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Review - Details

PICOSCOPE 2204A -  USB Oscilloscope - Review

Workshopshed

Table of contents

RoadTest: PICOSCOPE 2204A -  USB Oscilloscope

Author: Workshopshed

Creation date:

Evaluation Type: Test Equipment

Did you receive all parts the manufacturer stated would be included in the package?: True

What other parts do you consider comparable to this product?: 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.

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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.

 

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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%.

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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.

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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.

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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.

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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.

Anonymous
  • in reply to Workshopshed

    Good to see you are progressing with the Picoscope. It does do some strange things that take a bit of getting used to.

     

    I hadn't noticed that the maths channels are turned off when the trigger is in auto. I should be collecting data from a generator flux probe tomorrow / Thursday so I will see if it does it on my setup.

     

    Kind regards

  • in reply to three-phase

    Cheers Donald, I found the average function. Tools -> Math Channels, although when set the trigger to auto it seems to turn off the math channel

    imageimageimage

  • I enjoyed reading your review.

     

    You say it's comparable to the DSO Nano. I've got the quad channel DSO. What would you say the differences are? Which one is easier to use? My initial thought is that the DSO Nano might be useful for some things that require isolation. Your power surge leads me to wonder if the Picoscope isolates the USB from the probe. If not then that's a major issue and something that people should keep in mind. That's fine for little electronics projects but some motor controllers handle hundreds of volts. You don't any sort of noise from that getting coupled back to your USB port! But, I like the way the Picoscope looks on the PC and I could see a lot of uses for that as well. I'm wondering if it seems like they're mostly comparable but useful for slightly different things. Perhaps it'd be nice to have both?

  • Great review and experiments. You should be able to use the maths function to create a new trace with an averaging operation.

     

    Interesting that you got an over current alarm on the USB port. I have a Picoscope 3404A that I have used for four or five years to collect data when carrying out generator rotor tests and fault finding on static start systems. I have never consider that the USB socket was connected to the channel ground so went and checked and found out that it was.

     

    I have run the Picoscope on a couple of different laptops and a Macbook Pro and never had an over current alarm on any of them irrespective of what I was testing. The Picoscope I have came with dual USB plugs and I always make sure that both of them are utilised when powering the Picoscope from USB.

     

    Will be interesting to see if you get a repeat alarm when you are using it with an isolator.

     

    Kind regards.

  • in reply to Workshopshed

    As said, 100nF across the motor, and I'd go for 470uF plus 100nF across the rail.

     

    I suspect the diode will introduce all sorts of other issues, and the BUZ11 already has on across the drain/source pins.

     

    Your motor will only have a few connections on the commutator so anything is possible, but I doubt it is dirt or bouncing.

     

     

    Mark

  • Hi Andy,

     

    Great application for the PicoScope! I can imagine many people using it out in the field with a laptop, to check motors for electrical machinery etc. A friend exclusively uses the Picotech scopes, the automotive versions though. They are pretty much the market leader in that industry amongst others. I didn't realize they didn't have USB isolation built-in though - perhaps some of their other models do.

    For the capacitors Mark suggests, something like 100nF is ok. Also, it could be worth putting a diode across the motor (anode at the MOSFET) to try to reduce the voltage across it when it switches off. For a larger bulk capacitor., since your motor isn't large, maybe something like 100uF.

    I'm not sure what causes the measurement to drop to zero briefly, but it could possibly be the brushes bouncing off the commutator, or dirt on it, perhaps! Oscilloscopes always reveal interesting things during such experiments : )

  • in reply to mcb1

    The AdaFruit page https://www.adafruit.com/product/2107 does suggest that shared gnd is a common problem.

     

    I'll see if I have time to expand my experiment. Any suggestions on cap sizes?

  • in reply to DAB

    I agree, definately easy to use although for people who've never used a scope before then it is worth doing some reading/video watching.

  • The overcurrent is definately a concern, and I'm surprised the probe ground is tied to the USB.

     

    Driving motors is always interesting, and there are lots of harmonincs, spikes and other nasties floating around.

    It would be interesting repeating this with a cap across the brushes and one from each brush to the body.

     

    I wouldn't be surprised if another across the 5v rail didn't help as well.

     

     

    Chers

    Mark

  • Nice report Andy.

     

    I have found the Picoscope to be a nice device for usual tasks and the software is reasonably intuitive and useful.

     

    As long as it is used within its limitations, they are an easy to use scope and function generator.

     

    DAB