0-12v Variable Power Supply

BY R.GIRITHARAN Power supplies provide the necessary power, voltage and current requirements for electronic devices. They usually change ac to dc voltage. For example, 230 volts ac is changed to 12 volts dc. Power Supplies Consist of: 1. Transformer steps ac voltage up or down. 2. Rectifier Diodes change ac to dc. 3. Filter Network includes capacitors and inductors, smooths out the DC. 4. Voltage Regulator keeps the voltage constant. uVm0 t 2t4 Power supply is a circuit that provides a steady dc voltage obtained by rectifying the ac voltage. The following is the block diagram of a power supply stage. Step-down transformer High ac voltage Rectifier Low ac voltage Ripple Filter High ripple dc voltage Voltage regulator Low ripple dc voltage Load using dc voltage Pure constant dc voltage uVm0 t 2tu0 t 2t uVm0 t 2tVdcu 0 t 2tVdcTransformer The input coil is primary and output coil is secondary. The transformer operates only ac supply. The input voltage is 230V ac and output voltage is step down 16V ac. Power Supply Specifics Half Wave Full Wave Rectifier Power Supply Specifics Bridge Rectifier Rectifier Comparison Rectifier The are several ways of connecting a diode to make a rectifier to convert AC to DC. The bridge rectifier is important and its produce a full wave varying dc output. The bridge rectifier made four individual diodes, but it always available in special package containing the four diode required. The bridge rectifier uses 1.4V because each diode uses 0.7V. Bridge Rectifier Construction Important Concepts Half Wave Rectifier: Simplest. Hard to filter well. Full Wave Rectifier: Easier to filter. Requires transformer with center-tap. Transformer secondary must be twice intended voltage. Bridge Rectifier: Easier to filter (just like full wave). Center-tap transformer not required. Transformer secondary same as intended voltage. Higher parts count. Filter To smooth the output of the rectifier a reservoir Capacitor is used placed across the output of the rectifier and in parallel with the load. The capacitor having only two function charging and discharing. This capacitor charges up when the voltage from the rectifier rises above that of the capacitor and then as rectifier voltage falls, the capacitor provides the required current from stored its stored charge. The output from the transformer and rectifiers follows the sin waveform. The smoothing capacitor fills in the low voltage portions, so reducing the ripple voltage amplitude. The larger the capacitor (for a given load), the smaller the ripple voltage, but the higher the peak current through the rectifiers. Construction and waveform POSITIVE variable regulator NEGATIVE variable regulator These regulators function by maintaining a fixed level of 1.25V between the OUT and ADJ terminals and by ensuring that the current drawn by the ADJ terminal is very small. Provided the current through the potential divider from the output to the ground rail is large compared to the ADJ terminal, then the regulated output voltage is set by the resistors used for the potential divider. The recommended maximum resistor value between the ADJ terminal and the OUT terminal is 240O for the positive version (220O is OK) and 120O for the negative version. You want +12V out. R1 = 220O, so current through R1 = 1.25V/220O = 5.68mA V across R2 is 12V 1.25V = 10.75V, so R2 = 10.75V/5.68mA = 1.892KO which could be 1.8kO + 100O = 1.9kO. This will give an output of 10.792V + 1.25V = 12.042V. Regulator A regulator is used vary the output voltage. Here LM317 is used to vary the output voltage1.7V to 32V Regulator construction EE2603-14 Electronic Circuit Analysis 18 Adjustable Voltage Regulator IC The IC regulator LM317 can be used to produce any regulated output voltage between 1.2V to 37V. Typical IC values are: Vref = 1.25V and Iadj = 100mA Adjustable Voltage regulator IC 1.2V < Vo< 37V -Vi VIN VOUT ADJ LM317 R1 R2 Iadj = 100A Vref = 1.25V ( )2 adj12ref 21refref 2 adj 1 R ref 2 R 1 R oR IRR1 V RRVV R I I V V V V +||.|

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