Push-Pull Coil Oscillator questions - Page 1
Forum section: electronics / projects

author: lovemb
author_rank: Contributor
timestamp: October 20, 2015, 03:59:34 am
content: Hello everyone.I have this small circuit that I'm trying to understand and reproduce and I have a few questions.The circuit in question is a "push-pull coil oscillator" and it's being used to transmit energy to another "device".I made a LTSpice schematic of it and after running it the measures I get are somewhat close to real-life measurement.Coil sine-wave frequency: 120kHz (however I'm using a crap usb oscilloscope and a friend told me it is actually 118kHz)Original circuit is supplied with 12V DC, I tried a few supply values to see how the Coil voltage peak behaves.VsupplyIsupplyCoil VmaxSpice Coil Vmax12V46mA21V24V9V35mA14Vn/a7.5V29mA11Vn/a5V19.4mA7.7V10VAs you can see, my real circuit coil voltage max is different from the ltspice schematic.About the values of the circuit components, they should all be correct however I'm not very sure about the coil.What I (think I) know:-L1, L2 and C4 control the frequency.What I want to know:-What controls the the Coil Voltage Max(other than the input voltage) or if there is a "simple" formula to calculate it.-Supplying it with 5V, is there a way to increase the Coil Voltage Max from 7.7V(or 10V from spice simulation) closer to the 12V input values (that is 21V(or 24V from spice)). I can pull up to ~100mA from the 5V DC supply.In attachment is the LTSpice schematic, Coil picture with some info.Any help is appreciated.(And yes the oscilloscope looks that bad but I had nothing else at hand.2.4MSa/s usb oscilloscope..)

author: alsetalokin4017
author_rank: Super Contributor
timestamp: October 20, 2015, 04:48:28 am
content: Can you answer a couple of questions, related to your actual construction? I've reproduced the LTSpice schematic below with a couple of notations. First, could you confirm that the coils (really one centertapped coil) are connected as I've indicated, wrt the colors on your actual image of the coils? Next, that both coils are wound in the same direction (see "dot convention" that I've put on the schematic). Next, can you indicate where the scope probes are placed, including the ground references, on the schematic?I have a little bit of experience using similar circuits (basic Royer/Mazilli type zero-voltage-switching oscillators and multivibrators) to do wireless power transmission.

author: lovemb
author_rank: Contributor
timestamp: October 20, 2015, 06:19:58 am
content: Thanks a lot for your interest.The colors are exactly like in your mod image.The wounding direction is the same on "both coils", like you said it's like one coil with a center tap.I got a bit confused to the "dot convention" so I edited the image to show exactly the wounding direction.I had the oscilloscope probes at the C4 pads, however I'm not sure which one was ch1/ch2. (If it matters I can measure again)The oscilloscope ground was connected to supply ground.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2015, 10:13:24 pm
content: No K (coupling factor) from L1 to L2???What dimensions are the coils and their separation distance? Offhand I would roughly guess k = 0.4 to 0.7.Also, note that the values of the coupling capacitors (all of them besides C4 itself) are still on par with C4 (i.e., within a factor of 10 or so), so will load down the frequency somewhat as well (by about, oh, 8% or so).Does the oscilloscope have a stated bandwidth?Tim

author: lovemb
author_rank: Contributor
timestamp: October 21, 2015, 02:56:57 am
content: I wish I had a better answer, but I don't know what is the coupling factor.The spacing between L1 and L2 is 1.5mm.As to the oscilloscope, it's a Hantek1008B, 8ch usb oscilloscope designed to work on cars basically. No bandwidth stated, just the samples..I thought about simply using the original design @ 12V (or 9V), by boosting it from 5V. The problem is design space for the boost converter and current that is pulled from usb. (not sure if I forgot to say it was powered from usb)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2015, 03:29:42 am
content: Well can you answer my questions? Dimensions and stuff?These things can be calculated...Tim

author: lovemb
author_rank: Contributor
timestamp: October 21, 2015, 05:09:52 am
content: Oh, the dimensions were in the first post in the annexed image so I forgot to answer that part. sorry :peach coil has 21 turns, wound in the same direction, 29mm diameter, 4mm length each coil, 1.5mm space between coils.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2015, 06:31:59 am
content: Ah right, I see it.Best calculator is:http://hamwaves.com/antennas/inductance.htmlI get about 22.8uH. According to (from Radiotron Designer's Handbook, 4th Ed.),the coupling is off the chart, but probably around 0.6-0.7.For coupled inductors in series (in phase), Ltot = L1 + L2 + 2*M, where M = k*sqrt(L1*L2). Which if L1=L2=L, is just k*L. So Ltot = L*(2+2*(0.6)), or 3.2 to 3.4 times 22.8uH, or around 75uH. (The 35uH in the simulation is pretty close to equivalent, for uncoupled inductors.)The total capacitance shown is equivalent to 26.1nF as seen by the inductor, so the resonant frequency should be 114kHz. It might be higher or lower depending on actual component values and tolerances, and how the transistors skew that (i.e., they may turn on and off earlier or later depending on component values, adjusting the frequency a bit versus the ideal point).Tim