Need 12 and 5 V power supply

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Charon102

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ml7812a circuit

Hello folks

I need a power supply for my atmel 8051 programer. The schematic looks like this:

**broken link removed**

The programer could also get the power from the computer, but I don`t like this solution cause of short cuts which can freeze the system.
I found some electronic parts at home which could be helpful to build a power supply. 12 and 5 Volt are required for the programer. I found 78A12A, 7805A and LM317T voltage regulators. What I still need is a usable mains adaptor. Only have 12.5 V with 600mA. Is this usable?
 

7812a regulator circuit

Some AC adapters are not regulated and their output voltage is higher than the rating, especially at currents lower than the rated one. Measure the output of your AC adapter. If it's much higher than 12V, you can most likely use it with a 7812 and a 7805 to build your power supply.

If it really outputs only 12.5V, then it works with a 7805, but for 12V, you would either need an LDO (if you are strict about it), or simply use it as is. 12.5V is not too high for the microcontroller.
 

lm7812 usage

for 12V use 7812. for 5V use 7805.
use same input adaptor for their input which is 12.5V 600mA.
12.5V is low input voltage for 7812, but , i think, your circuit will work this voltage. if not, you can use 12.5V directly instead of regulated it. 0.5V voltage difference is not important.
 

lm7812 schematic

Great, thanks for the quick reply

Because I didn`t find any datasheet for the ml7812A I will use LM317T adjustable voltage regulator.
There is a formula in the datasheet that leads to the correct resistor values.
It looks like this:

Vout = 1.25V (1+ R2 / R1) +Iaj (R2)
in my case this would be:
12V = 1.25V (1+X / 240) + Iaj (R2)
If I would know Iaj I could get the correct value for R2.

Any suggestions?

Greetings
 

lm317t voltage regulator, 12.5v


I don't see it is a problem to use the LM7812 only because you didn't find its datasheet .. i think you have to know few things about it .. such as : its TO-22 package has its left leg : unregulated input, the middle leg : ground , and the right leg : regulated output

also if you want to use instead the LM317 regulator .. you will find typical values for Rs you asked about in its datasheet

Added after 2 minutes:

I found the datasheet of the LM7812 series regulator ..
you can get the National datasheet on the following link :
**broken link removed**

if you faced any problems with it .. i can help you ..

Regards
 

7812a voltage regulator datasheet

Its better if you can find a power adapter with a voltage of 15 to 25 volts. Then use LM7805 for 5V regulation and LM7812 for 12v regulation. You can use variable linear regulators but the more simple, the better and less cost and troubleshooting. Both LM7805 and LM7812 only need 2 capacitors for the input and output filters.
 

lm 7812 connection pins

For your application I would go for "L" version of the voltage regulators.
The reason being, you will ensure that the current will not be > than 100mA in the supply and the programming circuits..
And, should you have difficulties in findind data sheets, here it is (on both ICs):
**broken link removed**
Regards,
IanP
 

7812 voltage regulator power supply

I noticed something in the schematic...Is it true that you dont need the 12V supply there because it is only used to reset the AT89C2051? The RST of the microcontroller is controlled by Pin14 of the DB25. So if everyone agrees, you could disregard the 12V supply, use an inverter instead.
 

7812 heat

hello folks

I found a new mains adapter in my garage. Is a AC powe supply with 12.5 V and 300mA, that gives 17.6V direct current.
Great!!!

I want to use a bridge diode, 7812A and 78L05A. I tried to make a schematic with xicircuit. It looks like this:

**broken link removed**

The bridge diode and 7812A voltage regulator gives me 12 V without massiv heat production, so I will build it into a little plastic box. But couln`t messure the correct voltage a the 5 volt pin yet. My multimeter displays
12 V between the pin and ground. Something seems to be not correct.

Any suggestions?

Greetings and thanks a lot
 

7812a variable

See the way you have connected the LM7812 pins.
The LM7805 pins connection should be the same if in same package.
 

ml7812a power supply

Check the conenctions...you shouldnt get a 12V reading in the pin 3 of the 7805. maybe you connected the output pin of the 7812 to the output of 7805. that may damage the device. Try to measure its AC voltage just to make sure it is free from unwanted ac voltages.
 

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