I'm having trouble triggering my scope to capture LC tank ringing.

[cvriv]

I have a Rigol DS2202A.

I have a 1uH inductor and a 47nF capacitor on a protoboard in hopes to see about 1kHz of ringing on my scope.

I charge the cap to 5v using a jumper wire that I later yank and place across the inductor so they are both parallel with each other. I have my scope across the cap.

Whats the best way to trigger this event? I know how to set the scope to trigger a single event but its not triggering. It's triggering when I charge the cap to 5v, but not when I dump that energy into the inductor.

Any tips? I tried setting the trigger level to various levels. I have the scope set to Edge and increasing slope. I tried decreasing slope as well. I was watching this guy on Youtube do this but I think his cap was much larger and he just used a coil of copper wire he coiled around a marker. What am I doing wrong?

[Benta]

With those component values, you're around 730 kHz. Perhaps you should review your maths?

[magic]

Chances are you are discharging the capacitor with your scope probe, particularly if it's set to 1x (1MΩ impedance). Do the math on that RC circuit first and check if you are switching those cables fast enough

As for triggering, enable auto trigger (already enabled by default?) so that you see what's going on even if the scope doesn't catch any trigger event. Observe what happens as you connect and disconnect things.

[cvriv]

 

Sorry I did screw up my math. I'm tired and frustrated. Same circuit except im using a 1uF ceramic cap with 30mH and this is the best I could get on my scope. I should be seeing a ring of about 1kHz but no matter what I do this is the best I can get. I increased the voltage to +9V and made sure the current limit was enough on my supply. Any ideas?

[bob91343]

You need to be set at decreasing slope to trigger on the drop in voltage.  However, 1 microfarad isn't much and will discharge through the oscilloscope input impedance rather quickly.

To do this more easily, rather than do a one-shot with moving a wire, try driving it with a square wave generator.  Otherwise, use a switch to make the transistion more definite.

[cvriv]

Thanks for the help. It does look like I'm discharging the capacitor but cant figure out why. All of what your saying makes sense. I'll try it. I do have my scope set to adecreasing slope trigger and its set to a single trigger. If I set it to auto I dont think it will capture what I want. I did notice that with a smaller capacitance, when I yank the jumper wire the scope will trigger. If I have a larger capacitance it won't. I'll throw a switch in there to see what happens. I'll try a square wave via the function generator.

[eblc1388]

If you are trying it with a square wave, do it like this:

1. place in series a 50R resistor before connecting to the tank circuit at the signal generator output
2. connect X10 probe across tank circuit
3. connect Ch2 or EXT directly to the signal gen output and use it to trigger the scope
4. changing the timebase to view the tank ringing in more details
 

[Circlotron]

Put the scope across the inductor so that when you apply the charged cap (with nothing else connected to it) to the inductor it will see a *rapidly* rising spike that then rings down.

[Ian.M]

Circlotron is on the right track, but rather than fumble around disconnecting the capacitor from its charging supply and transferring it to across the inductor, lets make it easy to work.

Charge the capacitor through a high value resistor so you don't have to disconnect the charging circuit.  Anything up to 200nF will pretty much fully charge in under a second through a 1 Meg resistor, which will have negligible damping effect on the LC tank circuit.  For your 1uF capacitor, give it five seconds.  Connect a x10 scope probe to the non-grounded end of the inductor, so it doesn't load the charging circuit till you connect the inductor to the capacitor.  Put a NE2 wire-ended neon bulb (without a resistor) directly across the inductor to protect the scope^* by clamping the back-EMF in case the inductor gets disconnected before the damped oscillation has died out.  Assuming a positive supply,when you connect the inductor to the capacitor you'll get a sharp positive edge the scope can easily trigger on.   The biggest problem will be contact bounce.   The traditional way to avoid contact bounce would be to use a mercury wetted relay, which can still be found NOS, but I wouldn't even entertain getting one unless it was a hermetically sealed miniature one.

* Caution: If your scope isn't rated for >150V max input, the NE2 neon bulb will NOT provide adequate protection if you use a x1 probe.  With a x10 probe it will be OK as long as the scope is rated for >15V max input.  Some cheap USB scopes and handheld scope kits have very limited input voltage ratings.

N.B. High value small axial inductors have considerable DC resistance.  Therefore, its likely that your LC circuit is heavily damped, possibly over-damped, hence the lack of visible ringing. Measure its DC resistance and post it, and given the inductance and capacitance, we can calculate if that is the case, from the equation for the series critical damping condition:

  R²=4L/C

which gives fractionally over 63 ohms for your choice of a 1uF capacitor and a 1mH inductor.

So you can see what you are up against, here's a LTspice sim with 11.5 ohms inductor's DC resistance. You'll clearly see four cycles of ringing before it decays into insignificance.  Change the DC resistance to 50 ohms and you'll only see a slight overshoot like you have.  Change it to 63 ohms and the overshoot will vanish.

[cvriv]

Thanks for the replies. I have to read and try everything your saying. I just spent hours messing around with it on my own and can't get it to work. I have to try again after readinfg the new posts. This is so frustrating. Im literally only doing this to see the ringing. Can't believe this.

[ledtester]

I would just try something simple like this:



V3 is just a square wave generator. It could be a 555 timer, microcontroller, two-transistor oscillator, ...

Make the period of the square wave generator many times larger than that of the LC tank (like 10 times larger). Your amplitude might not be very large but you should see some ringing.

In this simulation the amplitude of V3 is 5V and the resulting LC tank amplitude is about 200 mV peak-to-peak.

And for your scope you can just trigger on the square wave signal.

[alsetalokin4017]

Are you looking for something like this?



Scope across parallel LC tank, probe at 10x as to not load tank down
Power from 9v battery, wire dragged across battery terminal to provide dirty "pulse" to LC tank
Scope in single shot mode
Do it over and over until you get an acceptable ringdown trace, but it shouldn't take all night!
(Ringdown happens when circuit to battery is broken)

[cvriv]

Are you looking for something like this?

(Attachment Link)

Scope across parallel LC tank, probe at 10x as to not load tank down
Power from 9v battery, wire dragged across battery terminal to provide dirty "pulse" to LC tank
Scope in single shot mode
Do it over and over until you get an acceptable ringdown trace, but it shouldn't take all night!
(Ringdown happens when circuit to battery is broken)

I've seen that on my scope many times! LOL. I checked the frequency and it was WAY higher than what I calculated. I didnt think it was the ringing I was expecting. I thought it was something else. Let me try again.

[cvriv]

Ok. This is what I got. It's the best ring, if it's the ring produced by my LC tank. I used a 10uF cap and a 1mH inductor. I should've seen oscillations of about 1.6kHz, but the ring on my scope is about 850kHz. Why is this? Also.... the divisions on my scope are 100v!!! What is up with that!

I used a 1 MOhm resistor to charge my cap. I used a DMM across the cao to make sure it was charged. It's total charge was about 4.5V.

I used a SPDT switch to cutoff the supply and close the tank.

I used the oscilloscope's external trigger across the inductor. I don't know if this made a difference because I set it to trigger on an increasing slope the same way I did before with the probe. I don't know.

So the big question is why are the oscillations so high compared to what I calculated?

Thanks for your help.

[Ian.M]

That will be the switch bouncing after the current in the inductor has had a chance to build up a bit, then its ringing with its internal inter-layer stray capacitance.  I did caution you to put a NE2 neon bulb across the inductor to protect your scope against high voltage back-EMF! 

The actual initial contact closure is at the trigger point marked (T), then it opens again due to bounce 3.2us later, then closes extremely briefly at the discontinuity 5.4us after the trigger point.  The ringing you are interested in should be much lower amplitude and would need the scope timebase set to 2ms/div so you could get several periods of 1.6KHz ringing on screen.

Try a different type of switch - that one's too bouncy.

[Bud]

Did you properly set up the probe attenuation in the  Fisher Price scope?

[cvriv]

That will be the switch bouncing after the current in the inductor has had a chance to build up a bit, then its ringing with its internal inter-layer stray capacitance.  I did caution you to put a NE2 neon bulb across the inductor to protect your scope against high voltage back-EMF! 

The actual initial contact closure is at the trigger point marked (T), then it opens again due to bounce 3.2us later, then closes extremely briefly at the discontinuity 5.4us after the trigger point.  The ringing you are interested in should be much lower amplitude and would need the scope timebase set to 2ms/div so you could get several periods of 1.6KHz ringing on screen.

Try a different type of switch - that one's too bouncy.

Thanks, but I'm going to throw the towel in on this. I have a lot of reading to do on oscillators etc.

[cvriv]

Did you properly set up the probe attenuation in the  Fisher Price scope?

I set my Fisher Price toy scope to 10x. Thanks for asking bud.

[Circlotron]

Here's a method that worked for me. 220uH powdered iron toroid and 2.2uF film cap. Soldered them together in parallel and a 100 ohm 2 watt resistor off one end. Connected the scope probe across the LC. Connected the resistor to a 12 volt battery so 120mA flows through the inductor. When the resistor is disconnected from the battery the inductor current flows into the capacitor and the circuit rings. Inductor is always connected to the capacitor so no risk of scope overvoltage. Capacitor max voltage is predictable because it corresponds to the same energy that was stored in the inductor when the DC current was flowing. Capacitor voltage is directly proportional to inductor current. Any contact bounce when disconnecting the DC current is smoothed over somewhat by the capacitor, depending on it's size.

Test results 220uH and 2.2uF calculates to 7.234kHz.
Actual measured value = 6.896kHz.

[VooDust]

This is so frustrating. Im literally only doing this to see the ringing. Can't believe this.

That's the fun thing about LC tank circuits: Anybody can be smart on the internet and in simulators, but actually building a working circuit can be tricky. But you'll learn a lot from it!

For starters, charging the cap first will only give you little bang, since if you start with 5V, the oscillation can never get higher amplitude than that (think about it).

How I got it to work was: Charge the inductor up instead. Do this by applying DC to the tank circuit, it will flow through the inductor and build a magnetic field. If you don't have a lab power supply with current limitation, you can use a single 1.5V AA battery like I did. When you disconnect power, you will get a much bigger spike since you have more energy in your tank circuit.

Is your 1mH inductor marked? Because if it's not, you might actually be working with a different value and the frequency of the ringing that you see is telling you the true value of your inductor!

Happy ringing.

Did you properly set up the probe attenuation in the  Fisher Price scope?

Not helping.

[james_s]

I was going to suggest the same thing someone else metioned, put the scope probe across the inductor.

You do have to be careful about the voltage spikes, when you charge and discharge an inductor like that you have the basic principal of a boost converter and that can easily get you hundreds of volts.

[vk6zgo]

What I do is to use the (usually) 1kHz probe calibration signal, connect it directly to an LC network, trigger the scope off the cal signal, then look at the resultant ringing.

There are probably limitations upon the L & C values you can use, but if you just  want to see the ringing, a 2.5 mH RFC & a 1 nF cap should work.

Having an analog 'scope with a dual timebase, I can then look at the ringing "blown up".
A DSO should be able to something similar.

[cvriv]

Ok I'll try it again. Thats really cool. Thanks.

[alsetalokin4017]

The sharper the cut-off of the charging of the LC tank, the better ringing you will get. Just like striking a bell with a mallet. Use a soft felt mallet and you get a dull clunk. Use a hard wood or metal mallet you get a much better ring. Same thing electrically. Hit that tank with a hard pulse with sharp edges for best results. Also, if you are putting your tank together on a breadboard, you may be reducing performance due to poor contacts. Solder your L and C together and arrange some kind of fast interrupter for the charging current, either mechanical or electronic. Contact resistance and intermittency cause the ringing to dampen out more quickly or even vanish altogether as the stored energy dissipates.

[alsetalokin4017]

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