A 350VA transformer power, the primary winding 230 , secondary winding is 18V . The secondary voltage of 18V with the load of the transformer , the voltage at idle , it is customary in the case of such efforts is higher than 5 % to 10 % . A transformer is designed so that it is at 100% load may be strained 24ur/dan , 120 % can be loaded up to 30 minutes , 130 % to 10 minutes , the times they can be several times longer , if the transformer is at the beginning of the load greater than 100 % cold and after the load has a chance to cool again . In short, the transformer can be easily allows short periods of time as 26A , which is the maximum current rectifier . Maximum flow is determined by the emitter resistors as a drop on them opens the transistor BD708 , then patching the base – emitter junction of the MJ2955 that the rectifier is not leaking more current than allowed . Due to the short circuits do not harm the rectifier . The rectifier is designed so that it could withstand even through 30A , but here are the restrictions of bridge dioda 35A . But it is in such good things have enough power in reserve material that would not happen smoke signals .
The primary part of the transformer is connected to the network via a fuse 2.5 A , and switches . Since the metal casing , shock-proof , wear safety plug and 3 wire cable , housing must be connected to the grounding conductor .
Secondary transformer is connected directly to the Graetz ( bridge rectifier ) KBPC35xx KBPC3504 for example , Graetz ( GR1 ) should be mandatory 35A, 50A , and can also be cooled properly . The entire rectifier circuit with large wiper capacitors ( C1 – C10 ) value is on the PCB . Serial transistors (5x MJ2955 or Q1 – Q5 ) are mounted on the aluminum L-profile PCB and at the same time . Total power is 575W transistors , which means it easily withstand a short circuit, although in the case of a short circuit at the output of the rectifier dissipation in transistor max . In normal operation, the dissipation of much smaller , since the transistors in the voltage is somewhere around 6V , which is 3 times lower dissipation as in the case of a short circuit . The cooling fin is HR200 , aluminum L profile is 5mm thickness dimensions 50×30 mm length 200 mm , aluminum bracket special design has a thickness of 3mm for 78S12 ( IC1 ) and transistor ( Q6 ) BD708 , the latter must be compulsorily cooled heat associated with alumunij L profile . Instead of transistor BD708 can also be used another PNP transistor with similar characteristics , such as BD712 , BD912
PCB Layout design for 12V / 20A Regulated DC Power Supply:
And here for the component placement:
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