Voltage of an LR tank circuit - Page 1
Forum section: electronics / beginners

author: MathWizard
author_rank: Super Contributor
timestamp: October 28, 2020, 03:33:24 pm
content: I'm working on the 2nd ODE's for circuit analysis in the time/phasor/frequency domains. And so I made a series LR tank, R=200k, L is unknown, it's a ferrite core inductor from a printer, or TV, about an 1" tall, around 167mH I figure, it really likes 1.193MHz.So from my signal gen, in HiZ mode, I'm feeding a 4Vpp , 1.193MHz square wave into the inductor, then into R, to GND.But I measured 36Vpp , at the node of L-R, (now it's decent sine wave) on my scope. Both the probe and scope are on 10x.I can't get LTSpice to have gain, and I only measure 0.9Vpp on my DMM, @ 1.2MHz that's pretty good, but I kill the signal right down to normal levels, on my scope.So what am I measuring?, Vin still measures 2Vpp. What eqn would give me a gain of 9, from this lowpass LR filter ?

author: bob91343
author_rank: Super Contributor
timestamp: October 28, 2020, 06:53:56 pm
content: I would like to help but your description is so disjointed that I have trouble following it. Maybe rewrite it so a poor old engineer like me can follow it. No offense meant, just confusion.

author: MathWizard
author_rank: Super Contributor
timestamp: October 28, 2020, 08:51:48 pm
content: I thought maybe it was just the rise time of the sqr.wave that was causing such a voltage gain. So I changed to a 1.17MHz sine wave, and I still get 29Vpp out, from 4Vpp in. The voltage really spikes across a very narrow band, a few 10's of kHz, around resonance.I'm used to phasor domain eqn's, and I'm learning the ODE equations for this. But I can't say I remember, or know why, yet, this circuit has a huge voltage gain. What am I missing here.In LTSpice, I'm not using any ferromag core inductor. And I don't know enough eqn's about any of that.

author: ledtester
author_rank: Super Contributor
timestamp: October 28, 2020, 09:05:20 pm
content: What happens when you exchange your oscilloscope probes -- i.e. plug the probes you are using for channel 1 into channel 2 and vice-versa?

author: drvtech
author_rank: Regular Contributor
timestamp: October 28, 2020, 09:23:32 pm
content: I too can't quite follow what you're up to but it sounds to me as though you've found the self-resonant frequency of your inductor. In other words this is no longer an LR circuit but actually an LCR circuit where the C is the parasitic capacitance of the inductor

author: MathWizard
author_rank: Super Contributor
timestamp: October 28, 2020, 09:27:28 pm
content: I have books, with the ODE's to use, but for the last few weeks I've been deriving and re-remembering the ODE side of all this, I think I've gotton most the RC and LC right for AC and DC, but not for the AC of RL yet.But maybe I'm a fool, but I don't remember reading or calulating enough, about inductors, to say that a passive , low pass LR filter, does have a huge voltage gain at resonace, it certainly can't have power gain, unless it's getting it from my Siglent wave gen.IDK, I'm not using Fourier series/transforms yet to resprent a square wave. And I'm just using the basic models, no added Rs, Cp/Cs, etc, or breadboard Cap.But no phasor eqn I know of says I'd have a voltage gain there. Maybe I learned it wrong.Here's with probes swappedAlso, why doesn't LTspice have a big voltage gain, I stepped L, and added a cap across L, and barely got any voltage gain.Note, Red is the output, at 29Vpp, Yellow is the input, at 4Vpp

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 28, 2020, 09:39:10 pm
content: I suggest:1) exchange positions of R and L compared to your post 2, so it is a conventional low pass filter, with scope grounds always on ground (lower) side.2) switch signal generator to sine wave, so you don't have to worry about transients and harmonics.When that works, and measurements agree with simulation, you can proceed to re-arrange as necessary and go to square pulses.

author: MasterT
author_rank: Frequent Contributor
timestamp: October 28, 2020, 09:47:34 pm
content: You have series LC resonant circuit, where C is probe's capacitance. My Rigol scope has about 20pF for 200 MHz 1/10 probes. You can add a cap, 10-20 pF across, and see a changes in peaking frequency. Using bigger capacitance would help to re-calculate correct value of the inductor

author: MathWizard
author_rank: Super Contributor
timestamp: October 28, 2020, 09:51:32 pm
content: Ok so what about the voltage gain, over the input voltage, or are you saying it's probe/scope that just reading it wrong ?, and there's not supposed to be gain ?I have an LCR meter too, I haven't bothered to check the value...in fact,, I should see what a similar type/size inductor does.

author: MasterT
author_rank: Frequent Contributor
timestamp: October 28, 2020, 10:04:56 pm
content: In series LC circuits voltage gain is limited by dissipation power, and this factor could be very low with good quality components. 10 MOhm impedance also quite high compare to 50 Ohm output of generator , so roughly 200.000 voltage amplification possible, theoretically.

author: MathWizard
author_rank: Super Contributor
timestamp: October 28, 2020, 10:26:00 pm
content: Yeah I have to think more about it, plus I think I know, that an ideal inductor, through it's magnetic field, will make an equal and opposite voltage, to momentarely oppose changes in current, just like interia/mass will oppose changes in acceleration, which is the same ODE's. So yeah I'm forgetting an inductor can make large voltages.My SDG2042X AWG was on Hi-Z mode, IDK the valus though.I have to learn more of the deeper physics, I never learned enough of it I guess. And phasors at 1 freq. won't see the other higher freq stuff too, right. I'm just getting into the s-domain stuff too, for that. Trying to derive some of the math, for fun/practice. I have the books on it though too, but I guess I rather learn it a bit slower.I'm forgetting too, I've made real and simultated Colpitt's and Hartley oscillator's, that have voltage gains, way over the power supply, and I guess it's through the magic of magnetics, and impedance ratio's, etc. And of course SMPS use it all the time. I have more work to do.