it indeed does decrease -then-increases due to the constant current source?

It's just a project for myself(experimenting to learn). Where I'm trying to maintain the same current all the time, when back-emf is induced due to an exterior magnetic flux changing.

7. ### Gryd3

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Jun 25, 2014
I'm sorry but I do not have any specs on how fast a constant current source can compensate, how what it's limitations are as far as how hard or fast the flux changes.
I would try to employ a filter in the form of an inductor which should also assist (passively) in keeping the current consistent.

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
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Jan 21, 2010
yep.

That's the idea. Nothing is perfect, but depending on the design you might be able to regulate the current to 1% or better with load.

Well, there's only voltage and current, and you're trying to keep one constant, so (within reason) only the voltage changes.

It depends on the circuit. It may take microseconds to react, or it may be faster. If really poorly designed it may be slower.

Then it sounds like a constant current source is what you want.

9. ### XRZ

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Jul 30, 2014
This concept can be applied to all kinds of conductors, and circuits? Even circuits that carry kA, to conductor's that are quite large(busbars)etc...

5,165
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Dec 18, 2013
Can you just clarify a few things please. You show the power source as a battery, are you planning on switch this on and off if so at what frequency. You show something that looks like a magnet, is this producing the changing magnetic field or is this just a horse shoe magnet of some kind.

Thanks

11. ### XRZ

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Jul 30, 2014
@Steve,to increase voltage wouldn't the circuit's resistance change to increase the voltage and maintain the same current?

Say we have a circuit that's resistance is 0.01 Ohms(that's the resistance of the conductors, so it should be fixed) the induced-emf(back-emf)is calculated to be 2V(it's always present so its a constantly induced, from a changing magnetic field, or motion in that magnetic field), the desired current with the present of back-emf is 100A, now the required voltage would be 0.001 x 100A = 1V, now the induced-emf is the problem. How would this be solved? o that the circuit will have the same current with the existence of back-emf always?

Of course, by adding more power correct? Okay, but how?
If more power is added to apply a higher voltage to maintain the back-emf, how would that work out?

sourceVoltage - inducedVoltage = totalVoltage?
Okay, if that is the case... 3V - 2V = 1V, but the resistance was previously increased, now the current will decreased?

Btw, do the source voltage cancel? Would it stay at 3V or drop to 2V due to back-emf?

12. ### Gryd3

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Jun 25, 2014
XRZ, you may be over complicating things.
V=IR is a nice little triangle. When you need to change one item, you can usually pick which of the other two you can alter.
This is the case with constant current sources... they cannot alter the resistance of the circuit, so they alter the voltage provided to it.
Constant voltage sources alter the current provided to keep voltage the same.
Even a source has a resistance though... so as you say, the back-emf induced by a magnetic field will induce a voltage in the wire, when seen externally though from a power supply point of view, this could be seen as the circuit changing it's resistance, and the source could then compensate.
Something to consider is the scale of your numbers you are using. 100A being induced by a 1V charge is highly unlikely in many known circuits. The exception would be a single loop of wire with no components. Like I said earlier... even voltage/current sources have a resistance, so you would need to have no resistive components in your circuit.

Alternatively, depending on the EMF, other properties such as inductance and capacitance which is present in all electronics to a degree can help mitigate small EMF.

There is something potentially tricky though... If your circuit includes items connected in parallel, the EMF could only affect one of the parallel branches and create it's own circuit within the complex circuit that the power source would not see as the sum of the voltages or currents could be 0 where the parallel circuits join together at the power supply.

13. ### XRZ

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Jul 30, 2014
Gryd3, In the case of a constant current source, how can the voltage be altered without changing the resistance of the circuit?
Using the example I provided, how would things work out?

To maintain 100A, what would be the result, how much power is needed before back-emf, and after back-emf is induced?
My assumption: If back-emf is 2V, the voltage has to rise to 3V, tripling the power from 100W to 300W now? Assuming resistance is the same.
But how can that be done without changing the resistance?

**The numbers used in the example are random.**
And yes, this concept has no components just a loop of wire connected to a power source, in the presence of a magnetic field, the goal is to maintain the same current, regardless of induced-emf.

14. ### Gryd3

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Jun 25, 2014
Typical constant voltage or constant current sources operate by reducing a raw input to a regulated output. So in a simplified sense, the constant voltage source would alter it's own internal resistance to adjust the voltage applied to the circuit. In your example, if the current source and circuit had a very small resistance, the back emf may be too much for the supply to compensate for for a sustained period of time.
In most applications, the circuit has a much higher resistance by the order of a magnitude or more, and the voltage or current regulator has a higher resistance and a bit of head room to allow itself to compensate. Add to that filters that redirect and mitigate noise and EMF and it becomes a non-issue for most devices.
your particular example could be present in a complex circuit where only a portion of the circuit is exposed to EMF in which an alternative current path through the circuit could bypass the voltage or current regulator and cause some damage...
It's difficult for me to explain, because although the numbers are made up, I can't think of a real world example to compare it to or use as an example.

15. ### XRZ

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Jul 30, 2014
I assumed things completely wrong.... I thought that a power source that has a supply of 300W can manipulate the voltage to the value required, and the current to the value desired.
Even if the resistance of the circuit is low, because if it is low... the voltage required to sustain such current would be: R x I , which is 1V.

So it's somewhat impossible to apply varying values of V( in our case beyond 1) to a circuit with low resistance, without applying massive power.
Because in theory to have the same current with that large amount of back-emf induced... we require at least 900W, based on ohms law.

16. ### XRZ

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Jul 30, 2014
But... then again im talking about the resistance of the circuit, where I should focus on the voltage and the internal resistance of a PS.
@Gryd3 , in my example isn't the internal resistance of the PS the important factor to control V? not the circuit's resistance?
I think I'm now confused.

17. ### Gryd3

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Jun 25, 2014
Almost but not quite. You can regulate Voltage or Current but not both, as you can not typically adjust the circuit's resistance enough to make a difference. The circuit's equivalent resistance can change during operation, which leaves only Voltage and Current to manipulate. Of course, the one that is required to remain constant will dictate the other one needing to change.

18. ### Gryd3

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Jun 25, 2014
In your example, you should factor in the resistance of the circuit as a whole.
The circuit would only include the current path of the EMF.

19. ### XRZ

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Jul 30, 2014
For my example I think a reasonable solution would be, adding a 1Ohm resistor that can handle such power.

20. ### XRZ

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Jul 30, 2014
Allowing the power source to compensate.