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Thread: Parallax USB Oscilloscope

  1. #1

    Default Parallax USB Oscilloscope

    Hello I have a question about the parallax scope.· I currently have a small brushed motor connected to an hbridge.· I have the input·pwm·duty cycle at·50% and I can view the voltage across the motor on CH1, with·CH2 disconnect from the scope.

    Conversely,with CH1 disconnected I can view the input signal to the bridge on CH2, but when I try to view both signals at the same time, the motor·goes to·100% instead of 50%.

    Whats up with that??

    btw:·The battery connected to the motor is 9.6V
    Last edited by ForumTools; 10-02-2010 at 04:07 PM. Reason: Forum Migration

  2. #2

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    The two ground pins on the 'scope probes are connected together inside the 'scope. When you connect one of the probes across the motor and the other across the input to the H-bridge driver, the ground path through the 'scope short circuits half of the H-bridge. Uh-oh. You can measure each one separately, because there is no short circuit.

    The term "ground" a misnomer sort of, because you don't have to attach it to true ground or to the (-) side of the battery. For example, as you know, you can connect it across the H-bridge, and that lets you visualize the voltage across the motor. But that "ground clip" is always the reference point and when you go to measure two signals with two probes, you have to choose the same reference point for both. And then, you can usually leave one of the ground clips disconnected, because the connection is made anyway inside the 'scope.

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    Tracy Allen
    www.emesystems.com
    Last edited by ForumTools; 10-02-2010 at 04:07 PM. Reason: Forum Migration

  3. #3

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    Thanks for the response I will experiment with the scope later today to verify this.



    I have another question, I was told that if I was to measure the voltage across a step-down transformer that converts the voltage from about 120VAC to 1.2VAC, I could fry my computer.· I'm not going to try it of course, but I would like to know if that is true or not and if so why?
    Last edited by ForumTools; 10-02-2010 at 04:07 PM. Reason: Forum Migration

  4. #4

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    If you have an Ohmmeter, measure the resistance between the two ground clips.

    The 1.2VAC output of that transformer is almost certainly isolated from the 120VAC input. So you could, safety considerations aside, use two probes with one across the output and the other across the input (which might need a voltage divider). But take care! When you do that, the ground clips do bypass the isolation provided by the transformer. There are lots of ifs, ands and buts, but it is true that under some conditions you could blow out your computer, your circuit, or your loved ones. A little knowledge is a dangerous thing.

    I have a portable Tek 'scope that does have complete isolation of the inputs, both from one another, and from the rest of the 'scope. The signals from each channel are transferred across an optical isolation barrier, so the ground clips are isolated from one another and from the 'scope body, and both ground clips have to be connected to the circuit, but each ground clip can be conneted to its own reference point even hundreds of volts apart. It is a very useful feature.

    However, most 'scopes are not like that. The grounds both have to be at the same reference point. And to compare two signals that have two different reference points you have to use a dfferential scheme. I don't know specifically how the Parallax USB oscilloscope is wired internally.

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    Tracy Allen
    www.emesystems.com
    Last edited by ForumTools; 10-02-2010 at 04:07 PM. Reason: Forum Migration

  5. #5

    Default

    If you have an Ohmmeter, measure the resistance between the two ground clips.

    The 1.2VAC output of that transformer is almost certainly isolated from the 120VAC input. So you could, safety considerations aside, use two probes with one across the output and the other across the input (which might need a voltage divider). But take care! When you do that, the ground clips do bypass the isolation provided by the transformer. There are lots of ifs, ands and buts, but it is true that under some conditions you could blow out your computer, your circuit, or your loved ones. A little knowledge is a dangerous thing.

    I have a portable Tek 'scope that does have complete isolation of the inputs, both from one another, and from the rest of the 'scope. The signals from each channel are transferred across an optical isolation barrier, so the ground clips are isolated from one another and from the 'scope body, and both ground clips have to be connected to the circuit, but each ground clip can be conneted to its own reference point even hundreds of volts apart. It is a very useful feature.

    However, most 'scopes are not like that. The grounds both have to be at the same reference point. And to compare two signals that have two different reference points you have to use a dfferential scheme. I don't know specifically how the Parallax USB oscilloscope is wired internally.

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    Tracy Allen
    www.emesystems.com
    Last edited by ForumTools; 10-02-2010 at 04:07 PM. Reason: Forum Migration

  6. #6

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    Well, I·WILL NOT·be hooking it up to a transformer anytime soon, because I'm not 100% sure of the internal workings myself.· I have two questions, 1) How exactly would the current flow when the ground is connected to remove the transformers isolation?? 2) What is the model # of your scope?

    Thanks again Prof
    Last edited by ForumTools; 10-02-2010 at 04:07 PM. Reason: Forum Migration

  7. #7

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    1) Under the best of conditions, no current will flow when the two ground clips are connected, one to the input and one to the output of the transformer. There is no DC loop.

    However, suppose that the output of the transformer is also connected to a circuit that is itself connected to, say, a machine tool in the next room that is also powered by AC and there is a connection to the safety ground pin on the AC socket. And the 'scope ground is connected to the safety ground of the outlet that the computer is plugged into. While the two safety grounds are nominally at the same potential, there will always be some small potential difference, and more likely so when the outlets are on different branch circuits. So a ground loop current will flow through the new ground connection. That will not usually be a large current, but it can enough to disturb the operation of sensitive circuits.

    However again, there exists a possibility that something has been miswired and one side of the connection goes to the hot side of the AC line--Oh no!--and when you make the connection all sorts of bad things involving smoke, sparks and screams will happen. Never make assumptions about these things.

    2) The 'scope is a Tektronix model THS720, dual channel 100mHz with additional multimeter data logger functions. It is more than 10 years old and its internal microprocessor is oh so slow. I can't help thinking how cool it would be if it had a Propeller as its guts.

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    Tracy Allen
    www.emesystems.com
    Last edited by ForumTools; 10-02-2010 at 04:07 PM. Reason: Forum Migration

  8. #8

    Default

    Well I don't know about the optoisolators, but parallax is coming out with a prop scope! I think I will get one!

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    Shawn Lowe


    When all else fails.....procrastinate!
    Last edited by ForumTools; 10-02-2010 at 04:07 PM. Reason: Forum Migration

  9. #9

    Default

    Shawn Lowe said...
    Well I don't know about the optoisolators, but parallax is coming out with a prop scope! I think I will get one!
    I am impatiently waiting. My only scope was an EBay dinosaur picked up 12 years ago and it was old, and sad, back then. Now I cant even find the special power plug it uses so it sits gathering dust. The PropScope is high on my to-get list when they release it.
    Last edited by ForumTools; 10-02-2010 at 04:07 PM. Reason: Forum Migration

  10. #10

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    When will the scope be released? It's seems like I've been hearing about it for over a year. But I do know that reliable designs dont pop up overnight
    Last edited by ForumTools; 10-02-2010 at 04:07 PM. Reason: Forum Migration

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