Digital Oscilloscope Maximum Input Voltage

[David Hess]

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 ?

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 ?

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

[mike_mike]

 I watched only a part from that video a few weeks ago.
I am still waiting for an answer for the question that I asked in my previous post.

[tautech]

I watched only a part from that video a few weeks ago.
I am still waiting for an answer for the question that I asked in my previous post.

DON'T probe mains with ordinary probes. Period.

The energy levels are too high to risk it. It's your choice but I tend to restrict myself to ~150V with 10x probes and anything higher brings out a 100x or 1000x probe.
With a modern DSO with good sensitivity such probe attenuation is no handicap.

[mike_mike]

I think I was not sufficiently clear when I wrote the first question.

My question is if I can power the oscilloscope from the mains (230Vac here in Europe) if the mains voltage is 247-250Vac, while in the manual is mentioned that the scope can be powered up to 240Vac ?
The value of the mains voltage (247Vac) was measured with an Uni-t UT52 digital multimeter.

[mike_mike]

When I was calibrating the probe, I saw that the calibration signal is having a little ”movement” on the screen. For example, I saw that the vertical lines of the signal are making some movement to the left and to the right as you can see from this video: https://www.youtube.com/watch?v=IOxCg90y3JI&feature=youtu.be

The settings on the scope are: 1V/div, 250uS/div

Is that movement of the vertical lines normal ?

[tautech]

I think I was not sufficiently clear when I wrote the first question.

My question is if I can power the oscilloscope from the mains (230Vac here in Europe) if the mains voltage is 247-250Vac, while in the manual is mentioned that the scope can be powered up to 240Vac ?
The value of the mains voltage (247Vac) was measured with an Uni-t UT52 digital multimeter.

Yes, it's safe to use it connected to your mains. All mains voltages worldwide have some variation and instrument design allows for it.

When I was calibrating the probe, I saw that the calibration signal is having a little ”movement” on the screen. For example, I saw that the vertical lines of the signal are making some movement to the left and to the right as you can see from this video: https://www.youtube.com/watch?v=IOxCg90y3JI&feature=youtu.be

The settings on the scope are: 1V/div, 250uS/div

Is that movement of the vertical lines normal ?

Normal.

[Damianos]

Additionally on what tautech says:
- the devices that are to be used on 220/230/240 Volt mains have a typical upper limit on 265VAC, that is the plus 10% of 240VAC. The lower limit, for proper operation, is around 198V.
- the horizontal "dancing" of the signal is just one pixel. Decreasing the time base, you can see that the line is not exactly vertical, so the instrument places the signal one time to one pixel and the other to the next one...

[mike_mike]

Hello,
I have a UTD2025CL DSO.
I was checking if there are oscillations on the output of a TL431 regulator and I got the attached screenshots.
It is normal to appear "multiple" wave forms on the same channel as in the image attached (MAP003 and MAP002) ?
The oscilloscope was set to AC coupling. The other settings can be read from the screenshots.

[vk6zgo]

Hello,
I have a UTD2025CL DSO.
I was checking if there are oscillations on the output of a TL431 regulator and I got the attached screenshots.
It is normal to appear "multiple" wave forms on the same channel as in the image attached (MAP003 and MAP002) ?
The oscilloscope was set to AC coupling. The other settings can be read from the screenshots.

Yes!
If you look at the display at 10ms/ div, you see effectively a 100Hz waveform, as would be expected from a linear supply operated from 50Hz Mains.
The "furry" stuff riding upon it is the high frequency stuff you see at shorter time/ div settings.

It is not unusual for a complex signal to look very different at widely different time/ div settings.

For instance, MAP002 is just MAP001 "zoomed in"

[radiolistener]

It is normal to appear "multiple" wave forms on the same channel as in the image attached (MAP003 and MAP002) ?

yes, it is normal. On first scan it has one waveform, on second scan it has another waveform, etc.

[mike_mike]

In the attached screenshot, the trace is a little bit lower than the mark on the screen for CH1. Why is this happening ? It is normal ? It has some relationship to the separation of the AC from DC signal ?
The signal source is the same TL431 power supply as mentioned above.

[mike_mike]

I have found that if I let the oscilloscope powered on for a few minutes, the offset between the trace and the CH1 mark is getting lower and lower. Is that normal ?

[Damianos]

Yes! All instruments have a "warm-up time", to become in the "normal conditions", where they are calibrated...
You can find that information in the specifications section of the manual.

[radiolistener]

Maximum input voltage is 300V no metter if you set scale for 10V/div or 10mV/div...

I suspect that if you put 300V 500 MHz, the input will be burned out like a match 

Some time ago, I asked Siglent support what is amplitude limit for high frequency for 1M input of their oscilloscope. I asked for 100 MHz frequency. They give me answer 40 Vpk. 

400 V is supported through 10x probe. 400 / 10 = 40 

I think 40 Vpk at 100 MHz is too much, I don't believe it can handle it with no damage. So, I don't want to test it   

[tggzzz]

Maximum input voltage is 300V no metter if you set scale for 10V/div or 10mV/div...

I suspect that if you put 300V 500 MHz, the input will be burned out like a match 

Some time ago, I asked Siglent support what is amplitude limit for high frequency for 1M input of their oscilloscope. I asked for 100 MHz frequency. They give me answer 40 Vpk. 

400 V is supported through 10x probe. 400 / 10 = 40 

I think 40 Vpk at 100 MHz is too much, I don't believe it can handle it with no damage. So, I don't want to test it   

Now look at the derating spec for the probe.

15pF at 100MHz is 100ohms. 400V into 100ohms means 4A is flowing