# I have discovered something called LR Resonance(may be not sure)

#### DD Ki Vines

Joined Apr 20, 2019
7
Today I was working with LR circuit.What I had done is I connected a 15mH inductor and a 15k resistor in series and applied a sinusoidal signal across it and was reading output from across inductor but when I was increasing frequency the voltage reached a Max value of 4.08v(supply voltage is 4.16v) across L between frequency range if 106k-110k and after that it started decreasing and at 250k it reached 1.44v.I am really confused with this kind of result and I can't understand what is going on it's like resonance but we all know that there exist nothing called LR Resonance.Please help me too much confused

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#### MrChips

Joined Oct 2, 2009
20,862
Show us a circuit diagram of how the L, R, and oscilloscope are connected.
Show a photograph of your oscilloscope probe and the 1X/10X setting.

What you are seeing is not resonance. Presumably, you have the R-L set up as a high-pass filter.
The oscilloscope has a 1MΩ,16pF input. At some point as you increase the signal frequency, the input capacitance has an effect which becomes a low pass filter, i.e. it attenuates high frequencies.

If your oscilloscope probe has a 10X setting, set the switch to this setting and try again.

#### DickCappels

Joined Aug 21, 2008
6,300
Remember, inductors have distributed capacitance and therefore a self-resonant frequency.

#### DD Ki Vines

Joined Apr 20, 2019
7
Show us a circuit diagram of how the L, R, and oscilloscope are connected.
Show a photograph of your oscilloscope probe and the 1X/10X setting.

What you are seeing is not resonance. Presumably, you have the R-L set up as a high-pass filter.
The oscilloscope has a 1MΩ,16pF input. At some point as you increase the signal frequency, the input capacitance has an effect which becomes a low pass filter, i.e. it attenuates high frequencies.

If your oscilloscope probe has a 10X setting, set the switch to this setting and try again.
Sir I am not using any active probe I am using passive probes with normal alligator clips

#### BobaMosfet

Joined Jul 1, 2009
1,057
Hi, and welcome. In order for you to understand what you're looking at, there are a couple of things to understand about inductors (and capacitors), that are usually understood only after someone has some experience and time invested in electronics-- which is sad, because this simplicity should be explained up front.

A resistor is a thermal (ie friction) device. It's job is simply to slow the flow of electrons down, and it will dissipate heat to do so. We will not discuss resistors here.

An inductor is a _field_ device. That means it does not use friction, but rather uses and electromagnetic field. The reason an inductor doesn't seem as magnetic as it is, has to do with things far beyond the yen of this discussion. The important takeaway is that it is a field. Like opposes like, so when you first apply potential to an inductor (that is, you drop or lose voltage potential across the inductor), it is a dead short. For the briefest amount of time. But very quickly, electrons start forming a cloud around the inductor (primarily in its core), This cloud continues to build so long as there are fewer electrons in the field than are being attracted through it by the power-source. When the number of electrons in the field equal the number of electrons going through the coil, it can't hold any more, and the ones it can't hold are now allowed to bypass the inductor. This is why ELI (voltage leads current in an inductor).

A better way to understand ELI, is to understand that phase between voltage and current is one way to look at it, but it's a bad way (IMHO). Sadly, it's the common way. What's actually going on is that you are seeing the field charge. Thinking in just a DC steady voltage at initially, you're going to see a voltage loss or drop across the inductor for the full amount- As the field grows, less potential is lost across the inductor because it's being resisted by the field. Until finally, the field equals the power-supply, and current is now max, but since it's flowing through the coil and bypassing the field, the charge on either side of the coil is the same- hence voltage is now not being 'lost' or dropped across the coil. It's just a conductor, like the inductor doesn't exist.

Now, when you apply a frequency wave to an inductor, what you're actually doing is charging the field, and then letting it discharge. It isn't that an inductor is 'opposing' change, it's simply releasing it's charge as fast as the surrounding potential of the circuit will allow.

When you increase the frequency going through an inductor you are giving the inductor less time to discharge. So it will fill to a point. If you give it too little time to discharge enough, it will fill up. So your voltage potential is less because the power-supply has almost charged the inductor's field, so more current flows, decreasing the voltage drop (potential loss) across the inductor.

'Resonance' is a sweet spot where voltage and inductance (and capacitance) can come together where you're not cutting one or the other off entirely. Depending on the physical characteristics of the component, it may operate better as you approach 'resonance'.

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#### MrChips

Joined Oct 2, 2009
20,862
Sir I am not using any active probe I am using passive probes with normal alligator clips
And that could be the source of the problem. You do not need an active probe.
What you need is a passive oscilloscope probe that has two attenuation settings,1X and 10X.
As a general rule, you should be using the probe on the 10X setting if you wish to realize the oscillosope's rated bandwidth.

#### Ylli

Joined Nov 13, 2015
933
An inductor, because of its physical construction, has stray capacitance across it. The point at which the reactance of this stray capacitance and the reactance of the inductor is equal will result in a parallel resonance. This is device's "Self-Resonant" frequency. In this case, it sounds like your inductor has about 150 pF of stray capacitance, giving it a self-resonant frequency of about 106 KHz.

#### MrChips

Joined Oct 2, 2009
20,862
Remove the inductor and do the experiment again.