Phase alignment of RLC circuit - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: March 24, 2020, 10:41:47 pm
content: Hi,with ref to the attached schematics, I have an AC bridge where the two coils reflect/measure metal density and their value changes miscroscopically (uV). The issue is that when trying to zero the bridge I am still getting a imbalance in the output (OUT_A - OUT_B).Two designs, one with a single op-amp drive for the whole bridge and one where both arms of the bridge are driven separately by two op-amps.When I tried balancing it by adjusting R1 (or R2) it start decreasing towards zero but then starts raising again without getting to zero.In the one with two op-amps (separate drive of each arm of the bridge) I also tried adjusting the phase and the same occurs. I also tried adjusting the amplitude of one side of the bridge but the same again.Any suggestions what it could be and how to zero it?Thank you

author: duak
author_rank: Super Contributor
timestamp: March 25, 2020, 02:58:33 am
content: If I look at the schematic, I see a 20% tolerance on the inductors and an operating frequency of 200 KHz. By varying the series resistors you're balancing out the resistive part of the impedance, but not the reactive part. At 200 KHz, the parasitic capacitance of the inductor will interact with the inductance. I would try placing a low value trim cap across one inductor and then alternately adjust it and the values of R1 & R2 for balance. If the trim cap doesn't help, then move it to the other inductor and try again To save time I'd also add a balancing trim pot of 1K or 2K with its ends connected to OUT_A & OUT_B and its wiper connected to the drive signal through a 1K0 resistor.

author: ricko_uk
author_rank: Super Contributor
timestamp: March 25, 2020, 03:56:46 pm
content: Thank you very much Duak!!The inductors tolerance is actually 10% (not 20% the schematic % is incorrect). What size and range trimmer cap do you suggest?

author: duak
author_rank: Super Contributor
timestamp: March 25, 2020, 05:32:25 pm
content: Ricko,I expect something in the range of 1 - 20 pF, but it could be more. It's hard to say without seeing the parts. If you have an RLC bridge that works at low RF, it might give you a more exact value.If you find the value needed s on the lower end, you can make up a variable cap by twisting two insulated wires together and soldering the ends across the cap. In the beginning you can untwist and retwist them to get close, and then later on cut bits off for the fine adjustment. BTW, these were called "gimmicks" and you can sometimes see them where low value fine adjustments are needed.This only compensates for the parallel capacitance. Depending on how well you want this to balance you may have to go further. By the way, what are the loads on the inductors. Do they add much capacitance?

author: ricko_uk
author_rank: Super Contributor
timestamp: March 25, 2020, 06:05:09 pm
content: Thank you Duak,that's very useful and interesting trick!! I will try that while waiting for the variable capacitors.To answer your question, how do I find out if they add much capacitance and how much they add?I measured the coils using Peak's LCR meter and at 200KHz I get the following values (they vary a bit while measuring so I had to pick a centre value):Inductance 83.36 uH7.84 ohm DCImpedance 6.410 +J104.8 OhmAdmittance +582 -j9508mSMag 105 ohmPhase 86 degreesMany thanks

author: duak
author_rank: Super Contributor
timestamp: March 25, 2020, 07:26:51 pm
content: Ricko, so what are the loads on the inductors? Scope probes? Opamp buffers, perhaps with cables betwixt? Is this on a PCB or lashed up on a breadboard? At 200 KHz, pF and nH matter.

author: ricko_uk
author_rank: Super Contributor
timestamp: March 26, 2020, 02:43:47 pm
content: Thank you Duak,no scope probes or anything else. The only other thing connected is the Instrumentation amplifier connected to OUT_A and OUT_B. They are fed directly into the instrumentation amplifier without any filters or any other components, just direct pcb traces.The whole circuit is very compact PCB so traces are only few mm long. Longest perhaps 5mm. The coil leads + the coil connector are a total of around 15mm.Thank you