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Forum Oscilloscope vertical position
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Related

Oscilloscope vertical position

krik
krik over 8 years ago

Bought my first oscilloscope a couple weeks ago, a Rigol DS1054 Z. And with some help from videos and blogs I am learning how to use it. While I have other things I plan to use it for, the reason that triggered buying it was to measure flicker in florescent and LED lighting. The reason I am testing them is that my niece suffers from seizures and after some reading I had to wonder if florescent or LED lights maybe contributing to the problem. Originally I thought just get some LED's that run on DC and the problem would be solved. It's not that easy, the 2 kinds I bought both flicker at 120 Hz, they are actually worse than florescent bulbs they were to replace. And after thinking about it, the LED's can be built without a good capacitor, so they may flicker. So basically now I am testing several different bulbs, fixtures and drivers.

 

So the issue is that the LED lights I am testing run between 12 and 36 volts, but to get the signal on my screen I am finding I have to spend several minutes turning the vertical position knob. I did eventually find that pressing the "Auto" button will put the signal in the middle of the screen, but it may set the vertical scale to 10mv, for example, so if I want to see it at 20mv I am back to spending a fair amount of time with the vertical position knob. In all the videos I watched it seemed to me that once they hooked up the test lead the signal was on their screen. Maybe that is the magic of editing but I doubt it as some do not cut way. So I am guessing either there is a setting I messed up at some point or there is another way that no one ever mentions when they are creating videos. So how do I get the oscilloscope to always keep the signal on the screen? Especially when I change the scale, but it would also be nice if it would also just automatically start out on the screen.

 

PS I hope this question is in the right place. These forums are very complicated to figure out. I have never seen forums setup like this.

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  • Robert Peter Oakes
    Robert Peter Oakes over 8 years ago +4
    A simple fact to bear in mind LEDs are not AC or DC. They are ALWAYS DC and are current devices. So even if your powering one from an AC supply, it will be rectified (All or partially) and current will…
  • michaelwylie
    michaelwylie over 8 years ago +2 suggested
    Not sure if I'm reading the problem correctly but it sounds like you're trying to observe an AC waveform on top of the DC signal. If this is the case simply set the coupling on that channel to AC.
  • jw0752
    jw0752 over 8 years ago in reply to krik +1 verified
    Hi Shane, I suspect that you want to look at the ripple. this is the inconsistency of the DC voltage. To do this you have to do what Michael told you and switch to the AC coupling. This causes the scope…
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  • michaelwylie
    0 michaelwylie over 8 years ago

    Not sure if I'm reading the problem correctly but it sounds like you're trying to observe an AC waveform on top of the DC signal. If this is the case simply set the coupling on that channel to AC.

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  • michaelwylie
    0 michaelwylie over 8 years ago

    Not sure if I'm reading the problem correctly but it sounds like you're trying to observe an AC waveform on top of the DC signal. If this is the case simply set the coupling on that channel to AC.

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  • krik
    0 krik over 8 years ago in reply to michaelwylie

    No all the lights I am testing are DC. Only DC LED's will not flicker, that is if the driver circuitry is built correctly, and many are not so I am having to test a ton of lights. Now 2 of the lights the driver circuitry is inside the lights, in those cases I am using a solar panel (its DC), to detect fluctuations.

     

    I may have failed to explain what I need, let me try a different approach. If I am measuring a 12v DC, I can set the vertical scale to 10 volts and only need to make a small adjustment to center the signal. But if I jump down to a 10mv vertical scale, it may take me 5+ minutes of furiously spinning the vertical position knob to get the signal back in the center of the screen. How do I set it so that the signal is always in the middle of the screen no matter what vertical scale I select?

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  • jw0752
    0 jw0752 over 8 years ago in reply to krik

    Hi Shane,

     

    I suspect that you want to look at the ripple. this is the inconsistency of the DC voltage. To do this you have to do what Michael told you and switch to the AC coupling. This causes the scope to ignore the DC voltage and only see the ripple. Once you are on AC you can decrease your vertical volts per cm until the ripple is readable. It will be almost impossible to totally eliminate all the ripple as the LEDs themselves will introduce ripple or what is more appropriately called noise. Give this a try and see if it shows you what you are looking for.

     

    John

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  • krik
    0 krik over 8 years ago in reply to jw0752

    That clarification did the trick. I just fired up a quick test and it works great.

     

    As for eliminating the ripples I fully expect to have some, its a matter of how much it varies and the time from peak to peak. I powered up a DC LED using a good quality PC power supply, as the driver, and it had ripples but the total variation was about 10mv in just under 8ms (120Hz). Where as the first 2 LED lights I tried, that had drivers built in, had over 2 volts in just under 8ms, not good for seizure sensitive person. The most recent external driver I tested had a variation of about 17mv in about 3ms, that would be over 300 flickers per second, but the voltage is so minor it will be almost like no flicker at all. And from what I read, anything over 200 per second the human mind cannot distinguish.

     

    Now I have to figure out who gets marked with the correct answer.

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  • mcb1
    0 mcb1 over 8 years ago in reply to krik

    I suspect you'll actually need to 'read' the change in light level.

     

    LED's are not linear and therefore may not actually produce much change in intensity.

     

    It is the current that drives a LED, and LED drivers may be pulsing the LED which has a much greater light flickering, even if the voltage remains constant.

    You could see this by inserting a low value resistance in the cathode side and measuring across it.

     

    To measure the flickering effect you'd need to use a phototransistor or similar since LDRs will be far too slow.

    Try to get the level to a point where the transistor is not in saturation.

     

     

    Mark

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