LC resonant circuit for voltage amplification - Page 1
Forum section: electronics / projects

author: toyonline
author_rank: Contributor
timestamp: February 17, 2014, 06:40:41 am
content: Hi, I recently have built a LC circuit for voltage amplification purpose. The circuit is shown in the attachment. Through frequency sweep, I observed resonance behavior of the circuit. The problem is the voltage increase is only ~8 times larger than the original input voltage, which is not enough for my purpose. And through Spice I know the simulated increase of voltage of this circuit is 50dB, i.e. 300 times larger.I know Q factor maybe a parameter that influence voltage increase of my circuit. So maybe the limited amplification is due to low Q factor of some of electronic components in my circuit? If my hypothesis was right, maybe I need to change some old components in my circuit with brand new components.Would any one help me to point out other possibilities? and suggestions to improve that voltage increase?Thanks so much.

author: Odysseus
author_rank: Regular Contributor
timestamp: February 17, 2014, 06:51:33 am
content: Exactly, your simulation model does not incorporate any resistance in the inductors or capacitors, which will decrease the Q of the circuit. Luckily, the inductors will be the primary source of resistance, which you can measure directly using a multi-meter. This won't account for high-frequency loss, but should get you in the ballpark.Since inductor construction is highly variable, it would most helpful if you could post a picture of your constructed circuit. Then we can make suggestions about which components to upgrade.

author: JackOfVA
author_rank: Unknown
timestamp: February 17, 2014, 10:07:30 pm
content: What impedance load will be connected to the output? As you load the output, the loaded Q decreases and hence the voltage magnification.Depending on the frequency of operation, appropriate core material may range from permalloy to ferrite (mix 61 from Fair-Rite, for example) to carbonyl iron.Polystyrene capacitors are also a good choice for high Q, but the inductors are almost always the limiting factor.Also, I assume you have some way of tuning and adjusting all the resonant components to the correct frequencies. With a Q of 300, at 100 KHz, the -3 dB bandwidth is only 333 Hz. It may also be necessary to consider temperature stability of the tuned circuits as some types of inductor cores will have a large temperature coefficient. And worse yet, the tempco of the cores may not match even though they are of the same type.

author: codeboy2k
author_rank: Super Contributor
timestamp: February 18, 2014, 12:41:57 am
content: App note from LTAN118 - High Voltage, Low Noise, DC/DC ConverterOne of Jim Williams' -- a 10 page app note with a 40 page appendix describing how he built it, how he measured it, and tools and techniques to dealing with high-voltage, low noise and EMI...A good read

author: toyonline
author_rank: Contributor
timestamp: February 18, 2014, 05:16:09 am
content: Thanks all for your suggestions.Attached is a photo of the circuit construction. The Q factors of the capacitors are manufactured to be >100,000, I am not much worrying about them. What I am considering are Q values of the inductors and the variable capacitor.The inductors are made of AWG #24 and #26 copper wire, wound on the iron powder toriod cores. The resistance of the inductors are estimated to be 2ohms for the two 265 uH, and 0.8ohm for the two 134 uH. Since the Q factor of a inductor could be estimated by its reactance devided by resistance, the Q factors of these two pairs are 1000 and 1500, respectively.The variable capacitor is a second hand product, which was measured to have a 15 pF capacitance up shift from original tunable range. If it was a brand new product, the Q should be ~5000. But I am not sure whether it changed much for a second hand product.I added equivalent resistors to the inductors to monitor real experiment by Spice, and found only when the resistors increases to as much as 10-100 ohms, there will be significant drop of output voltage.Take all these into consideration, the total Q should be larger enough (at least several hundreds). Is that correct?

author: toyonline
author_rank: Contributor
timestamp: February 18, 2014, 06:05:15 am
content: That may affects my experiment. Since the resonant frequency is ~700kHz, and if Q=1000, the bandwidth at half of its maximum is 700Hz. I need to have a more accurate tuning of the circuit to see if it would help.Quote from: JackOfVA on February 17, 2014, 10:07:30 pm

author: toyonline
author_rank: Contributor
timestamp: February 18, 2014, 06:07:37 am
content: The output is connect to a parallel stainless steel plates, i.e. simply a capacitor. The only thing that dissipate power is the 1500k resistor.Quote from: Dawgie on February 18, 2014, 05:05:48 am

author: toyonline
author_rank: Contributor
timestamp: February 18, 2014, 06:08:40 am
content: Thanks. I will read that interesting article.Quote from: codeboy2k on February 18, 2014, 12:41:57 am

author: johansen
author_rank: Super Contributor
timestamp: February 18, 2014, 06:58:56 am
content: think of high permeability iron powder inductor cores as a shorted one turn secondary with around one ohm of resistance.switch to ferrite cores.

author: toyonline
author_rank: Contributor
timestamp: February 18, 2014, 01:56:00 pm
content: Would you please recommend me a product of that ferrite cores? I have no idea of which company's product is better.Quote from: johansen on February 18, 2014, 06:58:56 am

author: johansen
author_rank: Super Contributor
timestamp: February 18, 2014, 08:46:10 pm
content: I will get back to you on that...you may be forced to use air core inductors.I just read an appnote from micrometals for their carbonal iron poweder cores (4 to 35 permeability) they suggest 250 maximum Q.Consider rewinding those inductors as two layer windings, across 160 degrees or less of the core, rather than single layer as you have it now.Increased capacitance will lower the Q, but the increase in inductance (requiring fewer turns) may compensate. this will only work below a permeability of 10, do you know what cores you have?most signal generators are 50 ohm output impedance, you will need to measure the actual delievered voltage under load to determine the voltage amplification.i would speculated you did a lot better than just 8 fold gain.

author: G0HZU
author_rank: Super Contributor
timestamp: February 19, 2014, 12:31:13 am
content: If the big powdered iron toroids are something like T200-2 (red) then you can expect to get a Q of about 300 at 700kHz for inductance values around 260uH. This info is taken from the Micrometals databook that shows numerous Q curve charts.I don't know if you can still get equivalents but if you try and find something like the old 18x11 Siemens (ferrite) pot cores you could make up something with Litz wire and the M33 SIFERRIT material and get a Q of about 500 at 700kHz for 260uH.I must also add that your circuit topology looks a bit strange to me. Why so many components in such a strange topology if you just want to get a high RF voltage? I was expecting to see something along the lines of a Q meter circuit with a series LC (to ground) driven by a voltage source. The big RF voltage would be at the centre of the series LC circuit.If the source is a voltage source (eg Zs <0.1 ohm typical) delivering a voltage Vs then the voltage at the centre of the series LC circuit will be Qu.Vs where Qu is the Q of the inductor (assuming the Q of the cap is very large).So for a 1V signal at Vs you would see 300V at the centre if the Q of the LC circuit was 300. However, the 1.5M ohm load resistor would spoil this a bit. Also the bandwidth of the peak would be very narrow... maybe a couple of kHz.It's late and I'm tired so maybe I'm missing something obvious here?

author: toyonline
author_rank: Contributor
timestamp: February 19, 2014, 12:08:53 pm
content: Thank you for the suggestion.The core I used is T200 and T157 iron powder core, and the wire is copper wire of #24 and #26. I have read about Q of T200 and T157 cores from micrometals book. The limited Q values of inductors seems to be the key here.I am not sure which component is more important for a high Q inductor? the core or wire?If using Litz wire, which size should I take? is there any reference?Thanks so much.Quote from: johansen on February 18, 2014, 08:46:10 pm

author: toyonline
author_rank: Contributor
timestamp: February 19, 2014, 12:17:53 pm
content: The topology here was used to produce double resonant frequencies. I need a circuit to magnify two voltages at different frequencies simultaneously.Now I have obtained that two resonant frequencies. Although the half maximum width is quite wider, due to low Q of the circuit.I have no idea of the performance of homemade simple LC circuit for high frequency voltage magnification. Take a simple LC circuit as a example, what is the voltage gain after magnification? and the corresponding Q factor?I will try to rewind my inductor, try both single layer but more compact, and double layer.Quote from: G0HZU on February 19, 2014, 12:31:13 am

author: toyonline
author_rank: Contributor
timestamp: February 20, 2014, 02:43:40 pm
content: I changed winding geometry of the inductors. I used one layer winding, but leaving more unwinding core area. I found this time the voltage magnification was improved, ~3 times than former geometry. That's amazing.I think maybe I could improve more by increasing unwound core area, i.e. winding across smaller degrees.Any possible explanation for that?Quote from: johansen on February 18, 2014, 08:46:10 pm