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inductance meter?

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haulinick

Fri Apr 20 2007, 03:36AM

Registered Member #619 Joined: Sat Mar 31 2007, 05:26AM
Location: San Jose, CA
Posts: 16

Maybe this is a dumb question but I cant seem to find the answer. I am trying to find the easiest method of measuring inductors. I have found many schematics and how-to's for making your own, but alot of those involve programming and stuff. Is there a simple circuit to use, just as an example, a voltmeter as an inductance meter. Or can I just buy? (I'm not willing to spend too much..maybe $100).

MikeT1982

Fri Apr 20 2007, 03:51AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119

I'm actually looking for the same darn thing, I am going to wind an inductor for my ruby lasers pulse forming network. Dick Anderson of Anderson lasers said he had this type of machine, but he's 4 hours away. I didn't get to actually see it if its a small contraption, big one or whatnot. When I was down there to visit he told me as we were headed back to our cars. I'd hate to wind the inductor, send it in the mail, measure it, send it back rewind it send it....hopefully not LOL!!

Steve Ward

Fri Apr 20 2007, 04:28AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

If you guys are winding air cored coils, or a common geometry, you can calculate the inductance by hand (look up inductance on wikipedia for examples). If the geometry isnt so nice (typically its not) then you can use an inductance meter. I bought one for about 40 bucks on ebay, it does capacitance too (which is very useful!). the range is usually something like 2uH to 20H.

Warning! If your inductor uses a core material (especially steel based) then the meter does not put out enough current to magnetize the core enough to read the working value for the inductance. If this is the case, depending on the situation, you can feed it an AC voltage, and measure the current through the inductor and work out the inductance in that manner. I found that when working with ferrite materials, the meter is better, but still not too accurate if you really need to know the value (its ok for ballpark figures).

Coronafix

Fri Apr 20 2007, 08:27AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

I found this at some point and bookmarked it to look at
it later, but never got around to it.
It may be what you are looking for.

Avi

Fri Apr 20 2007, 08:37AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

If you connect the inductor in series with a potentiometer you can create an impedance divider. Then you input a known frequency (across the series circuit) and adjust the potentiometer (or frequency) till the voltage across each is the same. You disconnect and measure the resistance of the potentiometer and place the resistance and frequency into the formula XL=2*pi*F*L
which you can rerrange to XL/F/pi/2=L

If you have an oscilloscope it is often a good idea to watch the voltage across each with the 2 channels and allign them visually. Note: you only need to match the amplidutes, not the phase.

Dr. Slack

Fri Apr 20 2007, 08:46AM

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

Make/buy depends on how many you're doing and whether the measurement conditions are important.

The easiest way to check whether an alledged inductor is the correct value is to buy an inductance meter. Mid-price multimeters often include an L and C capability, and you can never have too many DMMs in our hobby.

The drawback with that, apart from the cost, is that it usually chooses the operating frequency and power level. With air-core, that usually doesn't matter. With a cored inductor, whether ferrous or ferrite, its performance is both frequency and power level dependent. Chances are, a large inductor will auto-choose a low frequency, and that you are interested in the small signal inductance, but those aren't always true.

It's fairly quick to roll your own measurement, with an AC source, a resistor in series with the unknown, and an AC reading DMM or oscilloscope. Measure the volts across the resistor, then the inductor, then across both. Solve the vector triangle for the three readings to give the ratio of the inductor inductance and series resistance to that of the resistor you used. Maybe write a spreadsheet or a short program to do it if you have more than a few to measure. If you know the inductor has high Q, or are feeling lucky, then you can reduce the number of readings to 2. It's as accurate as an LCR meter would do anyway, and you get to choose the excitation conditions.

Best is to have an LCR meter, and understand how to lash up the above measurement when you need to.

<edit> clashed with the previous two replies. Comments - building a oscillator is OK, but much more work than I suggest. Adjusting a pot in series is OK, but if you match the amplitudes and ignore the phases, then you measure the *impedance* of the component, not the *inductance*. If you are feeling lucky, and the Q factor is relatively high, then there are the same, for low Q inductors you will get an error </edit>

Sulaiman

Fri Apr 20 2007, 10:14AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Since I have a signal-generator and an oscilloscope I do this;

Put a KNOWN capacitance in series with the inductor across the signal generator,

and find the frequency that gives the minimum signal,

then calculate L = 1/[C x (2 x PI x F)^2]

Very quick and accurate.

uzzors2k

Fri Apr 20 2007, 11:57AM

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

I would suggest building your own L/C meter, as it can be done for under 20 dollars. I built my own just last weekend, and it can measure both inductance and capacitance, with no more than 1% error depending on the timing components you use. Use a high Q inductor in the timing circuit, otherwise the capacitance measurement will only work up to 100nf or so. I wound my own coil, and it measures up to 1Âµf reliably. The range is from 0.1ÂµH - 10mH and 0.1pf to >800nf. The meter is also self calibrating, so it will measure correctly time after time. It has already paid for itself measuring 0.1ÂµH coils for an FM transmitter, which would have been painstaking work otherwise.

There are plenty of L/C Meter projects out there, this is the design I used

with troubleshooting help and all its a real easy project.

Looks great doesn't it.

Seen here measuring an unknown inductor.

kell

Sun Apr 29 2007, 02:25AM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

Here's another meter project:

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