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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I designed this circuit because my LCR meter blew and I needed to measure inductors. Inspired by a meter on a friend's website ( ), I improved it for better accuracy and resolution with small inductances. This simple meter uses just a single IC, a transistor and a few passive components. It can measure inductances down to 0.1 uH.
The measurement error and temperature drift is unknown, if anyone has a precise measuring instrument, it would be nice to see how it compares. The IC might change the flip voltage threshold a bit with temperature, but this is the price you pay for simplicity.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
nice. i even have that ic... somewhere. it's listed in my inventory. i'll have to try it out. i've always wanted an inductance meter so i can actually work out some math within designs, instead of just winging it. thanks
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
As I don't have any precise inductors, I made a small test - I measured three individual inductors, then connected them in series and compared with calculated result, then I did the same with the inductors in parallel. The measurement error with series inductors was 0.9%, with parallel inductors it was 0.6%. I also did a simple not-so-scientific test by heating the IC up with a finger, then blowing on it. I also let it running for some time, and the measured value did not change. So from this limited testing, the meter seems fairly accurate.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Proud Mary, The inductance is determined from the LR time constant where R is the series resistor in the meter, 100 ohms. So if the inductor has a 1 ohm or more series resistance, the reading may be off. However this meter is designed for small inductances (2 mH maximum) which usually don't have significant resistance.
If anyone has ideas how to improve the circuit or use a different principle for measuring inductors with high resistance, feel free to post them.
Registered Member #1822
Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300
Why couldnt you replace the 100ohm resistor with a potentiometer to compensate for large resistances? Is the device simply not suitable for large inductances?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
cduma: With bigger resistor, the pulse duration shortens (which probably means worse linearity) and parasitic capacitances manifest more. If you mean to compensate the resistor for inductors with larger ESR, that probably wont work, because increasing the collector resistor decreases the reading but bigger ESR increases the reading. Anyway this is just a "symptom compensation", the cause is still there.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
i'm about to build this circuit onto one of my meters. I don't believe that i have any of the 1n4148 diodes. would the UF400 family be fast enough? (50nS)
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Simple small-inductance meter addon for a multimeter
