Full Bridge Rectifier - Output Voltage Saturation

Question

I am putting together a full bridge rectifier, see schematics. The load is connected to V+ and V-. Capacitors as low ESR and with respect to the standard formula.

^{simulate this circuit – Schematic created using CircuitLab}

The simulation went as expected. However, when I physically realized this circuit on PCB, something strange happened. Connecting 5 V AC sinusoidal function generator provided about 4 V at output. But when I increased the amplitude (peak-to-peak voltage) of input signal, the voltage at ouput did not grow "linearly" and instead started to saturate. For example, for 10 V at input, the output voltage was only 6.5 V. I returned back to the simulation and tried it again, but nothing strange occured.

I measured the ouput voltage with oscilloscope with input impedance set to 1 MOhm. Am I missing something? I am adding the layout of my bridge rectifier.

EDIT: Board schematic.

Answer 1

Yes like I suspected, the diodes are in wrong polarity.

The electrolytic capacitors do not handle negative polarity well and will leak current, sometimes very heavily, in reverse direction.

The 50 ohm output impedance of signal generator limits current so because caps leak so much current the generator cannot charge the caps to full voltage and even if it could the caps would leak current and discharge while the diodes do not conduct.

And it may still be that your scope and signal generator short out V- and GND.

Answer 2

Can you please elaborate on the "short out one diode"

If the scope and signal generator share a ground, this happens even if you don't measure input and output at the same time:

All capacitors are connected, as you see them in the PCB drawing (+ wire connected in the top hole, - wire connected in the bottom hole). If this results in reversed polarity

Yes, the polarities in the PCB are inverted. When you remove the capacitors to correct this, check if they were not damaged due to the polarity inversion (or, even better, replace them).

These are the cathode and anode terminals:

Source

V+ and V- are inverted (in the full schematics and the PCB, compare with the symbols in the first one).

Is there some way how I can uncouple the grounds of the generator and the oscilloscope

If both are connected to the earth pin at the power plugs, no direct way to change this (don't cut them). A few options:

If you need simultaneous measurement (just the scope ground clips will short both sides):

• 4 channel scope with a single ground at the input and differential measurement at the rectified side;
• isolated probe(s)

Without simultaneous measurement:

• get a scope or signal generator which can be powered by an isolated power supply
• measure the rectified side in differential mode, with ground clips at the input
• use an isolation transformer to float one of the devices
• forget about the signal generator and just use a transformer as the input of the rectifier

Answer 3

Use a battery powered oscilloscope as the one of the easiest ways to "float" the oscilloscope. Or use a transformer as the source for the rectifier.

You are fortunate that you used a signal generator to initially power the circuit, in the real application, the current flow through reverse biased capacitors fed from a transformer might cause enough heating to cause the capacitors to rupture and vent an unpleasant smelling steam. Larger capacitors have designed rupture points, the cases of smaller capacitors can leave the circuit board as though fired from an air rifle.