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3.3V psu from 7805 power supply

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Rohith_elec

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is it good to make a 3.3V psu from 7805 using diodes to powerup a 3.3V uC ?

it is difficult to get 1117 voltage regulator here,
 

There are many types of regulators with 3.3V output, maybe you can find another type that's available in your area? There's also a 3.3V output version in 78xx series (LM7833).

Another option is to use an LM317 with output voltage set to 3.3V.
 

RetroTechie said:
There are many types of regulators with 3.3V output, maybe you can find another type that's available in your area? There's also a 3.3V output version in 78xx series (LM7833).

Another option is to use an LM317 with output voltage set to 3.3V.
Click to expand...


Yes but with big differences switchers or linear and what is minimal input voltage. What if input voltage is high and we need 3,3V what with dissipation power if linear ? ;-)

Also LM2576 have 50 µA (typical) standby current compared to linear 5mA standby current. ;-)

Of course this is not only one solution. From original question he need 3,3V from 7805 (5V), with basic regulator we need at least 3V upper voltage then 3,3V that means 6,3V! He should use some LDO regulator which will accept 1,7V input voltage and regulate to 3,3V. Also he can use diodes with 0,7V drop voltage in serie of three (with 2,1V voltage drop 2,9V which is low voltage) or two diode in serie (1,4V voltage drop and get 3,6V).
 
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LM1117 are used very ofter in mcu development boards to generate the 3v3 , 2v5, 1v8 voltage from the 5v input.
The current that the mcu used is usually quite low so it doesn't matter that these are linear regulators and you don't get much heat on them.

Alex
 

the best way is to use LM317............. it is the brother of LM7805 but it is adjustable. the main benefit is it is cheap and easily available.

so the best option in my point of view is LM317...... 3 pin adjustable regulator
 

Is there enough input voltage ?

Do we know what happen if input voltage level under needed minimum input voltage, what voltage will be on output ?

LM317 need at least input voltage higher for 2,25V-2,5V (read 3V) then output voltage.

Of course if input voltage is not 5V, and is higher 6,5-7-8-9V then its ok.
 
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muhammadali_16 said:
the best way is to use LM317............. it is the brother of LM7805 but it is adjustable. the main benefit is it is cheap and easily available.

so the best option in my point of view is LM317...... 3 pin adjustable regulator
Click to expand...

The discussion of this thread is "3.3V psu from 7805 power supply", there is no way to get 3v3 from 5v input with LM317, you need a low dropout regulator and that is why LM1117 can be used, it just needs a 1.2v in-out difference.
 

alexan_e said:
The discussion of this thread is "3.3V psu from 7805 power supply", there is no way to get 3v3 from 5v input with LM317 (..)
Click to expand...
True.

However "3.3V psu from 7805 power supply" could be read as "3.3V supply using a 7805's output" (=low dropout regulator needed) but also "3.3V from a power supply that includes a 7805" (=an LM317 could tap into same input voltage as the 7805). Of course original poster is in the best position to determine what's the case...
 

You may be right , maybe he just wants to get 3v3 and he has trouble finding LM1117 so he is looking for alternatives.
If the input voltage permits it he can use a LM317 regulator but he must be careful that the output is 3v3 before connecting the power to the mcu becasue a mistake in the LM317 resistors calculation or connection could create a high output voltage and damage the mcu, in this area a fixed regulator is more failsafe.
 

yes,
I really want to get the 3.3V after the 7805 regulation, since it is more failsafe,

I have LM317 power supplies, but i fear to use it as the resistance changes with temperature, the voltage may not be the same always.
 

if u care about the temperature than go for switching. if u r using Linear stuff, the temperature would be alwayz problem.
 

can u point me simplest switching PSU schematic ?
 

i hv used LM2676....... download its datasheet. the circuit is in it. default, it is set to 5V but it can easily change into adjustable. the circuit and adjustable method is on its datasheet. u can easily calculate it and use it
 

Rohith_elec said:
is it good to make a 3.3V psu from 7805 using diodes to powerup a 3.3V uC ?

it is difficult to get 1117 voltage regulator here,
Click to expand...

To answer the original question:

If you have an input voltage of 5V, then you can use diodes. Thing is, you won't always have 0.6V drop. It changes with current. Check the datasheet of the diode.

eg. Take a look at the 1N4007 datasheet from Fairchild. With a current of 1A, the forward voltage is 1.1V

Datasheet: www.fairchildsemi.com/ds/1N/1N4001.pdf

Just take a look at Figure 2 in page 2 of the datasheet and you'll get what I mean.

For 3.3V output, you can use the LM2937. It's a fixed 3.3V LDO regulator for 5V input. It's quite common here, although slightly expensive. I've used it to power dsPIC microcontrollers and you can use it without worries. Use it like you use a 7805. Pinout and circuit are available in the datasheet. Make sure you use an output capacitor of at least 10µF. Otherwise the output may be unstable.

**broken link removed**

Hope this helps.
Tahmid.
 

    V

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can i use LM2937 in parallel to obtain more current

---------- Post added 03-04-12 at 00:37 ---------- Previous post was 02-04-12 at 23:40 ----------

can i use LM3927 in parallel to obtain more current
 
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yes these can be used but with current limiting resistor. otherwise more current will passed through a LM2937
 

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