I built a simple bridge rectifier.


simulate this circuit – Schematic created using CircuitLab

I connected the output of the function generator with CH1 of my oscilloscope and with the input of the rectifier (using a T-Piece). The output goes into CH2.

The original wave looks like this: original wave

After connecting CH2 the picture looks like this: enter image description here

While CH2 looks good, CH1 now shows the wave I would expect without capacitator but not the expected original wave.

Could somebody explain why? I'm using a Gwinstek GDS-1072A-U Oscilloscope and a Gwinstek SFG-1003 function generator.

  • \$\begingroup\$ add where you connected the oscilloscope to the circuit (both the probe and the ground) \$\endgroup\$ Commented Jun 26, 2018 at 16:41
  • \$\begingroup\$ what about changing your diodes for rectifiers 1N400X, some diodes can't work as rectifiers. \$\endgroup\$ Commented Jun 26, 2018 at 16:52
  • \$\begingroup\$ 1st: 1n5404 is a rectifier diode. So i would expect that it should be able to work as a rectifier. 2nd: My question is not about the rectifying. As you can see in the second picture the rectifying works pretty good. The question is about why CH1 doesn't show the original wave. \$\endgroup\$
    – TimSch
    Commented Jun 26, 2018 at 16:54
  • \$\begingroup\$ When you are measuring the voltage across capacitor the ground probe (CH2) is connected to negative terminal of a capacitor right? So where is connected the CH1 probe during this measurements ? \$\endgroup\$
    – G36
    Commented Jun 26, 2018 at 17:05
  • \$\begingroup\$ :D ah, you are using the same equipment to test both signals, when you do that you are bypassing the sine+- to the ground of probe 2 and to negative of C1, this is called a shorcircuit. remember -CH1 is = -CH2. Disconnect -CH1 to see your normal output. \$\endgroup\$ Commented Jun 26, 2018 at 17:12

1 Answer 1


Your oscilloscope ground connections are shorting out one of the bridge rectifier diodes (D4 by the looks of it).

enter image description here

Oscilloscope inputs share a common ground connection unless you are using special differential probes.

  • \$\begingroup\$ Thanks for your answer. I have one question. While oscilloscopes got a bit neglected in class I once learned that I had to connect + at one end of the point to measure and - to the other. Why should I see the correct signal when I connect + to the + generator output and using the ground of -C1? I would think that I will get the voltage between +V1 and -C1 and not the expected +V1 and -V1. Or in other words: Am I only allowed to measure one point at the same time in the same system? \$\endgroup\$
    – TimSch
    Commented Jun 27, 2018 at 9:33
  • \$\begingroup\$ You are only allowed to have one common ground connection but you can measure a voltage with both probes simultaneously with respect to that common ground. \$\endgroup\$
    – Andy aka
    Commented Jun 27, 2018 at 9:38

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