[SOLVED] regulated -12v .5A power supply using 7912

[flash23]

I have designed a circuit to deliver -12 V , 0.5A using 7912 . However Im doubtful about the current limiting section. Also Im not able to get near exact -12V im getting -12.223 V o/p.
I also need to use a fuse prior to the transformer but I dont know how to calculate its parameters .
I think that overvoltage ( crowbar circuit) will not be required as the 7912 provides it , however Im not sure.
I would really appreciate any improvements and solution .

 

[eem2am]

hello,

i see your transistors are the wrong way round., amongst other things

a good way to regulate current linearly, is using an opamp.................

....use the fact that with negative feedback, the (+) and (-) voltages will be virtually the same.

......the above words are worth remembering, its a secret of easily evaluating opamp behaviour

...............then fix the (+) voltage at some value.
.....then attach your current sense resistor to the (-) input, knowing what voltage it will have across it (the same as you made on (+) )......................put the opamp into a BJT and it will magically bias that BJT to give you your required current.......which in fact you end up fixing with the current sense resistor value and opamp reference voltage on the (+) pin.