# How can I calculate the delay time of a choke?

1. Jan 23, 2016

### BHY-BK

I have a choke which has 2.5mH inductance and 2pF of capacitance.

I have been looking at formulas for calculating the delay time and found
d = sqrt Lo Co

Where Lo is the intrinsic inductance per unit length
and Co is the intrinsic capacitance per unit length

Since I only know the inductance and capacitance of the whole coil how do I calculate the delay time?
Do I need to break it down into per unit lengths-if so how would I do that?

2. Jan 23, 2016

### meBigGuy

That formula is for a distributed transmission line, not an inductor with parasitic capacitance.

I think the parasitic capacitance is considered in parallel with the inductor rather than distributed as in a transmission line. But, I'm not entirely sure.
http://www.cliftonlaboratories.com/self-resonant_frequency_of_inductors.htm [Broken]

Here is a lot of information on transmission line delay
http://www.montana.edu/blameres/cou...ation/TDR_AppNote_Agilent_MeasCL_apr96a11.pdf

Last edited by a moderator: May 7, 2017
3. Jan 23, 2016

### BHY-BK

Thanks for the articles. I read them both and they both had a lot of info.

The second one stated you need a TDR to be able to determine the delay....

So, I'm still wondering, how can I determine the inductance per unit length and capacitance per unit length? And what units and lengths are to be used.

I know the length of the coil is 352.2 inches, maybe that might help......

4. Jan 23, 2016

### davenn

ummm so is it a coil inductor or a transmission line ?

for coil inductors, inductance /unit length isn't something that is considered

a coil has inductance that is dependent on its number of turns, closeness of the windings of the turns, thickness of the wire etc

and similarly for capacitors, their values are not considered as a unit length

that is a freakin' long coil ... 29 feet !!

time you told us more about what you are playing with, the equipment etc ... some uploaded pics would help

Dave

5. Jan 23, 2016

### BHY-BK

Sorry that length was just a guess...

That's what I don't understand, if a coil can act as a delay line because it has a high capacitance then can't it be considered a transmission line?
If so then shouldn't I be able to use the total length of the coil and divide it by whatever the per unit length is then use the formula to calculate the delay?

I just need to understand how to figure this out, or if it can be calculated without the use of a TDR? Or if my line of thinking is completely off?

6. Jan 23, 2016

### davenn

a bit difficult for anyone to give good help without more info
again I ask for info on the choke, preferably with pic's

7. Jan 24, 2016

### meBigGuy

Regarding the length, I think maybe he was saying how long the wire is that is wound on the choke.

The choke is a lumped element and will not behave as a transmission line. You can use the model in the link. If you put in a pulse, you will not get a pulse out. You will get a spike from the parasitic capacitance followed by a ramping or exponentially rising voltage (V = Ldi/dt). It will have a time constant (just like an RC curcuit) if it drives a resistive load. A choke has a self-resonant frequency, a transmission line does not.

Think about the physical construction of a choke vs a transmission line. A choke has multiple layers (possibly) with capacitive coupling across layers. Even series capacitors between turns providing a series capacitor between input and output. A transmission line is a single wire with inductance (all wires have inductance) and capacitive coupling to a (more or less) low impedance shield. The scenarios are completely different.

BTW, the tdr link I provided isn't so much about using a tdr as showing the delay results of different configurations. None of them are chokes, though.

8. Jan 24, 2016

### tech99

You do not know the length of wire, but very roughly speaking, I think you will find that the resonance frequency of 2.5mH and 2pF will correspond to a frequency where the length of wire is a quarter of a wavelength long. So this will give you the delay. It is hard to design an inductor which does not behave as a transmission line.

9. Jan 24, 2016

### Staff: Mentor

What value of delay would you like to produce, and what signal would you like to be able to delay?