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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
This is a small project I've worked on the past few weeks on and off.
Here's the basic schematic I used;
It uses a simplified royer converter as the oscillator, that draws about 50 mA or so when running. Total system draw is about 150 mA when the oscillator is running and the teensy is on.
It feeds into a 30 pF capacitor, then through to a schmitt trigger inverter, then finally into the Teensy 3.1 for frequency measurement.
The code is relatively straightforward, since all it does is some simple math and spit out the inductance value, with some unit conversion thrown in. It initially turns on by flipping a latching relay, then it measures for about 10 seconds, then shuts off the oscillator to save power, waits another 5-6 seconds, then shuts itself off by triggering the latching relay back off. It was "easier" than trying to implement a restart in the code.
There's also a calibration setting I stuck in there, which measures the inductance of the leads when shorted and then subtracts that value from subsequent readings, by storing that value in the EEPROM.
It's powered by a small li-ion battery, which can be recharged with one of those simple charger boards from china. It's boosted up to 5V using one of my homemade boost circuits I designed (uses a MIC2250).
I'm not 100% sure on the accuracy of the measurements. It does low inductance values pretty well (at least ballpark anyway!). The oscillator seems to be capable of running in a wide range of frequencies (with the leads shorted, it runs at 8 Mhz!). The capacitor in the tank circuit is a 1% WIMA 1 nF one, so that part should be pretty spot on.
I'm not totally proud of the case. I'm not very good at fabricating stuff to put my projects in, and my tools are kinda limited. I'm also pretty sure a Teensy is overpowered for this, but I had it lying around and it wasn't doing anything else.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
You will definitely need to calibrate this device as the capacitances of the Q2 & Q6 mosfets will be significant compared to your 1 nF, how significant depends upon the actual transistors used. For this reason the 1 nF capacitor does not need to be a precision (1%) component.
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Inductance Meter
