How do you calculate an inductor's impedance to pulse dc?

[Sephiroth]

Basically designing a new coil and trying to calculate what the current draw would be at different frequencies...

I'm sure there must be a formula for this somewhere but can't find it!

Need to be able to calculate it from inductance, resistance, voltage and frequency.

Cheers!

[Bit's-n-Bytes]

[Sephiroth]

Thanks B&B

But I'm pretty sure those formulas only apply to AC and not pulse dc...

for example, my 255uh coil draws around 800ma at 6khz but according to that formula it will have around 9000 ohms reactance at that frequency!

I think there might be a different way to calculate pulse dc

[nvisser]

I also think they applies to sine waves
With square waves it is a different story. Your duty cycle will mostly determent the current you draw
A very short duty cycle with a sharp rising edge looks like the way to go to.

[Sephiroth]

ok how about this...

first we use the formula to calculate the power going through a coil in a given time which is:



where dt is 1/frequency and di is V/R

then multiply that figure by the duty cycle expressed as a decimal.

then divide that figure by the voltage...

and that should give us amps, no?

think I'm getting close but have to catch a bus so no time to test it!!! back soon with teh results

[gyula]

...
for example, my 255uh coil draws around 800ma at 6khz but according to that formula it will have around 9000 ohms reactance at that frequency!
...

Hi,

The formula for the inductive reactance of coils is 2*Pi*f*L.

This gives a 9.6 Ohm reactance value for me, using your 6kHz and 255uH.

How did you get 9000 Ohms?

As others say it, you have to consider the duty cycle of your pulse too. Did it have a 50% duty cycle when you measured the 800mA current?

rgds, Gyula

[morpher44]

nice little calculators

Impedance

Complex Impedance

[Sephiroth]

Hi,

The formula for the inductive reactance of coils is 2*Pi*f*L.

This gives a 9.6 Ohm reactance value for me, using your 6kHz and 255uH.

How did you get 9000 Ohms?

As others say it, you have to consider the duty cycle of your pulse too. Did it have a 50% duty cycle when you measured the 800mA current?

rgds, Gyula

2 x 3.141 x 6000 x .255 = 9611.46 ohms

yes it is 50% duty cycle.

[gyula]

2 x 3.141 x 6000 x .255 = 9611.46 ohms

yes it is 50% duty cycle.

With due respect, 255uH is not .255H.

Did you mean 255mH?

uH is microHenry, mH is milliHenry. You wrote uH.

[Sephiroth]

With due respect, 255uH is not .255H.

Did you mean 255mH?

uH is microHenry, mH is milliHenry. You wrote uH.

quite right! I meant .255mH not uh

eitherway... I'm sure that formula doesn't apply to pulse dc

[gyula]

quite right! I meant .255mH not uh

You lost me again. .255mH is 255uH and in the formula you wrote .255 which SHOULD be in Henry... and if it is .255H, then it is 255mH and not .255mH, ok?

It is important how you got 9611 Ohm, we the readers of you question cannot figure out any correct answer if you communicate wrong data.

[morpher44]

Non-Conservative Fields

eitherway... I'm sure that formula doesn't apply to pulse dc

You might check out the Non-Conservative Fields thread.
Your instinct is correct that pulsed DC into a coil is NOT
the same as AC into a coil.

I still can't wrap my head around this Non-Conservative Fields
concept ... but the hint here is to use Faraday's law and
never use Kirchoff's laws, when doing circuit analysis,
when magnetic fields are utilized
in the circuit.

[Sephiroth]

You lost me again. .255mH is 255uH and in the formula you wrote .255 which SHOULD be in Henry... and if it is .255H, then it is 255mH and not .255mH, ok?

It is important how you got 9611 Ohm, we the readers of you question cannot figure out any correct answer if you communicate wrong data.

lol... sorry... your right, I'm a bit distracted trying to solder this circuit and the fumes have probably gone to my head... just got it finished and YES I meant .255H, or 255mH and NOT 255uH.

Morpher : thanks for the links... I'll check them out!

[gyula]

lol... sorry... your right, I'm a bit distracted trying to solder this circuit and the fumes have probably gone to my head... just got it finished and YES I meant .255H, or 255mH and NOT 255uH.

Hi,

Ok, to err is human.

Here is my 2 cent to answer your original question...

so you have voltage, coil's inductance (.255H), resistance and frequency (6kHz) as known data,ok?

When you switch a voltage source onto a coil, current is retarded (starts from zero while the full voltage is across the coil) and later when a steady state condition is established, a maximum current flows through the coil, determined by the coil's copper resistance AND the residual series resistances of the switch and the voltage source.

Here is a link to see how these depend on each other:

Transients in an Inductor

I assume your total resistance in series with your .255H coil should have been a maximum of 16 Ohm if the battery or supply voltage was around 12.7V or so. This 16 Ohm includes the total resistances in series with the coil, including the coil wire DC resistance too.

Because with this 16 Ohm value your measured, current was around the 800mA you mentioned and I assume here a steady state condition has developed already while you were doing the current measurements.

With the formula embedded in the inductor transient figure of the above link (the known exponential curve for the coil current to approach the steady state value, you can calculate the impedance in a particular moment from the instantenous current via the coil at that moment and from the voltage. In the bottom part of the link a calculator is included which actually uses that transient formula.

Notice that if your coil's DC resistance were ,say, 10 Ohm, then the L/R time constant were .255/10=.0255s i.e. 25.5ms and compare this time to the half period time of your 6kHz pulse you used, this latter gives 83.33us (microsec), a much shorter time.

If you substitute this t=83.33us into the formula to learn about the current at this time moment, also using V=10V, R=10 Ohm, (these values chosen for simplicity in the calculation), then you get a current I=93.3uA microAmper), this current would flow via the .255H coil after switching ON at zero sec and letting the switch ON for 83.33us.
Notice: one full time period of the 6kHz frequency is 1/6000= .00016666s and you mentioned 50% duty cycle, so full time period should be halved, i.e. the half time is .000083333s, this will be the ON time duration of the switch if driven by 6kHz 50% duty square wave.

I hope this helps to know what current at any moment in a coil may flow.

rgds, Gyula

EDIT here is another useful link: ftp://ftp.ni.com/pub/devzone/tut/lllab6.pdf

[Sephiroth]

thanks gyula, that sounds more like it

I'll try the method with my new oscillator tomorrow as a test.

thanks again