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PICOSCOPE 2204A -  USB Oscilloscope - Review

Scoring

Product Performed to Expectations: 8
Specifications were sufficient to design with: 10
Demo Software was of good quality: 9
Product was easy to use: 10
Support materials were available: 10
The price to performance ratio was good: 10
TotalScore: 57 / 60
  • RoadTest: PICOSCOPE 2204A - USB Oscilloscope
  • Buy Now
  • Evaluation Type: Test Equipment
  • Was everything in the box required?: Yes
  • Comparable Products/Other parts you considered: BitScope Micro, IkaScope, DSO Nano
  • What were the biggest problems encountered?: Power surge on the USB port.

  • Detailed Review:

    My road test was to look at the Picoscope for examining small electric motors. I was interested to know about the properties of small electric motors and wondered if the scope could help with that, specifically looking at the waveform generator and scope triggering.

    Setup

    My experimental rig was to use the waveform generator to control a mosfet that drives the motor and use a resistor to monitor the current. I had initially thought I'd get away with a simple circuit but soon discovered that the waveform generator (AWG) was 1v peak to peak so I added an additional transistor to switch my fet.

     

    Circuit

    My first experiment was to use the AWG to generate a PWM signal to drive the motor. As my signal was inverted hence was my duty cycle so the 80% in the diagram below is actually 20%.

    AWG

    The scope was set for automatic triggering and I created a "custom probe" that simply displayed the volts as amps (because I was measuring the voltage dropped over a 1 ohm resistor). This produced a steady signal and there was an interesting negative spike when the circuit was switching off. This performed well, adjusting the duty cycle changed the speed as expected although I had to close this screen to get back to the scope controls. I quickly discovered that there was a sweet spot for the motor / control circuit of 1Khz where the motor speed was highest and it responded best to low duty cycles.

    OPWM Current

    Around about this time I got an over current warning on the laptop's USB port and had to reboot that to reset the ports. I'm not sure what caused this but it is a bit worrying. I had already spotted that the USB cable's gnd is connected to the gnd of the probes and wondered if that might be a problem.

    Earthing

     

    My second experiment was to look at the switch on current. For this I swapped the scope triggering to be single and manually set a level of 20mA (the little diamond on the screenshot), I also moved this from the middle to 10% to capture the best waveform. Rather than using the AWG, I manually switched the transistor with a wire link. The graph for this was quite interesting the current rises then suddenly drops. I suspect this is the point that the motor begins to move. There is also a second negative spike but I'm not sure about that one.

    Startup Current

    So with regards to looking at the triggering and AWG of the Picoscope I did find it was easy to use and in the process I've learnt a bit about these little motors. The software seems robust and captures the data well.  Examining the traces on the laptop screen is a lot easier than the small screens of a scope. I've marked down the performs to expectations due to the power surge. I think it should be possible to mitigate for this, perhaps with a powered hub or an isolator. USB isolators seem to be around £30 which I suspect why this circuitry was not added to the scope but I think it's a good investment, particularly if working with inductive circuits. I've also marked down the software as I was hoping to be able to run display an rolliing average on the screen at the same time as the signal. This might be possible but I could not work out how to do that. There are features to give an overall average (measurements) and to average at the probe (custom probes).

     

    So it seems a good device but I'm afraid it's going to be kept in the box till I get an isolator.


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