help with voltage regulator circuit

[majd-ghadab]

hi i have question about 7805 voltage regulator so can i put 2 of them like 7805 and 7806 in parallel to the same IN and of course with different OUT
because i want to use one transformer then rectifier the AC voltage to DC then have from the tow voltage regulators to different voltages
you can see the attachment to see the circuit
and thanks

 

[alexan_e]

Yes this works fine, it doesn't matter how many 78xx you have connected as long as the transformer can supply the total current needed for the output loads of the regulators (assuming that they work simultaneously).

Alex

 

[majd-ghadab]

thanks for fast reply because i am building big circuit which consist of 8 voltage regulators and layout it on eagle so i was very confusing cause it may not works so now i can build it with confidence

 

[alexan_e]

why so many , are you making a bench power supply?

 

[majd-ghadab]

it is my first real power supply and i want to make it can supply any circuit that i may make

 

[alexan_e]

The most common voltage are 3.3v and 5v for microcontrollers and digital circuits and 12v used in many other devices, also make sure that you include a variable regulator like LM317.
If you use 7805, 7812 and one or two LM317 you can probably cover all your needs.

LM317 VARIABLE POWER SUPPLY - circuit diagrams, schematics, electronic projects

 

[majd-ghadab]

i have used {7805,7806,7809,7810,7812,7815,7818,7824} the problem is if i want to add LM317 for example i want to change my finished eagle design and it is very difficult because there are many elements in the circuit !
at first i didn't find voltage regulator to give me 1.25 v but now i saw so can you give a name of ic the give a supply of 3.3 so it worth to change the design and thanks

 

[alexan_e]

Most of these regulators will never be used, just include a 5v, 12v and two LM317 variable outputs and you are fine.
For 3.3v I have only used LM1117.

Alex

 

[majd-ghadab]

actually i may use 9v for a radio circuit for example and 12 or 18 or 24 for a small DC motor so thanks for your advice but i don't want to put my self in trouble if one day i needed 18 volt supply and didn't make the circuit in its time so thanks

 

[alexan_e]

It is up to you but if you include two LM317 you can output any of these voltages, my suggestion was for a 5v and 12v fixed regulators which are very common values and two 1.2-28v variable outputs so that you can generate any additional voltages (like 3.3, 6, 8, 9 18, 24 etc).

Alex

 

[majd-ghadab]

i didn't under stand how LM1117 gives many different voltages you mean by a variable resistor or how can you explain or lead me to web page that includes circuit gives 3.3 and 5 volt from this ic
because a way like this will make my circuit very easy to replace {7805 and 7806 ....) in one !!

 

[alexan_e]

**broken link removed**

The LM1117 is available in an adjustable version, which can set the output voltage from 1.25V to 13.8V with only two external resistors (like LM317). In addition, it is also available in five fixed voltages, 1.8V, 2.5V, 2.85V, 3.3V, and 5V.

If you want an adjustable regulator then LM317 can output more current and higher voltages, if you don't like to use a pot to adjust the output you can use a switch to get preselected output voltages using different resistors.

 

[majd-ghadab]

OK at last i will add LM317 with adjusted value to 3.3 because i don't want to adjust the output every time i want to use the circuit finally thanks for your patient

 

[alexan_e]

You can use a calculator like to calculate the resistors.
LM317 / LM338 / LM350 Voltage and Current Regulator Calculators

For 3.3v you can use 270 ohm and [470 in parallel with 6k8] which give 440 ohm
or a 270 ohm and a 442 ohm (only available in 1% tolerance)

another alternative is 220 ohm and [470 in parallel with 1K5] which give 358 ohm

 

[Raza]

Hi majd,
Absolutely no problem for this circuit as said by alex the Trafo must be capable of supplying the required current if all work same time and the output current is kept into limit of regulators, i,e; 1 amp for 78xx series.