Measuring the capacity of a coil.

[Keruskerfuerst]

What´s the best way to measure the capacity of a coil?

 

[Nigel Goodwin]

An inductance meter - but your question is really far too vague to answer?, and coils don't have 'capacity', that's a property of capacitors.

 

[dknguyen]

Go to your local university, and see if they can't let you use an RLC meter they have sitting in one of their labs for a couple seconds.

 

[Keruskerfuerst]

I have several inductances. I want to measure the capacity of the inductances, since they are used in a parallel/serial crossover network.
The relevant inductances are in the range of 0,5mH up to 11.50mH.

I have a RLC meter.

 

[evandude]

I have several inductances. I want to measure the capacity of the inductances, since they are used in a parallel/serial crossover network.
The relevant inductances are in the range of 0,5mH up to 11.50mH.

I have a RLC meter.

It's semantics, but you measure the inductance, you don't measure "the capacity of the inductance".

If you have an RLC meter, it should be completely trivial... just hook it up and take the reading!

 

[OutToLunch]

are you looking for saturation points?

 

[Keruskerfuerst]

No. I am not looking for saturation points.

The main point is: since I have a serial/parallel crossover network, i would make sense to consider the capacity of the inductances.

 

[Hero999]

The maximum current rating?

That will depend on the wire thickness and core material.

There is no such thing as capacity in relation to inductors.

 

[Keruskerfuerst]

No. I am not looking for maximum current rating.

You can measure the capacity of an inductor using a 25000€ equipment of HP.
But I do not have such an equipment.

 

[Roff]

No. I am not looking for maximum current rating.

You can measure the capacity of an inductor using a 25000€ equipment of HP.
But I do not have such an equipment.

Do you want to measure inductance, capacitance, or some other property?

 

[bogdanfirst]

well, technically you cold build an oscillationg circuit, a LC oscillator.
If you know the inductance of the coil and the frequency of the oscillationm then you know the capacitance involved in the circuit. take out the capacitance of the capacitor you added and you have tha capacitance of the coil.
i can't think of another way now....

 

[Hero999]

You still haven't clearly expressed what you mean by capacity!

Could you mean power factor?

Self resonant frequency?

Look at what Google give you:
https://www.google.com/search?clien...+an+inductor&sourceid=opera&ie=utf-8&oe=utf-8

 

[bogdanfirst]

here is the idea:
there are no ideal components, every component has resistance, capacity and inductance.

inductors have inductance as their main property, but they have also a resistance(the resistance of the wire itself) and they also have a capacity called parasitic capacity. this is what he wants to mesure. the equivalent schematic of an inductor can be viewed as an ideal inductor in series with ca resistor and all this in paralel with a capacitor.

 

[RadioRon]

If, by capacity of the inductors, you mean the capacitance of the inductor then this is the way to approach it. Consider that the windings of the inductor have parasitic capacitance between them. We model this in the equivalent circuit of the inductor by placing a capacitor in parallel with the ideal series RL network of the inductance. This capacitance is supposed to be quite low so that it does not become significant, but as frequency increases the impedance of the inductance begins to approach the impedance of this capacitance until you reach the point called the "self-resonant" frequency. At this point the impedance of the network peaks. As frequency increases beyond this point, the network now looks like a capacitor.

An instrument would measure the capacitance indirectly by measuring the self-resonant frequency of the inductor and then calculating its parasitic capacitance.

If you are using inductors at audio frequencies in a crossover network, it would be wise to insure that your inductor's self-resonant frequency is much higher than the audio range so that it remains an inductor over the frequency of interest. So measuring the capacitance is not a dumb idea, but professionals usually look to the manufacturer's specification for the inductor to see this value, rather than measuring it.

You can measure the self-resonant frequency yourself if you have an oscillator with a wide frequency range. If you put a resistor in series with the inductor and connect this up to the oscillator, you can watch the amplitude of the voltage across the inductor and find where it hits a maximum and then begins to fall again. I believe it quite likely that you may reach the limit of your scope or oscillator before you get to this point depending on the inductor. If you don't see it happen anywhere below 500KHz then the self-resonant frequency is much higher than audio and you won't have to worry about the capacitance at all.

 

[Roff]

here is the idea:
there are no ideal components, every component has resistance, capacity and inductance.

inductors have inductance as their main property, but they have also a resistance(the resistance of the wire itself) and they also have a capacity called parasitic capacity. this is what he wants to mesure. the equivalent schematic of an inductor can be viewed as an ideal inductor in series with ca resistor and all this in paralel with a capacitor.

Well, I think the rest of us are thinking the same thing. If he had asked for capacitance, it would be clear. Capacity is an ambiguous term, and could mean several things in relation to an inductor. A clear statement from our OP would help a lot.

 

[Hero999]

Right, now we know at least what he's talking about.

You could build an LC oscillator and use the paracitic capacitance of the inductor as the capacitor; it's easy just remove the real capacitor in the circuit and just use the inductor.

**broken link removed**

 

[Sceadwian]

I have several inductances. I want to measure the capacity of the inductances, since they are used in a parallel/serial crossover network.
The relevant inductances are in the range of 0,5mH up to 11.50mH.

I have a RLC meter.

How about "Use the RLC meter"
Use the same resistor and capacitor under the same conditions to test the different inductors.
It seems a curious question to be asking as well because Inductors aren't something that need to be matched usually. As long as they're the same wire size and wrapped on the same form they're going to be the same.

 

[Alan Hall]

Hi Guys,
I've just joined. I'm located in the U.K. and am a retired electronics engineer. I was looking for a method of measuring the self-capacitance of a coil and found this site and this article. Am now reading it . . .very interesting . . . .
March 2015

 

[cowboybob]

Welcome to ETO, Alan hall!

As you can see, semantics (as always) sometimes plays havoc with understanding .

Now that your retired, what will you do for fun??

CBB

 

[BobW]

Hi Alan,
The self capacitance of a coil is a fairly constant value at low frequencies, but can start to shift significantly as the frequency starts to approach the self resonant frequency of the coil. So, measuring the natural resonant frequency of the coil won't necessarily give you the self-capacitance at the actual operating frequency of the circuit. The generally accepted method of measuring self capacitance is to add an external capacitor in parallel with the coil. Measure the resonant frequency. Then, change the capacitor to a different value and measure the new resonant frequency. Then the self-capacitance is calculated:
Cs=(C1/F2^2-C2/F1^2)/(1/F1^2-1/F2^2)
where C1 and C2 are the external capacitance values, and F1 and F2 are the corresponding resonant frequencies.
For best results, you should pick the external capacitance values so that the resulting resonant frequencies are in the range where you intend to operate the circuit.