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High-precision L/C Inductance Capacitance meter Design

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as requested by OP of https://www.eevblog.com/forum/index.php?topic=1855.0;topicseen ... i have to move on here. So i hope some feedbacks...


--- Quote from: saturation on November 30, 2010, 10:02:34 pm ---You can make your own even simpler since the relationship in high pass filters are:
So you can build this very quickly and measure either capacitance or inductance; getting a meter really matters if you don't have the time to calculate it as listed here:

--- End quote ---
ok mr saturation! i think its time and place to discuss this more in depth. this looks nasty'ly simple (theoritically) and i've been astonished with it since the first time i opened the ee book and saw it. my basic rule is that when i cannot understand a simple system, then i should go even simpler. now, the RC and RL hi/lo pass filter is the most basic thing on earth, theoritically to build an LCR meter is great with the super simple 2 passive components circuit... but just how to practicality it is, is where i'm still stumbled at. so let me point out of what i understand...

1) all we need is a setup/schematic shown in your link (either RC or RL), and... Vin supposedly be connected to some Vac generator... mcu PWM pin maybe?
2) all on earth the basic equation is the famous w = 1 / RC. is this means, we need to setup a known w and R in order to get the C value?
3) since we need to feed alternating V, is this means Vcc to -Vcc peak to peak instead of Vcc to 0V? so how to get the -Vcc voltage? can we put the R or L ground (as in the link's picture) to some dummy reference at Vcc/2 so we have Vcc/2 and -Vcc/2 seen by the filter circuit?
4) and the rest of the math is done in mcu programming right? ok leave this for now, just assume i'm too good to program that "software'tically" 8)... as i said, i always stuck with analog side :(.

i'm looking forward/expecting/hope to see this basic questions evolve to a bigger thing like the many DIY circuits provided by dear fellow members here.

maybe i got it wrong for the equation...
continuation from http://www.play-hookey.com/ac_theory/hi_pass_filters.html
here goes the ugly sketching. hope i got clearer solution.


--- Quote from: saturation on December 01, 2010, 04:08:02 pm ---That's good math work, safri, but the equation to use is different and far simpler.

Notice how all LCR meters use a specific freuquency for a range of C or L, that's roughly how this works.

Resistor here is a constant, use any thin film or carbon composite.  Don't use wirewound as its a inductor too.

Now, put your unknown capacitor or inductor in the circuit above, as necessary.

Get a function or audio generator generator as Vin, and measure its output Voltage with a scope for say 500kHz or 500 Hz, just to copy the frequencies the device in the original post used.  Say adjust it for 1 Vp-p  so its easy to read.

If your cap is as big as your pinky or your inductor is fairly large, use the 500 Hz.

Now connect the generator to Vin and the scope to Vout.  Adjust the output frequency precisely until the output voltage is 0.707Vp-p.  That's the fc.

Now that you have fc, substitute it algebraically into the equation and that's your L or C.


fc= Hz
R = ohms
C = farads
L = henry

For example, you have an unknown small inductor.
Use the inductor high pass circuit above.  You have some resistor lying around that is 500,000 ohms.

You pass 500kHz into it and it comes out 1V p-p.  Adjusting frequency down [ since its a high pass filter its already passing everything ABOVE the cutoff] , at 300kHz the Vout is 0.707 p-p.


L = R / 2 x pi x fc

L = 500,000 / 2 x pi x 300,000 Hz

L = 0.265 H or 265 mH.

As alm and I suggested in a old post, most hobbyist only need a good scope, like the Rigol 1052E, a function generator, a good DMM and PSU.  An LCR meter is nice to have, but if you don't measure C or L often, and a good DMM comes with C meter already too, you can rig one up fairly quickly and get very accurate results with it.

Some modifications of the same rig can calculate ESR, DF, and Q too.  But they can be more tedious to do on-the-fly, so it would be worth it to buy or build an ESR meter than a LCR meter without ESR built in.


--- End quote ---

How about doing it this way?



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