Oops! i wanna learn Hardware Design

[vijay s]

hardware design in coimbatore

Today i realised that I'm very bad in hardware design.since i focused on Embedded Software i never learned even basic hardware design.now i supposed to design a hardware for one of my project and I'm facing lot of problem in my design.
please you guys help me to learn hardware even i ask some funny questions.

first i want to know how to design a regulated power supply.i used linear regulators like 7812 and 7805 to supply 12v and 5v.the problem here is it generates unbearable heat.i used 12-0-12 transformer to supply the regulator. i wanna know how to design a power supply without heat and also what should be the value of filter capacitor or how to calculate the capacitor value.actually it generates heat even without load when load is connected the heat goes untouchable

hoping the reply from experts

 

[Audioguru]

why to leaarn oops

Your transformer is 24VAC. Its peak voltage is 33.9V and the rectifier bridge drops it to about 31.9V which is way too high for a 12V regulator.

If the regulator has a current of 0.5A then it will heat with (31.9V - 12V) x 0.5A= 9.95W. It would need a pretty big heatsink. Without a load it should not be hot, just barely warm.

Try half the transformer, 12VAC. Then the peak voltage is 17.0V and the rectifier bridge reduces it to 15V which is close to the minimum allowed input voltage for a 7812 so the filter capacitor must have a pretty big value to reduce the amount of ripple.
If the current is 0.5A then the 7812 heats with (15V - 12V) x 0.5A= 1.5W. It would need a small heatsink. It will not be warm without a load.

The 7805 also heats with the voltage across it times the current through it.

The capacitors at the outputs of the regulators should be at least 0.1uF as shown on the datasheets.

 

[vijay s]

7812 without heatsink current

thanks for replying audio guru.. and also i want to know how to calculate the value of filter capacitor we use next to bridge diode

 

[Audioguru]

how to learn hardware design

If your mains frequency is 50Hz then with a full-wave rectifier the p-p ripple will be 1V if the current is 1A and the capacitor is 10,000uF.
I use a graph for my 60Hz mains frequency:

 

[vijay s]

i have some more doubts

how to calculate systems total current consumption.assume that my system has a microcontroller connected to LCD and some 8 led.i will be happy if somebody explains with circuit

 

[Audioguru]

The total current consumption is simply the total (add them up) of the current used by the microcontroller which is small, plus the current used by the LEDs (which is determined by their brightness and if they are connected in series) plus the current used by the LCD (its backlight uses a lot of current).

 

[vijay s]

another one
the gain of CE configuration transistor Depends on