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Can oscilloscopes measure circuits with different voltages at the same time?

Photomultiplier

Nov 22, 2022
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Note: I haven't attached a schematic because it's not part of the question. I'm trying to solve my circuit's problem on my own. I'm making my problem a learning exercise.

I have an arduino hooked up to my PC. It is at 5v. I have a servo hooked up to a buck convertor powered by a separate laptop power brick. The servo is at 7.2v. In order to ensure that the 5v USB doesn't interfere with the 7.2v servo, I have used a photocoupler/Opto-isolator.
Currently, the circuit is not working correctly. That is to say, the servo does not move. I have verified each output and input using a multimeter. I'm currently of the opinion that I have a signaling problem between the arduino and servo. The servo does, of course, work.

My question is, can I safely connect my oscilloscope, using 2 channels, to both sides of the photocoupler using normal (non-differential) probes?
My oscilloscope does not have isolated inputs. My oscilloscope is an MSO5074.

Thanks!

PS: I'm using a photocoupler instead of a transistor because I wasn't sure how the buck convertor and the PC's PSU would interact. I did not want to take any chances. I will learn more about these kinds of things in the future, of course.
 

Bluejets

Oct 5, 2014
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Show your circuit anyhow...easier to follow a schematic than a 1000 word essay.

For many servos, anything over 6v will blow the guts out of it.
 
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Photomultiplier

Nov 22, 2022
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Show your circuit anyhow...easier to follow a schematic than a 1000 word essay.

I'm so tempted to argue that point considering I was just asking about voltage differences when taking measurements. I mean, I'm asking in general, not just for this particular circuit.

The voltage divider outputs 1.2v to the optocoupler.
I forgot to write on the schematic that I'm running the TTL adapter via a USB 3.0 header. Therefore, it has 900mA to it and subsequently the arduino.
 

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Bluejets

Oct 5, 2014
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I'm so tempted to argue that point considering I was just asking about voltage differences when taking measurements. I mean, I'm asking in general, not just for this particular circuit.
Fine...work it out for yourself then if you know so much.

In the meantime, you need a common between the servo and the arduino and chuck that opto isolator...does nothing
 

dragon

Oct 31, 2022
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If you took out the buck converter and just powered the servo with a battery like ordinary does it work?

<edit> about the question, i dont know sorry, ive only ever used multimetres.
 
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Harald Kapp

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Your main question: No you can't. If the two circuits are at different potentials, then connecting a scope to both will short-circuit these potentials. Depending on the potentials and the impedances present this may cause a high current to flow via the now common ground of the scope . Thsi may have any of a bunch of effects, up to the destruction of the scope or the circuits under test.

Concerning your circuit:
  1. The resistive divider to drive the LED makes little sense. Usually you need a single series resistor (Got a question about driving LEDs?)
  2. The output (transistor) of the photocoupler is connected in the wrong way in separate aspects:
    • The emitter of the NPN transistor needs to go to negative potential (ground), not to the positive potential. This is assuming that you used conventional coloring scheme (red = '+', blue or black = '-').
    • With the emitter connected to ground ('-'), you will need a pull-up resistor to the servo's '+' potential to create a defined high potential. With only the transistor you can pull the control input low, but there is nothing to pull it high. You can easily verify this with a multimeter or a scope.
 

dragon

Oct 31, 2022
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I don't know what I'm looking at in the picture, but I bet it makes sense to you, my diagrams are the same, I dont have much luck showing them to ppl.
servos have an angle input, and a power input, it doesnt look like its wired right,
it looks like the photo coupler is sending a constant voltage to the angle input?? that needs to come
out of the digital out of the arduino and it varies it with software.
 

Photomultiplier

Nov 22, 2022
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I don't know what I'm looking at in the picture, but I bet it makes sense to you, my diagrams are the same, I dont have much luck showing them to ppl.
servos have an angle input, and a power input, it doesnt look like its wired right,

The + (red) and - (black) leads are connected correctly. I tested it.

it looks like the photo coupler is sending a constant voltage to the angle input?? that needs to come
out of the digital out of the arduino and it varies it with software.

That is how I wanted it to operate.
 

dragon

Oct 31, 2022
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Buck Converters dont just increase the voltage like a transformer, they increase the power, so if u were running off a battery ud be draining it faster. why not just have batteries in series - then youve got the extra battery there for the extra amp hours ur going to need.
 

bertus

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Nov 8, 2019
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Hello,
Buck Converters dont just increase the voltage like a transformer, they increase the power,
That is not true. A buck converter will have losses. The power out will ALWAYS be lower as the power in.
The efficiency is given in a percentage.
Most buck converters have an efficiency between 70 % (for a bad design) and 93 % (for a good design).

Bertus
 

dragon

Oct 31, 2022
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Voltage doublers increase the power, if you have ac, and it turns into solid dc at twice the voltage, it is actually an increase in power... theres no amp loss, it doesnt come out of a heavier resistor, it just shorts out twice the voltage.

<edit> oh nah maybe im wrong. hhahahaha </edit> sorry!!!
 

bertus

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Nov 8, 2019
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Hello,
Voltage doublers increase the power, if you have ac, and it turns into solid dc at twice the voltage, it is actually an increase in power... theres no amp loss
Again not true. A voltage doubler will double the voltage, but half the current.
Show a circuit of your voltage doubler.

Bertus
 

danadak

Feb 19, 2021
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When you have two circuits that are not at the same potential best way to
manage with an oscilloscope is to use differential probing :



Regards, Dana.
 
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