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Old 11-29-2018, 06:43 AM   #1
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Default Scope and Signal Generator question

So, one of the things that Ajax is having me do is resonate the tank circuit, the load circuit on the secondary of an isolation transformer. Essentially the bus, coils,bulk cans, and vernier cans. I have a Tektronix THS720A oscilliscope and Tenma 2Mhz function generator. Correct me if I am wrong. The ground lead goes on the common collector bus and the probe goes on the other bus. When I do this the waveform goes flat. I can isolate one of the taps from the bus and read the wave with it out of the circuit but as soon as I drop the tap back on it the wave disappears. I fear that I'm not reading my probes right. Coming out of construction we didnt use scopes and generators, so my experience with this type of equipment is limited. Any advice or guidance is greatly appreciated.
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Old 11-29-2018, 07:52 AM   #2
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The Function generator is in parallel with the scope. Is there a way to edit posts?
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Old 11-29-2018, 01:57 PM   #3
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Default not after a set time

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Originally Posted by TheDude_Abides View Post
Is there a way to edit posts?

The edit function is only for a set amount of time.


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Old 11-29-2018, 02:00 PM   #4
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The edit function is only for a set amount of time.


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That's three hours. His problem is not enough posts to have the edit button.
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Old 11-29-2018, 07:53 PM   #5
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A benchtop generator like a Tenma is meant for small signal work. It isn't a power driver. It requires a minimum sink impedance, usually 50 ohms. Below that impedance the signal simply gets swamped out by the source impedance in the generator. Too far below it and the signal just gets lost in the noise. You haven't stated exactly what you are working on but if it's in this particular forum I'd bet the load is far too great for the generator.

This would be somewhat like powering a 1000 VDC locomotive with a 9V battery. With the locomotive off you'll get the 9V on the rails. As soon as the locomotive is switched online you will not have 9V on the rails any longer.
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Old 11-29-2018, 08:29 PM   #6
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Thanks for the reply Fish, it’s a 600kw induction heater with a load coil voltage of 1250vdc. So in your opinion what would the minimum size generator I would need to produce a useful waveform?
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Old 11-29-2018, 09:23 PM   #7
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1250vac

Sorry
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Old 12-01-2018, 04:26 PM   #8
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A resonator needs both inductance and capacitance. Is there a capacitor in there somewhere?


If you are trying to find an electrical resonance, the signal generator should be just fine.
Most likely, you will resonate with a series L and a Shunt C. At resonance, the LC will appear to be a short circuit. Connect the scope across the two lines, the ground clip on the "collector bus" is OK I guess. The power is disconnected right?

Connect the ground end of the signal generator to the connection point of the scope ground. Connect the other other signal generator lead to a 500 Ohm resistor. The remaining end of this resistor connects to where you connected the signal input of the scope.

As you sweep the signal generator in frequency, you see a varying sinewave amplitude on your scope screen. 10 Hz to 1 kHz should work for the frequency range.

The set-up you describe has low enough impedance that the signal generator output is getting squashed. The 500 Ohm resistor will prevent the signal generator from being overloaded. The system impedance is still quite low, but you can turn up the scopes sensitivity to make up for that.

A wiring diagram of your system would help me understand what you have.

If you are trying to find a mechanical resonance that you can hear as well as see on the scope, you will need lots more power.
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Old 12-01-2018, 08:21 PM   #9
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So, I was finally able to resonate the circuit on Friday. The induction heater runs at a range of between 1800 and 3300 hz. It resonated at 2050hz. So if I can resonate the LC circuit then the problem wasn’t in the load. I started looking elsewhere. The logic uses fiber optic cables so I started looking into those. Lo and behold two of the fiber optic leads were backwards. Causing the SCR’s to fit out of sequence. Once I swapped the cables it fired up and purred like a kitten. When I swapped out the logic board I must’ve got the two cables swapped. Chock it up to a lesson learned and we’re back in production. Thanks for the help everyone.
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