Rectifiers (Half & bridge)

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Rectifiers(half & full wave)


Moving on to applications of diodes, an interesting application is Rectifiers. They are devices which convert an AC signal to a DC one. They are primarily of two types- half wave rectifier and full wave rectifier. The names would make sense once we have a thorough look at them.

  1.    The Half Wave Rectifier



    • This rectifier uses a diode and a capacitor for doing its job.
      Half Wave rectifier with load R_L
    • We have to analyse its operation in two parts - one for the positive cycle of AC signal, other one for the negative cycle of signal.
    • During the positive cycle of signal, diode D1 is forward biased, hence it allows the current to flow with ease. Also notice the signal source is parallel to C1 and R_l, so potential drop across them also follow the input voltage.
    • Capacitor also gets charged upto (Vin-Vd).
    • Now as the signal volt starts to drop after its peak, diode D1 goes to reverse bias state( since its +ive side is at a lower volt wrt its -ive side).
    • This disconnects the source from the circuit, leaving behind a discharging RC circuit.
    • Now although the load is cutoff from the source, voltage across it doesn't drop to zero, thanks to capacitor C1 which now provides the load with current which was stored during first half of positive cycle.
    • For the remaining parts of the AC signal cycle, C1 continues to provide load with charge, this thing continuing till the time when diode is once again forward biased.
    • To know when exactly is D1 once again forward biased, we take a closer look at it. N-side of D1 is at a potential Vc (say volt across C1 at any time is Vc), P-side is at a volt Vin(the input signal volt). So, as soon as the signal reaches a value (Vc+Vd), D1 enters forward bias state.

    Vin & Vout against time.Notice the drop between two successive peaks

    • Now similar process repeats itself for rest of the cycles, yielding a near constant (not exactly constant) voltage across the load.
    • Although this model has rectified AC and generated a fairly constant DC signal, the fluctuations between peaks are too big to ignore, even with high values of C1. The difference between peak of the obtained waveform and its lowermost point is called  the Ripple Voltage (as indicated by Vr in above figure).
    • The lower the value of ripple volt be, the better it is.


      2.    The Full Wave Rectifier

    • This rectifier does not just simply ignore the negative parts of an AC cycle. Rather it converts the negative to positive parts using the tricky circuit it incorporates.

    Bridge rectifier
    • The complex looking square mesh of 4 diodes is actually very simple to understand if we approach +ive and -ive cycles seperately. Also lets first ignore C1 for further simplifying our task.
    • During +ive half cycle, diode D3 is forward biased and D2 is reverse biased, indicating current will flow through D3. Now the current has option to flow either from D1 pr through Load. But it can be easily seen that D1 is reverse biased for +ive cycle, thus leaving Load to be the only path for current to flow.
    • And after traversing through Load, only path left is through D4 (since D2 is reverse biased as stated earlier). Thus during the positive part, current flows through Load via D3 and D4. The Volt across Load being (Vin-2Vd).
    • Now for the -ive part of cycle, D1 is forward biased and D4 is reverse biased. Thus like the positive part of cycle, current flows through D1, gets blocked by D3 (its reverse biased during -ive part), traverses through Load and Finally crosses through D2 (D4 is reverse biased, explained earlier).
    •  Summarising once again, current simply flows through Load via D1 and D2.
    • If we plot this, we would end up with something like this ( y being the output and x being the input ) :
                   
    Output of a bridge rectifier without capacitor.

    • Now to complete our analysis with C1 included, we recall what the effect of C1 was in half wave rectifier. It simply provided Load with current between two peaks. So it will do the same job here, difference being that the peaks are very close to each other in this case, thus the drop (ripple voltage) would always be low wrt a half wave rectifier. 
                  
    Vout and Vin against time for full wave rectifier.

    • Thus a full wave rectifier is obtained.

    Next post will be on other applications of Diodes!

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