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I am trying to read the output wave of a full bridge rectifier circuit on my oscilloscope, but the waveform looks really funky.

I've set it up as shown in the spice simulation where resistor R1 represents where I have connected my probes:

enter image description here

When I breadboard it, I get this waveform:

enter image description here

I'm pretty sure its just a matter of me being an idiot and not setting up the scope correctly or something. I've tried using different diodes, probe resolution, and coupling settings but they all produce the same shape.

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  • \$\begingroup\$ this looks ok. what's wrong about it? \$\endgroup\$ Jun 25, 2020 at 17:17
  • \$\begingroup\$ What's the expected voltage at the transformer secondary? What happens to your signal if you add a small load (a resistor across the probe connections, sized for say 1mA RMS)? \$\endgroup\$
    – marcelm
    Jun 25, 2020 at 17:18
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    \$\begingroup\$ The transformer in your circuit can never work at 60 Hz.and 120 volt RMS. \$\endgroup\$
    – Andy aka
    Jun 25, 2020 at 17:44
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    \$\begingroup\$ Yes what Andy means is your transformer impedance is as low as the diode resistance and needs to be 1000x bigger inductance. That's why it is nonlinear at 60Hz. and probably burning hot too unless wrong values. \$\endgroup\$ Jun 25, 2020 at 18:25
  • \$\begingroup\$ I forgot to load the output...big senior moment for me.... \$\endgroup\$
    – jm567
    Jun 25, 2020 at 19:24

3 Answers 3

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Assuming you are using a real transformer with your real rectifier bridge and your real oscilloscope, then yes, that looks wrong.

Here's the output of a full wave bridge rectifier driven by a small transformer I had here in the junk box:

enter image description here

That's with a rectifier made of some 1N4001 diodes, and a "load" made of a 4k resistor.

The pulses are at 100Hz - I live in 50Hz land, so the rectified pulses are at 100Hz.

Here's something similar to what you got:

enter image description here

That happened when I was using a signal generator that shared a ground with my oscilloscope. The shared ground is shorting one of the diode pairs, leaving me with a sort of mangled half wave rectifier.

Given how yours looks, you've got a "not quite shared" ground between your transformer and your oscilloscope ground.

I don't know where or how it's there, but you can verify it by disconnecting the transformer from one side of your rectifier. If the signal stays just as it is when you disconnect one wire from the transformer, then that's the shared ground. If the whole signal goes away, then that's not the shared ground - try the other one.

For comparison, here's the output of a half wave rectifier:

enter image description here


If you're not using a normal transformer, then the above doesn't apply and I don't know what's wrong.

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I have faced the same problem. The problem has resolved by measuring the voltage with a suitable resistance instead of measuring without resistance as stated in the question.

(# Question: I am trying to read the output wave of a full bridge rectifier circuit on my oscilloscope, but the waveform looks really funky.

I've set it up as shown in the spice simulation where resistor R1 represents where I have connected my probes:"Scope Figure")

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I am not qualified to answer, so only a comment:
the shape "feels" like extra inductance being in series some where in the circuit. The transformer may be very non ideal.
Measuring and plotting the primary side of current can give more informative clues.

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