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

 

I sure would like to hear of an example of an inductor or a capacitor that
it's value changes because of frequency.

__________________
The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

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.

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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.

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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.

__________________
stevez

 

another homebrew LC meter with good accuracy:

http://ironbark.bendigo.latrobe.edu.au/~rice/lc/

 

-Cough- That's "high-speed" as in "high-speed CMOS family." Everybody knows there are faster chips out.

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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.

__________________
The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

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.

__________________
stevez