I have an LD1117V33 connected to a 12V powersupply. When I plug the output to my osciloscope I see about 280mv peak-to-peak voltage, sometime more.

Is it normal? Is there anything I can do to improve it?

My circuit is pretty simple. 12V - LDO - Output with 2 capacitors in parallel in input and output. 100nF input and 10uF output.

enter image description here

  • 1
    \$\begingroup\$ Is the glitch periodical ? Are you sure that your 12v is stable ? \$\endgroup\$ – Mathieu G. Feb 27 '20 at 8:09
  • \$\begingroup\$ what is the load current? type of load? \$\endgroup\$ – User323693 Feb 27 '20 at 8:10
  • \$\begingroup\$ No load, straight to the oscilloscope. \$\endgroup\$ – LD1117 Feb 27 '20 at 8:15
  • \$\begingroup\$ Is the 12V power supply linear or switch mode? Is the 12V power supply earthed? Is the oscilloscope earthed? \$\endgroup\$ – Justme Feb 27 '20 at 8:15
  • \$\begingroup\$ @MathieuG. good point. didn't check.. Let check \$\endgroup\$ – LD1117 Feb 27 '20 at 8:15

As the spikes are present on both 12V input and 3.3V output, so they come from the 12V switching mode power supply that is ungrounded.

The switching spikes couple capacitively from the mains side to the isolated 12V side, and the charging currents loop via oscilloscope probe ground wire back to mains earth. The voltage bounce comes from inductance of the scope probe ground lead which is not able to handle fast currents.

So part of the issue is how the probes are connected.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.