# Full-Wave Bridge Rectifier

It is used in many products and is de facto the standard rectifier: the full-wave bridge rectifier. Let's discover how it works.

## Full-Wave Bridge Rectifier

The full-wave bridge rectifier is the de facto standard rectifier circuit. It allows us to make use of both the negative and the positive half-wave of an AC signal. How is that possible? Let's find out!

## The Rectifier Circuit

The standard full-wave rectifier consist of four diodes that are used in a bridge configuration: two identical arms with two diodes each are bridged by a load in between them.

During the positive half-wave the diodes D1 and D3 conduct and power the load. D1 connects the positive the upper rail with the positive pole of the load. D3 does the same for the lower rail and the negative pole.

During the negative half-wave the two other diodes conduct. D2 brings the positive voltage from the lower rail to the positive pole of the load and D4 connects the negative pole with the upper rail.

For demonstration purposes I replaced the diodes with LEDs and lowered the frequency of the AC signal to 1 Hz. This allows us to watch the rectifier circuit in action.

The two alternative current paths allow to power the DC circuit in both half-waves. If we look at the signal we see, that the negative half-wave is turned into a positive one instead of cutting it away like a half-wave rectifier does.

Like with the half-wave rectifier, the rectified signal has roughly the same amplitude as the AC signal. Again there are losses over the diodes that can be reduced by using Schottky diodes. As we now have two diodes per path, the losses are twice as high as with a half-wave rectifier with only a single diode. However, as we can use both half-waves, we now have a higher output power for the same load resistance. The root-mean-square voltage is now $$V_{RMS} = {V_{p}\over \sqrt{2}}$$. This is not coincidentally identical to the general definition of RMS voltage for sinusoidal AC signals. Except for the small losses, we can use the whole power of the AC source with this rectifier.

## Adding a Capacitor Filter

To stabilize the output voltage we can again use a capacitor like we did for the half-wave rectifier:

The capacitor stabilizes the voltage, but causes currents spikes while it charges. Like with the half-wave rectifier you need to increase the capacitance value for bigger loads or less ripple. The big difference is that the capacitor doesn't need to provide energy for a full half-wave anymore. This reduces the ripple and makes it easier to use bigger load without the need for an unreasonably large capacitor.

## Conclusion

With a full-wave rectifier, we get power in both the positive and the negative half-wave for our DC circuit. The load on the AC source is evenly distributed and not just on one half-wave. We still need to stabilize the rectified signal to get a constant DC voltage. However, for the same load and capacitance, the full-wave rectifier produces less ripple than a half-wave rectifier. If the ripple is still too big for the DC circuit, a voltage regulator can be used to produce a fixed and more stable voltage for it. Compared with a half-wave rectifier the full-wave rectifier is the better choice in most cases. The drawbacks compared to the half-wave rectifier are a higher component count and twice the diode losses.