This is a circuit for power supply that has an output of 0v to 12v at 700mA with a transformer that is rated at 1-amp (such as M-2155) or 1.4amp for a transformer that is rated at 2-amp (such as M-2156). This is a simple design for power supply. This power supply can adjusted. This is the figure of the circuit.
Operation of the circuit is begin, first the mains voltage is reduced to a usable level by the transformer. Two different low-cost transformers can be used. The M2155 is a 1 amp type and M2156 is a 2 amp type. This is the AC rating and when you connect any transformer to a DC power supply circuit you must de-rate the current rating by 30% to give the maximum DC current that can be delivered by the power supply. The BD 679 regulator transistor must be heat sink if any more than 100-200mA is required and will certainly need a large heat sink when the full rated current flows. The heat generated in the transistor is due to two factors. One is the current flow. Obviously, as more current flows, the transistor will get hotter. But the other factor is the voltage across the transistor. If you are drawing 100mA at 12v, the transistor will rise to a certain temperature. If you reduce the output to say 6v, while still drawing 100mA, the transistor will get hotter because the voltage across it will be greater. In the first case the voltage across the transistor will be the voltage from the bridge rectifier minus the output voltage. Our figures were 22v - 12v = 10v across the transistor.
Operation of the circuit is begin, first the mains voltage is reduced to a usable level by the transformer. Two different low-cost transformers can be used. The M2155 is a 1 amp type and M2156 is a 2 amp type. This is the AC rating and when you connect any transformer to a DC power supply circuit you must de-rate the current rating by 30% to give the maximum DC current that can be delivered by the power supply. The BD 679 regulator transistor must be heat sink if any more than 100-200mA is required and will certainly need a large heat sink when the full rated current flows. The heat generated in the transistor is due to two factors. One is the current flow. Obviously, as more current flows, the transistor will get hotter. But the other factor is the voltage across the transistor. If you are drawing 100mA at 12v, the transistor will rise to a certain temperature. If you reduce the output to say 6v, while still drawing 100mA, the transistor will get hotter because the voltage across it will be greater. In the first case the voltage across the transistor will be the voltage from the bridge rectifier minus the output voltage. Our figures were 22v - 12v = 10v across the transistor.
