# LC circuit

Discussion in 'Electronic Basics' started by Ken, Mar 3, 2006.

1. ### KenGuest

Hi,

I am looking for a method to keep an oscillation going in a tank circuit (
inductor parallel to a capacitor). Therefore I would just like inject into
the circuit the power loss created by the natural resistance everyone so
cycle or a percentage of the peak value..
Yes I tried google and nothing comes close to what I m lookign for.

thanks
Ken

2. ### KenGuest

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

Ken

4. ### KenGuest

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

John

6. ### John PopelishGuest

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.

http://my.integritynet.com.au/purdic/lc-meter-project.htm#circuit-osc

7. ### John LarkinGuest

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

John

8. ### John PopelishGuest

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.

9. ### John PopelishGuest

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.

10. ### BrianGuest

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

11. ### KenGuest

I would think in terms of adding a high input impedance buffer to the

... 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:
http://www3.sympatico.ca/lerameur/

Ken

12. ### John PopelishGuest

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.

13. ### KenGuest

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

14. ### KenGuest

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:
http://www3.sympatico.ca/lerameur/

ken

15. ### KenGuest

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

16. ### John Jardine.Guest

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

Version 4
SHEET 1 880 680
WIRE -576 -48 -576 -128
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SYMATTR Value 2N2907
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SYMATTR Value 461m
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SYMATTR Value 15.27µ
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SYMATTR Value 100k
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SYMATTR Value 1µ
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SYMATTR Value 1µ
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SYMATTR Value 1
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TEXT -584 288 Left 0 !.tran 10
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TEXT 64 480 Left 0 ;60Hz

17. ### Sjouke BurryGuest

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

18. ### Guest

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

The very first hit of the 612,000 is:

http://www.electronixandmore.com/articles/oscillators.html

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

19. ### KenGuest

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.

K

20. ### Bob MyersGuest

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.