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I am making the LC meter V2 designed by VK3BHR Phil Rice.

https://sites.google.com/site/vk3bhr/home/index2-html

I have seen many have done this and it works fine, but in my case it is not measuring inductance properly. For example a 90uH coil shows 52uH, 3.3uH shows 0.3uH. The measurement is completely wrong. Capacitance measurement is looking OK. Calibration was done using a 1% 100pF NP0 capacitor. I checked F1 and F2.

F1 = 00042879 (little lower than recommended value of 00050000 +- 5%)

F2 = 00032223 (within 71% of F1)

I have used all new components and L is a 100uH +/-20% toroid whose internal resistance is 0.3 ohm. C and Ccal are 1000pF NP0 +/-5%. I am using the PCB designed by VK3BHR. The relay is driven by a BC548B transistor as I cannot find a low current relay and I have checked the relay, it's clicking and contact resistance is less than 1 ohm.

I am thinking that the +/-20% tolerance for the 100uH toroid is too high. What else could be wrong? I will provide any other info that is useful for troubleshooting.

Can anyone help me to find out the problem?

Circuit diagram:

schematic

PCB tracks

PCB component layout

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  • \$\begingroup\$ Are there errors in your code? Perhaps in your wiring? You should copy relevant schematics and code segments to your question, your circuit board layout and pictures of what you've actually put together. \$\endgroup\$
    – K H
    Feb 6 '21 at 3:40
  • \$\begingroup\$ I have added circuit diagram. The code was tested by many successful builder, so thinking that code is not an issue. The PCB layout also designed by VK3BHR and its tested by other builder. \$\endgroup\$
    – Ashok Das
    Feb 6 '21 at 4:15
  • \$\begingroup\$ What does the problem guide tell you to do about calibration errors? What does 0uH read? \$\endgroup\$ Feb 6 '21 at 13:23
  • \$\begingroup\$ Nothing mentioned specifically about calibration error. This unit is calibrated in software, I cannot understand the code as I am completely blind about the PIC micros. Only clue so far I found is that the inductor used was tested at 10KHz. Operating freq in this circuit is about 500KHz, so it may be the inductor which is making problem. I am now waiting for a new inductor to arrive. 0uH read 0.1uH after 5-10 sec of shorting the terminal and pressing the Zero button and it keeps on increasing. \$\endgroup\$
    – Ashok Das
    Feb 6 '21 at 21:49
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I have replaced the inductor L with the TDK part, 100uH, 0.7ohm, 2MHz SRF. Also replaced both C and Ccal with 1% polystyrene capacitors. Removed the relay to a separate PCB (thinking that Relay coil might be interfering with 100uH inductor). Now the circuit is working as it should. Thanks to all for their valuable feedback to make this a success.

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  • \$\begingroup\$ Ashok Das - Hi, Thanks for coming back with the answer to your question. In order to mark the topic as solved, please "accept" your answer (click the "tick mark" next to your answer, to turn it green). Then it is shown as having an accepted answer in various lists, and we don't get nagged for it being a question with an unaccepted answer. Thanks! \$\endgroup\$
    – SamGibson
    Apr 17 '21 at 15:32

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