Graetz circuit is a two half-period rectifier. It is used for rectifying alternating voltage when consumer with constant voltage has to be powered. It consists of four diodes connected in series by two. If you mentally divide the diodes in two pairs D1 – D3 and D2 and D4, the alternating voltage is supplied between the anode of D4 and the cathode of D1 of one pair and between the cathode of D2 and the anode of D3. The constant voltage is obtained between the anodes of D1 and D3, and the cathodes of D2 and D4, where the load has to be put. In each half period of the alternating voltage the two pairs of diodes transmit a wave.
Through the first half period the upper end of the voltage source is positive. Diodes D1 and D2 operate and transmit the positive half-wave, and D3 and D4 are blocked and do not take part in the first half period because their resistance from cathode to anode is infinitely large.
First the current passes through the arm of diode D1, goes into the load resistance and goes back, passing through the arm of diode D2. The green arrows in the figure below show the movement of the current from the source to the load, and the red arrows indicate the movement of the current from the load back to the source, in this way closing the circuit.
In the second half of the cycle, the lower end of the voltage source has a positive polarity compared to the upper end. The current runs through the arm of diode D3 (the green arrows) and then flows through the load again in the same direction and returns to the source through the arm of diode D4 (the red arrows). As you can see on the schemes, the load current always flows from „+“ to „-„.
How to connect a Graetz circuit and smoothe the pulsations
At the output of the bridge (rectifier), the voltage isn’t rectified, but still has a wave character with a frequency two times greater than the input one, therefore a capacitor is used almost every time. It acts as a capacitive filter, i.e. smoothing the waveform. When choosing a capacitor two main parameters need to be observed: the operating voltage, which must be higher than the output value of the rectifier voltage without load and its capacity.
If the capacitor has too little capacity, its effect on the wave nature of the output voltage will be negligible, but if the capacitor is big enough, and the load current is not too large, the smoothing of the wave will be almost complete.
Choosing a capacitor.
When choosing a capacitor we can use the following simple equation:
I is the rectified load current in amperes;
f is the frequency after the rectification or twice the input frequency in hertz;
С is the capacity in farads.
Choosing a transformer
The Graetz circuit is almost always used with a transformer, which serves to reduce the mains voltage to the required level in order to connect the consumer.
To determine the output voltage of the transformer compared to the voltage that comes after the rectifier, we use the following formula:
We can find the average value of the load voltage by multiplying the load current by its resistance (Ohm’s law) or by dividing 2 into the maximum load current, its resistance, and the number pi.
Advantages of the Graetz circuit compared to the one half-period monophase rectifier
The effectiveness of the Graetz circuit is twice as large. The reason for this is that the one half-period rectifier rectifies only half of the input signal, while the the Graetz circuit transmits to the load both the positive and the negative wave of the voltage sine-wave;
The residual AC waves prior to filtering are much lower than the bridge with two diodes and a simple filter is sufficient to fully smooth the voltage. The one half-period rectifier has significant voltage pulsations and can lead to damages of the DC device that will be connected after the bridge.
The higher efficiency results in higher output voltage, higher power as well as higher factor of utilization of the transformer.
The filter in more modern circuits comprises one instead of two capacitors and inductivity, placed in a common housing of a filter, smoothing the pulsations to a minimum. The requested value of the pulsations is below 100mV.
Graetz circuit or the bridge two half-period rectifier of monophase voltage is the most common circuit for voltage rectification, it is used everywhere, so this type of circuit is produced in a single block and it can be found in this way in electronics stores.
The basic parameter is the breakdown voltage of the block, which can be even 1000V, as well as the direct rated current, which depends on the size of the load, the peak load in amperes is also shown. Graetz circuits emit high temperature under load, so they must have cooling radiators. To facilitate the installation of radiators, Graetz circuits often have a hole in the middle.
The diodes in the encapsulated block have the same characteristics, they are in constant thermal regime and work as one.When an electronic transformer is added to this block, the resulting device is called an adapter. For easier use the adapter has a cast body to which there is an attached cable with a two-pole plug on the side that will be plugged into the mains voltage and on the DC side there is a cable with different connectors at the end.