I need help/suggestions on choosing a USB Oscilloscope

Discussion in 'General Electronics Chat' started by electronice123, Aug 25, 2009.

  1. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
    302
    0
    I am in need of some advice on USB oscilloscopes.

    I have never used one so I don't know exactly what to look for.

    The circuits I work with operate below 100kHz, I need to be able to measure up to 20kV. I want to be able to save the waveforms on my computer and I need a 2 channel scope.

    Are the USB oscilloscopes on ebay any good? Can anyone recommend a good one for under 200$?

    And what about HV probes, Are all oscilloscope probe connections the same regardless of brand???
     
  2. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    20kV ?. My usb scope can handle 30V at most. You need some really good gadgets (really expensive) to handle 20.000V.

    I also doubt you'll get a good one under 200$. They tend to be quite expensive.
     
  3. Gustav180

    Member

    Aug 25, 2009
    17
    0
    Its is not so hard to do a probe for 20 kV, or i think you can get one from some old instrument. I have used HV-probe for 30 kV for measuring the HV on CRT:s for TV-set. It is only a big resistor, several Mohms and a good isolation.
     
  4. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
    302
    0
    Thanks everyone for the help. I looked up HV probes and yeah, they are expensive....Perhaps I can get some HV resistors and just make a voltage divider and measure off the resistor, of course after I do the math to be sure the voltage won't be too high!
     
  5. kkazem

    Active Member

    Jul 23, 2009
    160
    26
    Hi all,

    Yes, you can make a High-Voltage probe for 20KV to 30KV from a roughly 100 Megohm resistor, but be careful. At 30KV, even with a 100 Megohm resistor, you'll be dissipating 9 watts, so you'll need a 20-25 watt, 100 megohm resistor for it to be reliable. If you're only going to keep the probe on the HV signal for a limited amount of time, you might be able to get away with a 10 watt, 100 meg.

    There is another problem. The scope input amplifiers are normally 1 MEG Ohm input resistance and about 13pF input capacitance. You can make a voltage divider by using the 100 Meg, 25 watt resistor as the top resistor that will connect to the high-voltage and use a 1K, 1/8 watt, 1% resistor as the bottom resistor. The bottom end of the 1K is common (ground) for both the 20KV input and the scope input channel amplifier. The center where the bottom of the 100 MEG connects to the 1K resistor is the output to the scope input channel amplifier. This should work well and give reasonably good accuracy if the 100 meg is a 1% value, it may give you better than 1% overall accuracy. The divider ratio is: 20KV input = 200mV Output. The scope bandwidth will be somewhat limited using this homemade HV probe since there is no compensation circuit. However, since the output impedance to the scope is a low 1K Ohm, it will probably give good results up to a MHz or so. It's really hard to say as it depends on the scope input channel amp specs and the parasitic components in your probe, including the wiring from the probe to the scope. I suggest using 50 OHm Coax (RG58U) or equivalent.

    Good luck,
    Kamran Kazem
     
  6. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    I have not used a USB oscilloscope, so I can't comment with knowledge. However, I'll put my two cents in... Next to a digital multimeter, I consider my oscilloscope the most important and most-used electrical measurement tool I have. While a USB unit might be nice because they tend to be less expensive, I suspect I'd miss my regular scopes in short order if I only had a USB scope.

    You can buy older scopes on ebay for fairly low prices. I have a Phillips 25 MHz scope that I paid around $1700 for in the 1970's and they go for around the $50-$100 range on ebay. A very common scope in the 1970's was the Tektronix 465 and similar models; you can still find those for sale. These are analog scopes, but in some ways they still work better than some of the digital scopes I've used. If I could have any scope I wanted, my first choice would be a smaller vacuum tube Tek model I used in college in the 60's -- I think it was a 4 MHz scope and had the finest triggering of any scope I've used. Newer doesn't necessarily mean better.

    Of course, buying a used scope entails the risk that it might not work right. On the other hand, manuals can be found. My Phillips scope has the manual and complete schematics in the manual.

    In seeing how much I use these two tools (DMM and scope), I feel they're worth saving up enough money to get a good one.
     
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