Recently I was interested in LC meters as my both cheap DMM does not have LC meter function. However I have access to better tool - scope :) As time to time there are projects where you must know winded inductors inductance. For better results and faster developing some kind of LC meter or some helping circuity would be really handy tool.
There are quite a lot of different LC meters based on PIC around. But as it happens, not all of them suits ones needs completely.
Also it is quite important important to have your own software written. As it is possible change code anytime later, possibility integrate into other projects and learn something too.
It seems, to make PIC based LC meter is not that difficult. Wee need LC oscillator and then by metering frequency we might later calculate out capacity or inductance.
The most important here is - well calibrated oscillator as frequency might be measured using different tools and calculations might be done using hand calculators.
Here is already nice explanations of LC meter operating theory and formulas described here
PIC with internal comparator might be used for such a task as described and here .
Or oscillator might be done using LM311 (see fig1).
It might be handy for hobbyist just have such cheap oscillator for fast measurements via scope as "add on".
By simulating oscillator it in Proteus I was able calculate inductance easily while metering output timings on pin7 of LM311 via virtual scope, then calculating frequency and using well known formulas calculating back inductance of L1. There is quite a lot ready LC calculators around in web or as android app.
fig.1
And dirty breadboard model has been build out of this.
1000 pf. C3 was intend to use instead of 680pf but my dead cheap M890G DMM shows, C3 is 960 pf but not 1000pf as manufacturer has declared.
By specifications, M890G multimeter has 2,5% measurement accuracy in the range of 2000 pf. For example 100nf multilayer ceramic capacitor it shows as 78nf. Capacitors I own, are unknown accuracy class. So I am faced to fact - I do not know really values I am using in oscillator :) I have no also tool to measure inductance, so I must trust L1 I have is 68 uH +/- 5%. Let`s believe it is 68 uH :)
Here is shoots of what real life oscilloscope showed with C3 only connected.
In fig 2 It says, LM311 oscillates about 641kHz freq. Assumed L1 is exactly 68uH and freq then by backwards calculation, we have result as large as 907 pf
fig 2
And here is how it runs another 1000pf cap paralleled (925pf measured with multimeter) to C3. Now it oscillates at approx 459 kHz. Again, assumed L1 is exactly 68uH and freq then by backwards calculation, I have result as large as 1768 pf (960pf+925pf should be 1885pf) so inductor might be not 68uH if multimeter is better accuracy.
fig 3
And then was paralleled 100nF cap (fig 4) (cheap multimeter was not sure is it 78 or 80 nf) and here is shoot from oscilloscope (I also switched to cleaner li-ion power supplay :) ) Now I monitor 69Khz frequency as if backwards recalculated would be 78.241 nF ..
fig 4
When simulated circuit with used values of L1 and C3 in Proteus it would show 59Khz frequency instead of 69 Khz I observed... so here is huge space for error due of breadboard wiring, two not exactly known values of C3 and L1 and parasitic capacities and something I do not know. But the principle itself might be tested to understand how it works in real world.
By having oscilloscope, two exactly known value elements L1, C3 and simple circuity there is no need to build instrument based on PIC if inductance measurement are rare events for you. There might be no need to buy exactly high tolerance parts too. It might be much easier and cheaper just go to professional workshop and kindly ask some pro guys to measure these with professional calibrated or better accuracy instruments.
As I am not tech guru in electronics and just returned to this hobby after few decades of years it is really interesting to learn something and experiment. Because of this experiment results, I think it would be better to build some frequency meter with LC measure functionality add on up to 0,6-2 Ghz based on PIC. Such tool would be better and more useful instrument for hobby shop. I had made some experiments using two PIC12F629. One as input divider using asynchronous TMR1 input T1CKI and another as frequency counter. Even such small chips gives interesting results. But this is another story.
0 comments:
Post a Comment