reducing voltage

[JDW89]

im trying to reduce 5v to anywhere in between 3.3v to 4.2v preferably about 4v. how do i do this with resistors? any help will be appreciated, thanks

 

[MikeMl]

Need more info: What is the nature of the load? What sort of voltage tolerance does it require? How much current does the load draw @ 4V?

 

[JDW89]

It is for an electrinic cigarette, the battery works at 3.3v - 4.2v. Im trying to work it without the battery, through a usb. I cant get my probes in to measure the current from the battery when its used but when its on 5v it draws 0.85A, which is weird because i thought you could only get 0.5A from a usb, is this info any good?

 

[Hero999]

5V probably won't do any harm, just add a diode (1N4001) if you're worried.

 

[JDW89]

I think it will reduce the life of the atomizer if the voltage is too high. What will a diode do?

 

[Hero999]

Reduce the voltage by about 0.9V

Look at the graph on the top right hind side of page 2 of the datasheet.
**broken link removed**

 

[MikeMl]

A single Silicon rectifier will have a forward voltage drop of about 0.8V at 0.8A. A 1N5817 Schottky rectifier will have a forward drop of about 0.4V. Add one of each in series and you will drop the 5V to about 3.8V

 

[Hero999]

The maximum voltage rating is 4.2V so add the silicon diode, mission completed.