LC circuit

Liek I said, I tried Google= nothing I am NOT lookign for theory on it. I want a schematic.

Ken

Looks good, thanks here is a link with my circuit so far and my problem:

yes that is exactly it. I have been trying to find a solution to this a over a week now. You have any ideas? K

Google "lc oscillator"

John

This sort of oscillator is often referred as a negative resistance oscillator, because once the negative resistance gets higher than the loss resistance, oscillations ensue.

Here is an LC meter that uses a comparator to produce the negative resistance to keep an arbitrary LC tank oscillating.

You must be using a different Google than the one I use.

John

As I understand your problem (correct my errors) you want to cause an LC tank to oscillate. But you don't want it to be driven into oscillation, continuously, but given a blast of energy only when the oscillation has damped down to some minimum amplitude. I don't understand if the blast is to be a single pulse, of a negative resistance that feeds energy in, slowly, till the amplitude gets to a second, larger amplitude. Then there is some other feature about operating only when the supply voltage is larger than some value.

I would think in terms of adding a high input impedance buffer to the tank, so you can use that output for both the positive feedback that generates the negative resistance, and also feeds the rectifier, so you can use that to measure the amplitude, without loading down the tank. Then you need a comparator, with hysteresis (some positive feedback), to make the drive versus coast decision, based on the rectified and filtered tank amplitude signal. A second comparator and some sort of voltage reference would be used to shut the drive down when the supply voltage is too low. You might use a CMOS analog switch to turn the negative resistance drive on and off, based on these two control signals. This might all be distilled down to a few transistors, but doing it with nice, clean functional blocks, first, will probably get you a working unit, sooner.

When VComp's output goes low, does that turn the simple switch off. If not, that is your problem. Have you checked to make sure that VSen1 is turning on, when capacitor C3 has the positive charge on it? If VSen1 does turn on, does VComp turn off, when VSen1 turns on? The only thing that can discharge C3, is the input of VSen1. If VSen1 has a CMOS input, that current could be very small. Also, have you tried using another VSen1 device (in case the one you are using is bad)?

Brian

.. Sorry , I am not sure what you mean by that. I know you are talking about a voltage follower with unity gain , but what for.

I have added a second cirduit to the site:

Ken

Ken wrote: >>I would think in terms of adding a high input impedance buffer to the >>tank, so you can use that output for both the positive feedback that >>generates the negative resistance, >

A follower would be one example of what I am thinking about. Any non inverting amplifier configuration with a high input impedance might work. But you could try a follower if you have no particular need for more gain. You put energy into the tank, but you use the copy of its voltage from the follower for any need to have information about what is going on in the tank (phase feedback for the driver, or amplitude measurement).

Add the follower between the resonant circuit and the diodes.

Actually its an oscillator-resonator, cannot accomplish that with semiconductors

WOW this is a great program , thankyou for your time. You circuit is for sure better then mine. How does LT spice compare with Pspice or Orcad 10.5 , any idea ?? I am going to take a day to see how your circuit woks completely, but from the simulation , its not giving me a nice wave. I posted the output at this site:

ken

Sorry After zooming in , .. it IS a sin wave. sorry again.

"Ken" wrote in message news:U1gOf.7364$ snipped-for-privacy@news20.bellglobal.com...

about

Your first circuit was nearly there!. Needs a discharge path across C3. Probably more important is that the circuit has a perfectly stable state in the non oscillating condition that you noted. A sim needs the comparitor "+" input holding at 0V until the circuit has stabilised. Releasing it will then give the 'kickstart' to force the circuit into it's oscillating mode. Difficult to make a 'good' circuit given the small number of components and how they all notice each other. Really a couple of monostables and a flip-flop wouldn't go amiss. Below is similar but working circuit in the "LTspice" format. Includes a bit of hysteresis for the comparitor, a transistor for the 'switch' and a CR to give a rough LC energising pulse. Should though be buildable for real. Side note; Tuned circuit Q factor can be measured simply as counting the number of cycles to the point where the voltage has dropped to 4.3% of the initial voltage. [ InitialV / (e^pi) ] john

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You have just described an oscilator.... -)-)-) try googling for that!!!

John, like probably you,I get 612,000 hits on "lc oscillator using Google.

The very first hit of the 612,000 is:

Considering that even this first link contains schematics for the Armstrong, Hatlesy, Colpitts, Electron Coupled and other common oscillators. Evidently you know how to use Google, but Ken doesn't, or else he want's someone to hold his hand an spoon feed him very basic information.

...And to Ken, "we ain't you mamma"! :-(

Harry C

HI Bob, Good question: One of the thing I am trying to do well. I can only explain well using an analogy. You get on a swing set, you generate enough energy so that you start swing (oscillating) everutime you reach a peak value, you give yourself (or a friend..) a small push to keep going the same amplitude.BUT if you keep giving the same push everytime with the same strenght you did the first one, you will reach a point where you will no longer get to go any higher, or simply break and fall. In an oscillating mechanical system when you reach that point, the machine simply over heat or break down. This is what I want to do electrically.

I sure hope you can...vacuum tubes are getting harder to come by...:-)

What exactly do you mean by an "oscillator-resonator"? I looked over your schematic, and followed the thread so far - but unless you really, really need to have something that lets an LC tank "ring" all on its on, and only kicks it again when the amplitude drops down to some given point, this seems like an awful lot of trouble to go to for a sine wave - you can do THAT with a single transistor. What is it you're really trying to do here?

Bob M.