Convert 5v DC to 3.3v DC?

[kardzzz]

Hi Guys,
I want to take the 5v from a molex plug and create 3.3volts.
1) How do i do this with resistors? Please provide details of how I calculate the 2 resistor values.
2) If i use a LM3940 regulator, do i need capacitors?
thx

 

[Externet]

To do it with resistors you would need a very steady comsumption current on the 3.3V side.

Use 3 silicon diodes capable of the current consumption, will yield a very close voltage to 3.3V :

+5V---------|>|----|>|----|>|----------+3.3V

Miguel

 

[Hero999]

You haven't said how much current you require?

Diodes will work but the voltage regulation will be poor.

THe LM3940 will work but you need capacitors, as per the datasheet.
**broken link removed**

If it's a really low current, (200mA absolute maximum) the LM317 will just about do but ideally you need capacitors.

 

[kenixtee2003]

if u use resistor to compensate some of the voltage, it will be unstable and the resistor will generate heat which is unnecessary for ur pc. same as the idea of using diode, because the voltage drop on the diode itself drift unevenly, hence u are not sure whether the output u get is exactly 3.3V or not..

its stil better to use a LMseries step down component in ur design, include some capacitor at the output to smoothen any ripple.

 

[PGrogan]

He could also use a Zener like a BZX85C3V3 (1W) and a resistor to bias it! It would probably be easier and cheaper than a LM

 

[kardzzz]

guys, thanks for replies.
I went out and bought some L78L33ACZ (regulated 3.3). Hard to find so i stocked up a few more.
thx

 

[arhi]

I'm fond of BAxxCC0FP/ BAxxCC0T (1A LDO voltage regulator). They have voltage drop of 0.3-0.5V that makes them very usable you have versions for all popular voltages (3.0, 3.3, 5, 7, 8, 9, 10, 12V). They are fairly cheap and accessible.

Compare the 0.3V typical (0.5V max) voltage drop to the voltage drop of L78xxACZ of 1.7V typical and I do not believe 78xxACZ was much cheaper.

 

[arunb]

use LM 317, the datasheet contains the circuit for 3.3 V conversion..its very easy

 

[arhi]

LM317 is even worse, typical voltage drop is 2V