# What is the reason the output signal I obtained from full-wave bridge rectifier has changing amplitude?

I observed the output signal of full-wave bridge rectifier sending sine wave and square wave for an experiment. In both of the output signals it is clear that each cycle shows one of the two different maximum voltage values and I have no idea why this happens. The bridge rectifier was built with 4 x 1N 4001 diodes which supposed to be identical and a 100 ohm resistor. The output signal across the resistor was observed. The schematic of the circuit is this.

simulate this circuit – Schematic created using CircuitLab

To be more clear I will add the input and output signal images. These are the input signals displayed in oscilloscope.

And these are the output signals displayed in the oscilloscope

The experiment was conducted in a freshman physics lab.

• Besides the possibility that your diodes are not identical, it is also possible that your function generator (signal generator) is outputting some DC in addition to the AC signal. What happens if you pass your signal through a decent sized non-electrolytic capacitor before you rectify it? If the problem disappears, your signal has a DC component. Commented Dec 13, 2020 at 13:27
• Were the signal generator and scope floating or grounded to same reference potential like mains earth? What if you bypassed a diode by grounding both sides of it? Did you use differential measurement or single-ended measurement? The output sine wave also indicates 200mV offset so can it simply be a DC offset from generator if there is no other obvious issues? Commented Dec 13, 2020 at 15:15

Take a careful look at first two oscilloscope photos. Your 'scope shows measurements of function generator input signal... from the first sinewave photo:

Vp-p=10.7V
Vmean=202mV

That 202mV is an offset voltage that should not exist - it may very well be the cause of the rectifier's uneven output peak amplitude. The function generator's mean voltage should be zero. Do be careful to ensure that the oscilloscope is DC-coupled rather than AC-coupled.

Your second photo of square wave from the function generator shows a similar DC offset:
Vp-p = 9.2V
Vmean = 175mV

Your function generator should have a front-panel knob that adjusts DC offset from a negative-to-positive voltage. It should be set so that your oscilloscope shows Vmean=0V

1N 4001 diodes which supposed to be identical.

Found the false claim!

Either your diodes aren't identical, or your load isn't time-invariant, or your source has lower impedance on one polarity than on the other. All three scenarios happen in the real world.

Note that anything you attach to the circuit to measure voltages might be influencing it.

I'd recommend loading the output of your rectifier with a resistor and measuring the voltage across that, so that your scope isn't the dominant load on your rectifier. Measure the source with the same load to preclude the polarity-dependent source impedance.

• The output signal was measured across a 100 ohm resistor. Sorry nor to mention this. First time asking a question. They are all the same industrial type of diodes, is the difference normal? I conducted the experiment a few more times and obtained the same result every time. Commented Dec 13, 2020 at 13:28
• Don't worry, just edit your question (don't just post a comment) to say that there's a 100Ω load attached to your bridge rectifier. Preferably, you just use the schematic editor that's built into this site's question editor to just post the schematic of what you've built. Commented Dec 13, 2020 at 13:30
• "industrial" doesn't actually mean anything: unless you're building diodes in a lab, there's no non-industrially produced diodes... I can't tell you what the tolerances of these diodes are by heart, but I did say that happens in the real world. Commented Dec 13, 2020 at 13:32

Be very careful when connecting an oscilloscope to circuits using bridge rectifiers.

Almost all scopes have the ground of the probe connected to the AC ground of the power outlet. If your signal generator is also grounded to the AC power then one of the lower diodes in the bridge (D2) will be shorted.

When using a signal generator it will merely short the output which should not cause any permanent damage. However, if you were using the AC power as the input it could severely damage the oscilloscope or give you a fatal electric shock.