Question: MPP REV-D - Step 4

[Olof N]

Hello.
Im not very experienced in using a scope and in the build documentation it says: "monitor the signal across the damping resistor using a x10 probe, where you should see a flyback signal of about 350V."
Well... Exactly where do I connect the leads of my x10 probe to measure this?


The scope I have access to is a Fluke 105B, on the scope inputs it says 300V.. Does this mean I will fry the scope if I try to use it in step 4?
If so, are there any other build-steps that is "off limits" for my scope?

[Qiaozhi]

Hello.
Im not very experienced in using a scope and in the build documentation it says: "monitor the signal across the damping resistor using a x10 probe, where you should see a flyback signal of about 350V."
Well... Exactly where do I connect the leads of my x10 probe to measure this?


The scope I have access to is a Fluke 105B, on the scope inputs it says 300V.. Does this mean I will fry the scope if I try to use it in step 4?
If so, are there any other build-steps that is "off limits" for my scope?

Your scope can accept a maximum input voltage of 300V, but with the x10 probe the input will only rise to 35V (350V/10).
My scope also has the same limitation. I have accidentally connected it with the probe set to x1, and no damage occurred.

As it says in the documentation: "monitor the signal across the damping resistor". In other words, across the coil.

[Olof N]

Thank you very much for your answer, now I can continue the build.

[dotniet]

If you have to measure higher voltages, you can build your own voltage divider with a 100K + 10K resistor (10:1)

[Olof N]

A new question on step 4.
I built step 4 without access to the scope but the earphone test went well so I went ahead and finished step 5. Voltage on TP3 drops from 0.116 to -0.7 when I put a Coke-can in the coil…

Now I had access to the scope again so I measured the flyback across the coil to verify that everything was OK with the step 4, sadly I think it weren’t. I have huge fluctuations in the signal as shown in the attached video..

What might be the cause for this?
Bad scope/Probe (the only probe left is pretty well used)
Totally wrong damping resistor? I´m using the supplied 470 ohm resistor.
Bad component, I´ve checked all the components on the PCB and they seem to be the right ones
Or am I just lacking the skills needed to use a scope in the right way..

I’ve built a coil as described in the “Chance PI coil” thread, measured inductance of 296uH and 1.9 ohm

[Qiaozhi]

That's most likely an artifact of your scope. It looks like it cannot sample fast enough to capture the fast flyback.
What scope are you using?

[Olof N]

It´s a Fluke 105B Scopemeter 100MHz
I dont know any specs on the X10 probe but I´ve bought a new 200MHz X10 probe that should arrive any day.
I´ll paus the project until I can test with a new probe, or maby another scope if I can find one.

[Qiaozhi]

It´s a Fluke 105B Scopemeter 100MHz
I dont know any specs on the X10 probe but I´ve bought a new 200MHz X10 probe that should arrive any day.
I´ll paus the project until I can test with a new probe, or maby another scope if I can find one.

You'll probably find that changing the timebase on the scope, to show only one flyback pulse, will stabilise the display. However, the flyback pulse is very fast, so it may not be able to sample the waveform peak, and the maximum voltage will appear to be lower than it actually is.