* 3. 2131DC Power Supplies 5. VoltageDivider. Its function is to provide different dc-voltages needed by different electronic circuits. It consists of a number of resistors connected in series across the output terminals of the voltage regulator. Obviously, it eliminates the necessity of providing separate dc power supplies to different electronic circuits working on different dc levels. Comments. Strictly speaking, all that is really required for conversion from ac to dc is a trans- former and a rectifier (in fact, even the transformer could be eliminated if no voltage transformation is required). The filter, voltage regulator and voltage divider are mere refinements of a dc power supply though they are essential for most applications except for battery charging and running small dc motors etc. 55.4. Rectifiers We will consider the following circuits : 1. single-phase half-wave rectifier, 2. single-phase full-wave rectifier, 3. full-wave bridge circuit, 4. three-phase half-wave rectifier, 5. three-phase full-wave rectifier, 6. six-phase half-wave rectifier, 7. three-phase bridge circuit, 8. voltage multiplier circuits. Many semiconductor devices or systems (like car stereo systems) require a negative dc source or both a negative and a positive dc source. For the sake of simplicity, we will analyse only the positive dc power supplies. However, a positive dc supply can be converted into a negative one by simply reversing the two leads in the same way as we reverse the polarity of a dry cell. Quite a number of integrated circuits (ICs) require both positive and negative source with com- mon ground. In that case, the polarised components in the negative portion of the supply will have to be reversed. For example, its rectifier, filter capacitor and voltage/current regulation devices will have to be reversed as compared to the positive supply. 55.5. Single-phase Half-Wave Rectifier The basic circuit of a half-wave rectifier with a resistive load (but no filter circuit) is shown in Fig. 55.2 (a). The alternating secondary voltage is applied to a diode connected in series with a load resistor RL. Let the equation of the alternating secondary voltage be Vs = Vsm wt. (a) Working During the positive half-cycle of the input ac voltage, the diode D is forward-biased (ON) and conducts. While conducting, the diode acts as a short-circuit so that circuit current flows and hence, positive half-cycle of the input ac voltage is dropped across RL. It constitutes the output voltage VL as shown in Fig. 55.2 (b). Waveform of the load voltage is also shown in Fig. 55.2 (b). It consists of half- wave rectified sinusoids of peak value VLM. Fig. 55.2 During the negative input half-cycle, the diode is reverse-biased (OFF) and so, does not conduct i.e. there is no current flow. Hence, there is no voltage drop across RL. In other words iL = 0 and