Electronics > Beginners

Digital Oscilloscope Maximum Input Voltage

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David Hess:

--- Quote from: mike_mike on February 04, 2018, 04:21:07 pm ---Thanks for the reply.
When measuring the noise from an LM317 or LM723 power supply, I saw that in one day, the noise was about 24mVpp for the LM723 power supply and about 50mVpp for the LM317 power supply. A few days later, I measured again the noise from the same power supplies, and I found that the LM723 had noise about 40mVpp and the LM723 had noise about 55mVpp. Is that normal ? Why the noise whas lower in one day and higher in another day ?
--- End quote ---

You probably were not measuring noise from only the power supply.  At such low levels, a differential or coaxial connection should be made.  For a power supply, the bandwidth should also be limited because a majority of the noise from the power supply is at low frequencies.  Traditionally oscilloscope bandwidth is limited to 20MHz when making power supply noise measurements but lower would actually be better except when it is a switching power supply.

mike_mike:
Thanks for the reply.
In the next image (the image shows the ripple at the output of an linear LM723 power supply) the frequency measured by the scope is 115.2Hz because of the noise ?
--- End quote ---

The value from the first image also fluctuates randomly, sometimes it is 100Hz, sometimes it is 300Hz, sometimes is about 1.5KHz. Is that normal ?
In the next image the ripple on the filter capacitor I think is clear and the frequency is about 100 Hz as I expected: https://imgur.com/a/e4kdI

David Hess:
Regulators have a high but limited "line rejection" which allows some of the ripple at the input to appear at the output.  To see this, set the oscilloscope's acquisition mode to averaging to reduce the noise level and the triggering to line.

mike_mike:
 Thanks for the reply.
In the country were I live the mains electricity is 230Vac. When I measure the mains voltage using an Unit UT52 multimeter, the multimeter measures about 247Vac.
It is safe to use the oscilloscope at this voltage (247Vac) ? In the oscilloscope manual it is wrote that it can be used up to maximum 240Vac ? Also in the manual it says "the power supply voltage should not fluctuate more than 10%"

Dubbie:
Dave made a good video a few years back on the topic of measuring power supply noise and ripple. Worth a watch if you are new to this!

https://youtu.be/Edel3eduRj4

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