Power Supply Circuit

[ArdyNT]

HI, I've designed my power supply circuit as shown below:



First, if I use 1N5392GP diode as the bridge rectifier, then can I use 1A transformer? or I need more than 1A?

Second, are the values of those components (capacitor and resistor) correct?

Third, when I use it to supply my AVR, LCD, and optocoupler, the 7805 regulator is getting hot (i dont know why). So I modify it to become like this:



I add TIP32C to 7805 regulating circuit. Well, I know by adding this transistor will provide a higher current to the load, then do I need to change my transformer and diodes (related to the current)?


Thank you..

 

[tpetar]

You should have at least 3V higher before 12V 7812 regulator, I mean when load is present. After bridge rectification you will get 12V x 1,414, but on load this will fall down for few volts.

You dont need D4 diode.

Use heatsink for both regulators.

1A is fine for most regular projects with 7-seg LED displays, LCD displays, uC,....

 

[ArdyNT]

You should have at least 3V higher before 12V 7812 regulator, I mean when load is present. After bridge rectification you will get 12V x 1,414, but on load this will fall down for few volts.

You dont need D4 diode.

Use heatsink for both regulators.

1A is fine for most regular projects with 7-seg LED displays, LCD displays, uC,....

Ah, I connected the bridge to 12 VAC from transformer. So 12V x 1.414=16.96V and this is enough??

Next, 7805 is fed directly with 12V from 7812, is this too high or still acceptable??

 

[tpetar]

16V will drop down under load for 2-3V. You can try and see results.

7812 to 7805 is ok, for your needs, but if you want maximum from 7805, then input voltage should be no more then 3V.

 

[ArdyNT]

Ok. I've tested it. The 7812 regulator output is stable at 12.1 V and 7805 output is 5.02V, but still the 7805 is hot, any suggestion?

 

[tpetar]

Use heatsink.

 

[nitishn5]

You have 7V across your 7805 which is dissipated as head.
If you put an intermediate voltage regulator between the 7812 and 7805, say a 7809, the power dissipation will be split between two ICs and the heating would not be so much.

Are you using the backlight for the LCD? If so this is would be consuming some unnecessary current from the 7805 output which is heating it up.

 

[tpetar]

Its normal for 7805 to heat up, dont worry. Just use heatsink.

You can read datasheet for data.

7805 with input of much greater voltage can handle with needed current for your needs, efficiency is other thing.

 

[ArdyNT]

Are you using the backlight for the LCD? If so this is would be consuming some unnecessary current from the 7805 output which is heating it up.

Yes I use LCD with backlight. So, it is normal?

 

[nitishn5]

You can power the Backlight from the unregulated supply using a 5.1V/4.7V Zener Diode. Its voltage is not that critical.

 

[tpetar]

No need for zener, I think is not good solution, specially when backlight is off.

I use 78M05 DPAK, its for max 0,5A, and works fine. LCD backlight (2x16) is around 100mA.

Surf through Internet and see what and how people use parts in their projects. :wink:

 

[ArdyNT]

Ok, thanks for the advices