This is a simple design circuit for 2 A power supply. This circuit 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. This circuit is control using transistor BD679. This is the figure of the circuit;
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 reason for this the AC voltage is increased by 40% when it is rectified and to maintain the VA (volt-amp) rating for the transformer, the current rating must be decreased.
This means a 1 amp transformer will produce a 700mA power supply and a 2 amp transformer will produce a 1.4 amp power supply. For a low-cost power supply this output is quite sufficient and is all we can get as quite a lot of heat will be developed in the diodes, transistor and transformer when 700mA is flowing and literally burn your finger when 1.4 amps flows. The BD 679 regulator transistor must be heat sinked 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.
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 reason for this the AC voltage is increased by 40% when it is rectified and to maintain the VA (volt-amp) rating for the transformer, the current rating must be decreased.
This means a 1 amp transformer will produce a 700mA power supply and a 2 amp transformer will produce a 1.4 amp power supply. For a low-cost power supply this output is quite sufficient and is all we can get as quite a lot of heat will be developed in the diodes, transistor and transformer when 700mA is flowing and literally burn your finger when 1.4 amps flows. The BD 679 regulator transistor must be heat sinked 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.
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