# Another dumb inductor question

Discussion in 'General Electronics Chat' started by shortbus, Feb 26, 2015.

1. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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How is the amount of amperage in an inductor controlled/limited? Say that you have a source capable of 15A but you only want 1A through the inductor. How is this done? A limiting resistor before or after the inductor makes it into a L/R with a different time component, doesn't it? This stuff is so hard to learn, should have started younger in life.

2. ### MikeML AAC Fanatic!

Oct 2, 2009
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You put a switch (transistor, FET) in series with it, and then you PWM it.

And then you figure out that to keep the current flowing in the inductor during the off times, you add a catch diode...

Stare at this: Note the duty cycle required to make the average current through the inductor ~1A. Note that if the switch remains closed, the inductor current would be ~15A

Last edited: Feb 26, 2015
3. ### Stuntman Active Member

Mar 28, 2011
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If you are simply trying to limit current through a DC device, the same rules apply as if you were limiting current through a resistive load (your inductor will have an equivalent series resistance). Keep in mind, however, that kickback voltage (from switching the inductor on and off) can damage components, so plan accordingly.

If you are powering this with AC, you will have impedance changes related to the frequency, so frequency becomes a much more interesting topic.

4. ### alfacliff Well-Known Member

Dec 13, 2013
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how does the current in a car light get limmited to 1 amp when the battery is capable of a thousand amps? by the resistance of the lamp. how do you limit the dc curent through an inductor? by knowing the load charistics and setting up the load so the current draw isnt more than the inductor is capable of.

planeguy67 likes this.
5. ### crutschow Expert

Mar 14, 2008
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An ideal inductor has zero resistance. So the only way to limit the steady-state current from a DC source is to add some series resistance.
To limit the current without adding resistance requires an AC source so the inductive reactance can do the limiting.

6. ### Roderick Young Member

Feb 22, 2015
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Good question. It would help, though, to have context as to your application, or why you want to limit current through the inductor. If it's the primary of a transformer, for example, then it would be wound with enough inductance to have a high enough impedance to only draw the desired current, given the input.

For example, let's say there's an outlet capable of supplying a sine wave at 115 (rms) VAC, and although the outlet could supply 15 amps (rms) or more, we only want to draw 1 amp.

$Z = \frac{V}{I}
Z = \frac{115}{1} = 115 \Omega$

If we also assume that the frequency is 60 Hz, then the impedance of an inductor is

$Z = 2{\pi}fL
115 = 2{\pi}60L
L = 0.3 H$

At least that's the general idea. Or maybe I guessed wrong at your application?

7. ### DC_Kid Distinguished Member

Feb 25, 2008
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make the src fluctuate, then the coil becomes "Z", thus limiting the current.

8. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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Thanks to all of you. The source is DC. And unlike MikeML's circuit there will be a mosfet switch but it will be after the inductor and it will be to ground. This is to generate a small spark from the kickback voltage. Like was done in early hit and miss engines for ignition, but without a set of points. The set up will be like a boost regulator without an output cap. The spark gap will be where the output cap would be in a boost regulator.

I just assumed you needed to limit the amperage through an inductor some how. The resistance in an inductor is so low that I assumed you had to do it by another means than switch duty cycle.

9. ### DC_Kid Distinguished Member

Feb 25, 2008
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inline inductor that creates huge voltage spike, and "mosfet inline" as switch???? sounds like you are ready to fry some fet's. how do you protect the fet from the flyback voltage ??

10. ### MikeML AAC Fanatic!

Oct 2, 2009
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Even if you use a "switch" in place of the FET, it will only last a few sec. because of the arc that forms across the contacts.
Methinks that Shortbus needs to use a Kettering Ignition, with coil, capacitor and points.

Note that the resistor limits the initial coil current to 3A.

Sep 30, 2009
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12. ### MikeML AAC Fanatic!

Oct 2, 2009
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But it takes ~10Kv to ionize even a 1mil spark gap. Where are you going to get a FET that stands >~1kV?

13. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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Hmm, that may be true in open air, but they have been doing it in an EDM since right after WW2 with less than 100VDC. This is not taking place in the air, but in oil. The oil ionizes and conducts it needs ~100VDC to ionize and then it conducts the current at ~35VDC. They did it with ~100VDC and a RC oscillator circuit then with transistor pulsed voltage. The only problem with just using a pulsed voltage is that every pulse doesn't always ionize. They use an inductor to raise the voltage and kick start the ionization. But can't find a source of schematics from existing machines.

And one reference I found claims the old low tension ignition inductors make a 0.005 spark with ~600VDC from an induction coil powered with 6VDC dry cell batteries, this was being done around 1900 or so.

Last edited: Feb 27, 2015
14. ### KLillie Member

May 31, 2014
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Current divider?

Sep 30, 2009
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16. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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The same way it's done in a DC-DC boost/stepup convertor. But instead of a capacitor taking the spike it will go through the spark gap. The inductor output voltage I'm shooting for is ~200VDC.