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Homemade LC meter

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alisarhangpour

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Hello,
I wanna make a homemade digital LC meter, I want a rather precise one
How to do it? Or I'd better buy an RLC meter?
 
alisarhangpour said:
Hello,
I wanna make a homemade digital LC meter, I want a rather precise one
How to do it? Or I'd better buy an RLC meter?
afaik , an LC meter works by counting the freq generated , when the unknown L/C is inserted in the ckt , if L is to be measured , use a standard C(s) and count the freq , and Vice-versa..
 
Can you define what you mean by precise? Keep in mind that L and C do vary with frequency, temperature and a few other things. It's common to define inductors based on L at some standard conditions - I'd expect C to be much the same. For greater precsion one might make measurements at conditions of use.

Look up LC or RCL bridge to help get you started. Usually done at standard frequency where bridge is balanced against known values.
 
stevez said:
Keep in mind that L and C do vary with frequency,
I sure would like to hear of an example of an inductor or a capacitor that
it's value changes because of frequency.
 
k7elp60 said:
I sure would like to hear of an example of an inductor or a capacitor that it's value changes because of frequency.
It think stevez was referring reactance. And it's true: you can take advantage of reactance to produce a quality LC meter.

There are a lot of homemade LC meter plans on the web, but the best I've come across is the ELSIE Meter Kit. Both versions use the L or C to drive an oscillator. The signal is fed into an 8-bit microcontroller and used as an external hardware timer, which reveals the L/C value. It measures 1pF to 1uF, and 1uH to 500mH. They also added a frequency meter that goes up 30MHz, thanks to a high-speed 74HC4017 decade counter.

They didn't list any testamonials, but if word-of-mouth is any indication, this meter should be just about as accurate as the expensive store-bought alternative. No joke.
 
DigiTan said:
They also added a frequency meter that goes up 30MHz, thanks to a high-speed 74HC4017 decade counter.

30MHz, that's pretty poor really?.

An 'antique' PIC (the 16C54) in a very old MicroChip frequency counter application note goes to 50MHz (with no external chips) - later mods used the 16C84 in it's place, and changed from LED's to LCD as well.
 
I did mean to say that it's my understanding that inductance and capacitance change with frequency, temperature and other things.

In my search for information on capacitor heating a very helpful engineer from a major capacitor manufacturer passed along some information on capacitors. I might have misunderstood what I was looking at but it did appear that capacitance did change with frequency and temperature.

The behavior of an inductor is related to a number of things including the permeability (and other) of the core. Permeability does change (at least according to my references) with frequency, temperature, etc therefore the resulting inductance would change, so it would seem. Current flow is another factor - I thought a swinging choke was actually carefully designed to take advantage of these changes.

I may have my terms mixed up. At the moment I don't have the time to look back in my references to support what I understand but others might be able to add some helpful information. Some of these changes or differences are trivial, some are not. The original poster mentioned precision so it seemed important.
 
Nigel Goodwin said:
30MHz, that's pretty poor really?.

An 'antique' PIC (the 16C54) in a very old MicroChip frequency counter application note goes to 50MHz (with no external chips) - later mods used the 16C84 in it's place, and changed from LED's to LCD as well.

-Cough- That's "high-speed" as in "high-speed CMOS family." Everybody knows there are faster chips out.
 
When I made the statement,"I sure would like to hear of an example of an inductor or a capacitor that it's value changes because of frequency", I was thinking of the formulas as only physical values affect the outcome. Capacitors depend on the plate size, the spacing between the plates and the dielectric material. The plate size, and the spacing can vary with temperature. If the dielectric material is other than air, it can be affected greatly by temperature. But the effects of this change are unimportant depending on the purpose of the capacitor. If it is used as a frequency determining element then the dielectric materal can be a critical factor. That is why capacitors have temperature coefficients and temperature ranges.
At higher frequencies the the lead length of the capacitor may have too much inductance and
can affect the operation of the capacitor.

If one looks at the formula for calculating inductance the physical values determing the
inductance. One of those values is the permability of the core, which is affected by frequency.
These physical values can change with temperature. At various frequencies the capacitance
between windings can affect the the inductor operates.

It is my understanding that the swinging choke's inductance is affected by current flow.

I appologize for any confusion I may have caused.
 
Like others, I try to be helpful and offer what I might know or think I understand. There is always an amount of guessing in trying to figure out how to best respond to a query. Tossing things around in the manner that we do stimulates some thinking and I think it contributes to what I and others get out of the forum.

As a result of this thread a few have offered direct answers to the question at hand and we've hinted that a high level of precision in measurement might also reveal some of the variability that is well known to some but not to others. That is hopefully of some value to the readers.
 
Too old a post
i observed after posting a reply. However, I have given minor mods replacing the L/C selection switch, by a relay. This helps to prevent wires to&fro the switch . The revised schematic is attached. The artwork is ready for sharing to any one needy.
 

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another homebrew LC meter with good accuracy:

**broken link removed**

I've built this one, or to be precise, this variation of it:
**broken link removed**

It is an excellent meter. Probably the best homemade LC meter you're likely to find.
 
Google "circuit cellar lcr meter", that ones a bit more complex and I dare say a bit better than most of the "simple" pic based LC meters.
 
If you're referring to the one in Circuit Cellar Issue #214, that meter has a maximum test frequency of 10kHz, which will be fine for most capacitors and audio range inductors. If you intend to measure RF components then the test frequency is far too low to give reliable measurements.
 
A new method to accurately meaure the Cx with the LC meter

Dear all,

First of all I would like to thank all of you because with this forum you make the
great electronic hobby even greater and helpful

Please also consider that I'm not a native English speaker/writer.

This said, I would like to enter deep into detail on my development. Like you, I'm also
interesting in making my own LC meter and of course there is a lot of practical information
in the net worldwide, but only a few of them go deep in detail to justify the physical
equations to be used.

Starting from the well-known LC formula to get the oscillation frequency:

F = 1/(2π*√(LC))​

It is deduced the value of

Cx = Ccal * [(F1 / Fx)^2 - 1]​

By comparing the frequencies F1 and Fx that are computed by the PIC (F1 is using C1 and
Fx is using Cx). This comparison has to be done after the calibration process,
which compares F1 to Fcal (which uses Ccal). I don't want to go deep in detail with this because
it is already explained (and well explained) in the Internet.

At the end, the accuracy of the computed value Cx will depend on the accuracy of the Ccal, which can be expressed as follows: Ccal = Ccal_th + Ccal_tolerance

where Ccal_th is the theoretical value of Ccal (the one indicated in the capacitor case) and Ccal_tolerance is the tolerance given by the manufacturer (e.g. 2%).

Now I would like to present you a theoretical development to measure accurately the external
capacitor Cx on the LC meter. Note that only the Software is modified, not necessarily the
main Hardware.

Let's imagine you have a LC tank circuit, with real values for L and Ccal that can be expressed
as follows:

L_real = L_th + L'​
Ccal_real = Ccal_th + C'​

where L' is the additional L effect of the inductance (e.g. parasite + tolerance) over the theoretical
value L_th

and C' is the additional C effect of the capacitance (e.g. parasite + tolerance) over the theoretical
value C_th

The theoretical oscillation frequency is:

Fcal_th = 1/(2π*√(L_th * Ccal_th))​

and the real frequency is:

Fcal_real = 1/(2π*√(L_real * Ccal_real))​

Assumption: the effect of L' can be virtually replaced by an additional capacity in the
circuit --> C". Therefore:

Fcal_real = 1/(2π*√(L_th * (Ccal_th + C' + C"))​

Making

(Fcal_th / Fcal_real)^2 = (Ccal_th + C' + C") / Ccal_th​

We can isolate

C' + C" = Ccal_th * [(Fcal_th / Fcal_real)^2 - 1]​

Note that C'+ C" is computed using Ccal_th value which is without measurement error (ideal value of the component)

In the same way:

Fx = 1/(2π*√(L_th * (Ccal_th + C' + C" + Cx))​

Making

(Fcal_real / Fx)^2 = (Ccal_th + C' + C" + Cx) / (Ccal_th + C' + C")​

Therefore

Cx = (Ccal_th + C' + C") * [(Fcal_real / Fx)^2 - 1]​

where (Ccal_th + C' + C") is an exact value (without error) and thus Cx will be calculated in an exact way without error because of tolerances and/or parasites.

The question is quite simple: Am I doing an error anywhere?

I hope there will be someone out there willing to discuss about this and wanting to share his
knowledge as well about this way of facing the problem.

Waiting for your reaction ...
 
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