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measuring inductance

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Solidacid

Thu May 07 2009, 07:27AM

Registered Member #1430 Joined: Sun Apr 06 2008, 11:12AM
Location: Ã…rhus, Denmark
Posts: 102

i was wondering if you guys could help me find a way to measure the inductance of various inductors.
i have mutimeters but no oscilloscope(i might be able to borrow one)
i was thinking i could use
Schematic1

from powerlabs.org
on the site it says "The frequency of operation is automatically adjusted to resonance"
if i put my inductor in the flybacks place maybe i could somehow measure the frequency of the pulses?

i need to measure inductors both with air and various other cores

any ideas?

EDIT:
i have an arduino, maybe i can somehow use that? that would also eliminate the need for a screen, as i can just plug it into my USB port

Dr. Slack

Thu May 07 2009, 10:29AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Measurement of inductors is one of the trickier things to do, especially as there are many different types and they are one of the least "pure" components, having loss, stray C, saturation, all things which often make it important what frequency and drive level you use to excite them.

A good way to get an idea of the L is to make an AC current source. At low frequency this can be done by putting a big resistor in series with the LV outupt of an AC transformer from the mains. Put this current through the unknown inductor, and measure the voltage drop. Measure its DC resistance as well. If the calculated AC impedance is an order of magnitude or more than the DC, then you have a good measurement of the inductance.

Mains frequency is so low that it will only tend to be good for mains components, transformers and flourescent chokes for instance. The inductance of higher frequency components may be difficiult to measure this way.

Resonance is good for higher frquencies. Use a sig-gen program to drive the unknown L connected to a known C (most of us have a known C lying around somewhere, it's worth buying one if you haven't), sweep the drive frequency until the audio output peaks (or nulls depednign on how you have it connected), and do the sums from frquency and C to get the L.

Barry

Thu May 07 2009, 01:06PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Here is a well thought-out coil measurement system from Nuts & Volts magazine, Feb 2008. It can also measure capacitors and other devices. In one of the examples they measure the impedance of a 40m antenna. All it requires is a signal generator, voltmeter and a few components.

I've reprinted the article with permission from the author: Low Cost Impedance Analyzer

It could be what you're looking for.
Cheers, Barry

Life is like an analogy.

klugesmith

Thu May 07 2009, 04:56PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

You can measure the frequency of an oscillator in which the unknown inductor and a known capacitor form a tank circuit. One popular circuit uses a common voltage comparator, for example the core of the project at
http://electronics-diy.com/lc_meter.php

All you really need is the LM311, a handful of R's, one L and one C, and a way to measure the frequency. Many digital multimeters have a suitable "frequency" range. You can get within a couple of percent with headphones and a bit of musical talent. This particular circuit works over a broad, but limited, range of characteristic time -- sqrt(LC) -- and impedance -- sqrt(L/C). Helped me to make 50 uH 30 A air-core inductors for a 3-phase LISN (one-off, uncertified).

To echo Dr. Slack: physical inductors (esp. those with ferromagnetic cores) are imperfectly represented by the ideal circuit element. Beware of sensitivity to frequency and current.

-Rich

Dr. Dark Current

Thu May 07 2009, 05:52PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Solidacid wrote ...

on the site it says "The frequency of operation is automatically adjusted to resonance"

On that site it says a lot of things. The frequency of operation of that circuit has absolutely nothing to do with resonance, the circuit switches off when the transistor can no longer hold the collector current, which depends on its gain, base current, winding inductances etc.,etc...

Mads Barnkob

Thu May 07 2009, 07:20PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I bought one of these, they are actually pretty precise, but the leads are a bunch of crap.

Ive only been happy about that buy :)

Solidacid

Thu May 07 2009, 10:12PM

Registered Member #1430 Joined: Sun Apr 06 2008, 11:12AM
Location: Ã…rhus, Denmark
Posts: 102

i might buy one of those :D, i can add it to my collection of 5 multimeters :P

but i still need a "ghetto" way of measuring inductance :(
i was really hoping i could use my arduino..

uzzors2k

Fri May 08 2009, 08:45AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I've built the PIC based LC meter in the above link and it works well. The capacitance range isn't excellent, as it bums out somewhere below 1ÂµF, but it's great for inductors. Their are plenty of AVR LC meters you can build, also with the Arduino.

PanosB_GR

Fri May 08 2009, 09:51AM

Registered Member #1680 Joined: Fri Sept 05 2008, 04:19PM
Location: Greece
Posts: 43

One good oscilloscope is a sound card.

You can output sinusoids from the soundcard, ranging up to whatever is the card sampling rate (96kHz for good ones) and record the voltage to the input channel. Just put the inductor across.

Measuring the frequency responce of this setup is the best way of determining the inductance.

And it is "ghetto" enough.

twist2b

Sat May 09 2009, 01:27AM

Registered Member #2086 Joined: Tue Apr 21 2009, 02:33AM
Location:
Posts: 117

You sure you want to use that schematic? It really is not to friendly with the 2n3055. The author admits he, himself went through a couple, not to mention the heatsink it needs o_O

Klugesmith - That is awesome... I always wanted to build something like that :)
Anyways, There is a 555 Timer schematic that is alot better, and still simple.

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