# Inductance Measurement

#### viju

Joined Sep 28, 2008
120
I am trying to build a circuit for measuring the inductance of a choke coil.Range would be around 5 to 7 micro henry.Can somebody guide me how to build the circuit? I tried with the circuits which I got from google hits but it didn't work.

#### KL7AJ

Joined Nov 4, 2008
2,208
Hi Viju:

There are many ways of doing this, depending on what components you have available.

One way is using the resonance method, putting the inductance in parallel with a known value of capacitance (100pf would be a convenient choice) and then measuring the resonant frequency of the LC circuit. I usually use my trusty old grid dip oscillator for this sort of thing, but if you don't have one, you can use an r.f. signal generator and a high value of series resistance, 10K is a good choice. Feed the LC network through the resistor, and look for a peak voltage ACROSS the LC network with an oscilloscope or R.F. voltmeter. Once you detemine the resonant frequency, you can use the resonance formula to calculate the inductive reactance and then the L itself.

eric

#### studiot

Joined Nov 9, 2007
5,003
Whenever you set out to measure (or deal with in general) a reactive component you need to specify the frequency or frequency range of interest.

As Eric said there are many possible methods, but frequency often determines the most appropriate one.

The other question is is this a one off or will you be wanting to do lots of measurements?
And what are your resources? Do you for instance have access to a signal generator and voltmeter capable of operating in the frequency range concerned?

#### mik3

Joined Feb 4, 2008
4,846
Apply a signal of known amplitude and frequency and measure the current through the inductor. Then calculate its reactance (XL) and by using XL=2*pi*f*L find L.

#### viju

Joined Sep 28, 2008
120
Thankyou members:

I am trying to build an automatic setup in which lot of parameters are measured.One of the things is inductance.

I work in an automobile industry where I measure end cap of a DC motor.
It contains choke coil and a capacitor.

I need to measure the values of capacitor and inductor at 1 KHz frequency.

How can I generate 1 Khz frequency ( Voltage can be 12 V or 5V)? I don't have function generator?

Guide me.

#### Wendy

Joined Mar 24, 2008
21,840
Then you are going to have to build a signal source, but at 1Khz the method I proposed is toast. If you are going to make measurements then some test equipment is manditory, goes with making measurements. Signal sources are pretty simple, a 555 will do a good job of making a simple square wave, but you're going to have to define how you want to measure it. Alternate methods of determining inductance involve an oscilloscope.

If you want to keep it simple you may need to think more in terms of go/no go, an idiot light in other words. 1Khz at 3µH is a reactance of 0.01884Ω, which is pretty much invisable. Strikes me you may want to focus on the resonance, if there is any.

We're going to need more information than you've provided (schematics are always nice), otherwise we could spend all week proposing ideas and having you shoot them down with new information that you could have given up front.

#### studiot

Joined Nov 9, 2007
5,003
Seems to me the case for an old fashioned Bridge with a 1Khz source.

#### Wendy

Joined Mar 24, 2008
21,840
I thought of something like that, but still, that is going to be an awful low impedance to measure, even for a bridge.

It could also be possible to build an LC oscillator around the circuit, and then measure the freq.

#### The Electrician

Joined Oct 9, 2007
2,724
Thankyou members:

I am trying to build an automatic setup in which lot of parameters are measured.One of the things is inductance.

I work in an automobile industry where I measure end cap of a DC motor.
It contains choke coil and a capacitor.

I need to measure the values of capacitor and inductor at 1 KHz frequency.

How can I generate 1 Khz frequency ( Voltage can be 12 V or 5V)? I don't have function generator?

Guide me.
If you work for a company, you shouldn't be wasting your time trying to build something like this. Especially if you want it to be part of an automatic measuring setup. Not only will you have to design a circuit to make the measurement, you will have to get the measurement data into a computer, so you'll have to build a computer interface.

http://cgi.ebay.com/TH2811D-LCR-bridge-High-precision-meter-w-D-Q-10kHz_W0QQitemZ250460509055QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item3a509c137f&_trksid=p3286.c0.m14&_trkparms=65:12|66:2|39:1|72:1205|293:1|294:50

It can make measurements at 100 Hz, 1kHz and 10 kHz, and has a computer interface.

Buying it will cost less than they will pay you to spend the time to design what you need.

#### viju

Joined Sep 28, 2008
120
Thank you for the link Bertus.

It's a just a choke coil on a ferrite rod where I measure only the contact resistance( welded joint of a choke coil and the terminal).

I measure resistance by passing 500mA constant current source.

My requirement now is to measure the inductance value of the coil.

I have no restriction in measuring the frequency output from the circuit as my PLC can measure upto 30KHz signal.

Kindly suggest me a circuit to measure the inductance, please.

#### GetDeviceInfo

Joined Jun 7, 2009
1,623
if you have a scope, build an RL circuit and apply a square wave. Measure your time constant to resolve inductance.