| Electronics > Beginners |
| Need help Understanding Oscope vs Multimeter Readings |
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| MK14:
The video seems to mention that you are using a small 9V, almost totally flat, battery to power the thing. That may be the cause of your problem, because it will tend to have a high internal resistance, and give lots of voltage change, as the current consumption of the circuit changes. Ideally, you want to use some kind of very steady power supply, to power it. Such as a bench power supply. |
| skinny:
--- Quote from: MK14 on May 30, 2018, 02:09:04 am ---The video seems to mention that you are using a small 9V, almost totally flat, battery to power the thing. That may be the cause of your problem, because it will tend to have a high internal resistance, and give lots of voltage change, as the current consumption of the circuit changes. --- End quote --- Thanks for the reply. Switched it out for a DC power supply. Same phenomenon occurs. |
| MK14:
--- Quote from: skinny on May 30, 2018, 02:15:25 am ---Thanks for the reply. Switched it out for a DC power supply. Same phenomenon occurs. --- End quote --- Ok. Also, the ground lead of the (visible on video) scope probe (I can't easily see how the other probe is connected, at least at, that point in the video), seems to be NOT connected up. This might explain why the oscilloscope image is so shaky. Ideally, the ground would be connected, where the 555 gets its ground (as close as reasonably possible). |
| Brumby:
The signal is definitely AC coupled somewhere along the line. The proof is when you switched over to AC coupling on the scope and the behaviour did not change. For DC coupling of a signal - EVERY point along the path from source to scope must be DC coupled. ** For AC coupling of a signal - you only need ONE point along the path to be AC coupled. ** ** This can be affected by active stages --- Quote from: MK14 on May 30, 2018, 02:27:48 am --- --- Quote from: skinny on May 30, 2018, 02:15:25 am ---Thanks for the reply. Switched it out for a DC power supply. Same phenomenon occurs. --- End quote --- Ok. Also, the ground lead of the (visible on video) scope probe (I can't easily see how the other probe is connected, at least at, that point in the video), seems to be NOT connected up. This might explain why the oscilloscope image is so shaky. Ideally, the ground would be connected, where the 555 gets its ground (as close as reasonably possible). --- End quote --- Ah, I hadn't looked that closely yet - but I did notice the shaky trace. This would certainly explain the level shift since you would be relying on stray capacitance to complete the ground connection. This is automatic AC coupling by default. The really jittery trace is another indicator, since such a ground connection would be rather high impedance, allowing all sorts of electromagnetic hash to get into the signal ground. If you don't have a ground connection from the scope to the circuit, connect one to the negative of the battery - and watch the magic. (My punt) |
| Brumby:
Here it is ... flapping in the breeze and no other connection visible on the scope. |
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