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how to see a few MHz, 5000V peak to peak rectangular wave?

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steve_rb

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I want to check voltages connected to a few electrods. These voltages are about 100-5000 V peak voltage and rectangular in shape. Frequency is about few MHz. I wanted to use oscilliscope but it can't see more that a few hundred and probe cable also is not for high voltages. Any advice how to do this?
 

I think designing HV AC probes which include a resistor for DC measurements is not simple, in theory you need to add an extra RC group to increase voltage level. Due to high voltage presence, in practice it’s not so simple to do this. :-D The difficult part is locating a suitable resistor for Rx that has high enough resistance and physically is long enough such that arc-over is avoided. As a possible solution to be constructed from several equal lower value resistors in series. To maximize the bandwidth of the attenuating probe, the probe capacitor must be adjusted precisely such that the input capacitance of the scope is canceled. Because the Rx are so high, the probe becomes a good antenna, and a shield is mandatory.
Please read this :
Basics of High Voltage Probe Design
High Voltage Probe
 

In my case I don't have voltages that high. Maximum I have is 5000 V. Is there any comertial oscilloscope for this purpose? I don't want to be involved with making probs. Normal ohmeters can measure 1000 V DC which is a bit low for my case. I also have a few DC voltages up to 3000 V to measure. I want an instrument to be able to measure these voltages.
 

High Voltage & High Frequency Oscillosocpe Probe
HVP-15hf

tt-hvp-15hf.gif


http://www.globalmediapro.com/att/a/0/1/m/a01m10/hvp-15hf-manual.pdf

hvp-15hf - Google Search
 
Excellent. Thanks. I found this myself too via googling. Just one question. This probe ratio is 1000. Does this mean if I am measuring a rectangular wave with 3000 V peak for example with probe 1000 ratio oscilloscope will show 3 V on the screen?
 

You can also try the century-old "classical" method. Find an old vacuum-tube oscilloscope with still working CRT (cathode-ray tube).
With some care you can get directly to the vertical deflection plates of the CRT, and connect your 5 kV peak lines to them. All other components originally connected to those vertical plates should be disconnected.
Then if the time base of the oscilloscope can run to ~ few MHz of your signal, then you should see the real shape of your signal on the screen.
Working with high voltages in the scope as well as on your RF line is dangerous and requires a good knowledge of what you are doing. But this is valid for using the HV probes, too. Take care!
 

That’s a very complicated idea, but in practice don’t work. :cool:
Let’s assume we have all conditions required, no related problems due to additional high voltage wires in the scope circuit.
Even in the old CRT tubes the deflection factor (the reciprocal of the deflection sensitivity in a cathode-ray tube) is limited for tens (hundreds) of volts per inch. So, in this case it’s impossible to apply 3000-5000V on the deflection plates directly. See the example attached.
The Sylvania SC-3525 cathode-ray tube has a deflection factor :
Plates D1-D2 = 154-193 Volts/inch for a useful scan of 2 inches
and
Plates D3-D4 = 117-152Volts/inch for a useful scan of 4 inches
This implies D1-D2 = 200V x 2 = 400V or D3-D4 = 150v x 4= 600V.

Also do not mix the CRT anodes high voltages with deflection plates voltages. :grin:
See a diagram
 

Attachments

  • SC-3525.pdf
    1.7 MB · Views: 102
  • scope diagram.JPG
    scope diagram.JPG
    30.9 KB · Views: 80

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