Coil measure circuit - Page 1
Forum section: electronics / projects

author: ElBenjo
author_rank: Contributor
timestamp: May 12, 2023, 06:51:05 am
content: Hello, everyone. I want to make a coil measure circuit I found on this cool sitehttps://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/Homebrew_RF_Circuit_Design_Ideas.htmthe circuit is on bottom.I got almost all the parts but I got some doubts.1.- I marked some potentiometers that I don't know what they're for, I want to know what they control.2.- There are three inductors of 2.7mH that I don't have. Can I replace them with inductors of less value in series?3.- Can I spect good inductance measure? What error percentage can I espect?Thank you very much for your comments and your time.

author: moffy
author_rank: Super Contributor
timestamp: May 12, 2023, 07:30:08 am
content: 1. The 100k in top right looks like you are trying to get the ratios 1:10:1002. The 1k left hand side, just adjusting the maximum level of the sine wave.3. The 1k feeding the MC1496, adjusting carrier level input, one of the determining factors of sensitivity/scale factor.4. The 500R in series with the 1.5k going in to pin 12 of the MC1496, you are trying to match the 1.8k going to pin 6 of the MC1496.

author: ace1903
author_rank: Regular Contributor
timestamp: May 12, 2023, 09:26:18 am
content: One question from curious viewer:Lower connection of the inductor Lx is grounded or not?According schematics looks grounded but then that 20k resistor doesn't make sense to be there.

author: ElBenjo
author_rank: Contributor
timestamp: May 12, 2023, 04:43:17 pm
content: Hello, Moffy. Thank you for your answer. If I understand then I can replace the 100K and 500 potentiometers with fixed resistors if in the first case the 1:10:100 ratio is acomplished and in the second case if both resistors have 1.8K, right? I'll try to do it with 1% resistors is series measured to be equal or very close.In the case of the other two potentiometer, how can I determine the right levels after build it?Hello, Ace. If I understand the circuit the connection is like the image I post here at the end, but would be nice if someone tell us if I'm right.

author: jwet
author_rank: Frequent Contributor
timestamp: May 12, 2023, 05:32:03 pm
content: I googled this and found the original authors site with a few more details on this circuit, a block diagram and some theory. He has a good website.https://www.qsl.net/va3iul/I'm also confused by the 20K going to ground- there might have been a addendum/correction published, contact author? The overall circuit looks pretty reasonable, I suspect a typo.As far as accuracy goes- 5% would be an optimistic estimate- likely closer to 10%. Repeatability will likely be much better and is important for matching up inductors for filters, etc.

author: ElBenjo
author_rank: Contributor
timestamp: May 12, 2023, 07:53:37 pm
content: How you would design the LX part of the circuit? I'll try to simulate it to see the output that goes to the MC1496. I like to simulate the whole circuit but LTSpice does not have the MC1496.

author: Benta
author_rank: Super Contributor
timestamp: May 12, 2023, 08:42:52 pm
content: What's your problem with the 20k resistor? It's got nothing to go with LX, but belongs to the two opamps to the right of it.

author: moffy
author_rank: Super Contributor
timestamp: May 12, 2023, 10:29:06 pm
content: Quote from: ElBenjo on May 12, 2023, 04:43:17 pmI might want to keep the 500R because it might help with zeroing the ouput offset, not sure might be one of those try it and see what works. You should be able to replace the 10k in parallel with the 100k pot with a suitable fixed resistor and eliminate the pot. The other two pots are I assume part of the calibration process, perhaps his site will have more info.Quote from: BentaI agree with Benta, it has something to do with impedance matching/attenuating the signal into the opamps.

author: ElBenjo
author_rank: Contributor
timestamp: May 12, 2023, 11:13:58 pm
content: I think you guys are right about the 20K resistor. Now I only have two doubts about the circuit:1.- How critical are the 2.7mH inductors? I got cheap inductors and my chinese component tester give me between 10 and 15% less inductance that indicated in them. But don't know if my inductors are lower value than indicated or my tester is not calibrated.2.- I don't have the crystal and the 4011 i.c. and was thinking to replace that part of the circuit with a 555 with tested capacitor and resistor values to match close as posible 100Khz. As far as I know NE555 has 1% frequency tolerance with calculated external components and since the same signal is inserted into both parts of the circuit I see no problem to do it, but don't know if for some reason has to be 100Khz with crystal precision or can be 99Khz to 101Khz. What you think about this modification?

author: jwet
author_rank: Frequent Contributor
timestamp: May 13, 2023, 04:04:23 am
content: The 2.7 mH inductors are not super critical, they are designed to take the 100 Khz square wave from the clock oscillator and by band-passing and then hi-passing make into something sine like. A square wave is a fundamental sine wave with no even harmonics and all the odd harmonics diminishing at 1/n- Fourier expansion. If you excitation looks like a sine, I think this simple circuit will work better.A 555 once calibrated with decent components can probably hold 1% frequency tolerance plus component drift probably another 1%. You probably are reducng the accuracy by a couple of percent and the aging or long term accuracy a bit more. You can build it with a 555 and put the xtal oscillator in later if you find the drift unacceptable.One thing worth doing is to create some "standards" that you measure on a known good instrument and keep with the instrument. Perhaps a set of 3 that you know the value of so you can evaluate if the gadget is working over time.

author: ElBenjo
author_rank: Contributor
timestamp: May 13, 2023, 04:45:55 am
content: Thank you very much. One last question (may be), can I put inductors in series to make a larger inductance, right? Like 1mH + 1mH + 680uH to make 2.68 mH. Asking because 1mH is the biggest inductor I got.

author: moffy
author_rank: Super Contributor
timestamp: May 13, 2023, 06:05:37 am
content: Quote from: ElBenjo on May 13, 2023, 04:45:55 amYes.

author: ElBenjo
author_rank: Contributor
timestamp: May 13, 2023, 06:16:52 am
content: Thank you very much to everyone for aswer my questions. I ordered the parts I don't have and as soon as they arrive I'll start making the circuit. Don't know how much it takes me to build it but as soon as I have it working (or not working) I'll tell you guys.Again, thank you very much. You're a nice community.