# Non-changing current with C-EMF

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

1. ### Gryd3

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Jun 25, 2014
Adding resistance, is one way, you can also add capacitance, or inductance. They have a nice property where their effective resistance is dependant on the frequency of the current/voltage... so a sudden burst of EMF with a very high rise and fall would treat the inductor as a really high value resistor, or a capacitor as a very very low resistor. This allows you to mitigate or redirect these kinds of things to ground.

Now as far as the resistance of the circuit is concerned.. this is where I get hung up a little..
1 foot of 24 gauge wire will have almost 0.03Ω of resistance, any components in-line will increase the circuit's resistance even further. Can you give me an example of how you are thinking a circuit would be less than an ohm? (Only thing I can think of is multiple parallel components... and all the components get exposed to EMF while the power supply portion does not.)

2. ### XRZ

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Jul 30, 2014
Why did I assume it's impossible to solve such a problem , by adding a resistor alone would work but certainly a resistor that can handle the dissipation of 200W... I mean if current is going to be 100A, and resistance is 1 Ohm.

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

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Jan 21, 2010
That's I2R, not 2IR.

100 * 100 * 1 = 10,000W

4. ### XRZ

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Jul 30, 2014
Ops, well that's a lot.

5. ### XRZ

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Jul 30, 2014
It's a bit mind boggling that in order to maintain the same current (100A) when back-emf is induced(2V) almost about 10,000W is needed to maintain the same current, or maybe I missed something again @Gryd3, @(*steve*) ?

6. ### Gryd3

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Jun 25, 2014
Well, I'm thinking the math is right... but what if you apply the same situation to a 50Amp constant current supply driving a 1/4Ω load?
EMF causing a 2V spike would induce an 8A current through a 1/4Ω load which sounds incredibly high for EMF... I'm certain the examples we are looking at are flawed somehow. There has got to be an easier way to look at it.

7. ### XRZ

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Jul 30, 2014
But then again, if there is a constant resistance of 1 Ohm, and require 3V and 100A, we could use a dc convert and a 300W PS?

8. ### XRZ

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Jul 30, 2014
@Gryd3 what do you think of the idea of a dc-dc converter, that has power input of 300W controlling the voltage to 3V, and 100A?

9. ### Gryd3

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Jun 25, 2014
I'm thinking that is a LOT of parallel LEDs.

Have you played with circuit simulators?

I'm thinking maybe one of us should whip up an example circuit to explain what portion gets subjected to the magnetic flux to induce EMF and we can pick it apart and figure out what we are missing

10. ### XRZ

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Jul 30, 2014
What does this have to do with LEDs?
Well I have but the most basic ones you can imagine. Hm, it really just one portion of the circuit( you can pick any) that is exposed to the magnetic field, but the idea of using a dc/dc converter is playing with my mind now, greatly!

If resistance is 1Ohm, and the Input power is 300W, that is about 150V x 150A, without the converter. Add that bad boy in, and control the voltage up to a point where we need the current to be maintained to 100A @ 3VDC.
Instead of having 100V... and 100A which is just to much to maintain the current at low voltages. @Gryd3 you can whip up any example that has the same principle, a portion of the circuit is exposed to a magnetic field and induced emf(large one) and we need to maintain the same current without applying massive amounts of more power!

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

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Jan 21, 2010
I'm a little confused about what you're doing.

the trivial answer to how you get more magnetic field from a given current is to have the current pass the same way more than once. In short, multiple turns. However it also sounds to me like you're trying to do some hall effect thing, although where induced EMF comes in there I'm not sure.

5,165
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Dec 18, 2013
Can you explain why you are doing this, what's the idea behind the project. We have been doing this sort of thing with data packets on long cables for years with constant current, it's called a 4-20 current loop. As Steve said use a constant current generator if you want to maintain a current in a wire with induced voltages. The voltage will obviously go up and down but the current should stay the same.

13. ### Gryd3

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Jun 25, 2014
It's more so just understanding the mechanics involved if a low resistance circuit was subjected to EMF that produced a couple volts. The constant current source would need to compensate and this is what is leading to the funny math... is figuring out how the source actually compensates.
I also do not know enough about the subject of EMF to provide any real world examples or numbers.
So far the examples have been 1 or 2 volts on a circuit of an Ohm or less which results in a high current. I'm trying to draw a circuit that may be subject to EMF so we can get a better understanding from something that we can look at and measure instead of just using voltage, current and resistance examples.

XRZ likes this.

5,165
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Dec 18, 2013
This is what I think
A constant current source is just a high voltage source with very high resistance so no matter what resistance you attach to it is so small in comparison that the current appears to stay the same. As far as the source is concerned the varying voltage caused by the induced voltage is a change in resistance up and down and is sort of ignored because of the high resistance. That's why the current appears to stay constant.

15. ### XRZ

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Jul 30, 2014
Well said @Gryd3 ! I'm just thinking of solutions to such a problem, where back-emf is present and there is a circuit has low resistance, and require high current, and needs high voltages to manage with back-emf.
I think maybe a dc/dc converter could be an option

16. ### Gryd3

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Jun 25, 2014
Well.. I'm certain that was the part we were missing, I knew there was resistance in the source, but was too unsure to ballpark a value.
So even if the constant current supply was connected to a 1/4Ω circuit, the total resistance of the circuit would be much higher due to the actual constant current supply.

Do you know if it is unreasonable to expect EMF of 1-2V to be picked up in a circuit? To have a resistance that low, the wire would need to be really thick, or reasonably short.

Well, without understanding the mechanics 100% .. ie not knowing what to expect for a reasonable magnitude for EMF .. I'd just suggest using an inductor as an easy way to counter EMF to keep current in the circuit constant.
It sounds as though Adam may have other tricks for countering EMF for other portions of a circuit.
I'm interested in hearing them. (Twisted Pair networking cable, or shielded coax comes to mind)

17. ### XRZ

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Jul 30, 2014
@Arouse1973 but most constant current sources will apply massive power, in this case 10kW to sustain that current.
Maybe using a dc/dc converter could work! Applying 300W input and get an output in that range?

18. ### XRZ

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Jul 30, 2014
X2(me too!

5,165
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Dec 18, 2013
It works in the same principle as the example I gave, you will always need a good headroom of voltage so that when the resistance changes the same amount of current can still be drawn by the load. Modern constant current sources use different kinds of feedback to control this but the principle is the same. The example shown is a non active type and has limited applications due to the low current but it shows the principle mechanism of what happens. The load is stepped from 1R to 10R.