getting 3v from 5v source.

F

fever

New Member

• Jul 7, 2006

• #1

hello,
iam powering a digital speedometer using 7805 for 5v output.now from this output i want another supply of 3v for other ckt.i know i can use lm317,but i don't want to use another chip in that place.
so is it possible with zener diodes to get 3v output?
like this http://members.shaw.ca/roma/29-4.gif

 

N

Nab AC Analysis

New Member

• Jul 7, 2006

• #2

No man it is not possible to get 3v from zener u better use another chip or simply use a solar cell

 

audioguru

Well-Known Member

Most Helpful Member

• Jul 7, 2006

• #3

fever said:

hello,
iam powering a digital speedometer using 7805 for 5v output.now from this output i want another supply of 3v for other ckt.i know i can use lm317,but i don't want to use another chip in that place.
so is it possible with zener diodes to get 3v output?
like this http://members.shaw.ca/roma/29-4.gif

Click to expand...

A low voltage zener diode is a lousy voltage regulator. The voltage changes when the input voltage changes, when the load current changes and when the temperature changes. The transistor's output voltage also changes when the other things change.

 

justDIY

Active Member

• Jul 7, 2006

• #4

sure, a 3.3v zener is a very common part, at least online ... you don't have your location filled in, so who knows what for where you're at.

other options include a 78L03 (or is that 78L33), or something newer like an LP39xx or LP29xx from national, or a blue/green/white LED if you don't need much current

 

Hero999

Banned

• Jul 7, 2006

• #5

If size is an issue you can get LM317s in a TO92 package.

 

Dr.EM

New Member

• Jul 7, 2006

• #6

The 5v supply is regulated, why not use a potential divider to derive 3v from it? How much current does the 3v circuit need? If its low, then you may not need a buffer either.

 

GraveYard_Killer

New Member

• Jul 7, 2006

• #7

or experiment by putting diodes in series until you get nearly to 3V output.

 

F

fever

New Member

• Jul 8, 2006

• #8

thanks for ur replies guys.
well iam india,and i in my local stores i asked for these 2(78L03,78L33) but these r not available.i dono abt other national chips.(can i order samples).

and the other circuit iam talking abt is a remote contol car's transmitter which uses 2 AA batteries.

putting diodes in series...is it satable? may be it work like zener diode.

and for using potential divider...i exactly dono how much that circuit consumes current.

 

F

fever

New Member

• Jul 10, 2006

• #9

guys is it safe to feed 5v to a RF transmitter which uses 2 AA cells?
any more ideas to get 3.3v from 5v....

 

audioguru

Well-Known Member

Most Helpful Member

• Jul 10, 2006

• #10

fever said:

guys is it safe to feed 5v to a RF transmitter which uses 2 AA cells?

Click to expand...

It might blow up or it might work fine.
You didn't attach the schematic so are we supposed to guess?

 

justDIY

Active Member

• Jul 10, 2006

• #11

thats pretty much the only options; divider, diode(s) or active regulator (transistors).

you could look up the chips that comprise your transmitter, and see if they're rated to handle 5 v... my guess is no, most things that want 3v, will handle 2.8 to 3.7v

 

Oznog

Active Member

• Jul 11, 2006

• #12

Could put 2 rectifier diodes and one Schottky in series with 5v to make 3.3v. Doesn't draw quiescent current. Poor regulation, though.