How to run a 3.3V device off of USB 5V

[Pneumothorax]

I recently upgraded my Dell laptop's bluetooth to version 2.0. Now I have the old Bluetooth module collecting dust. I have the pinout available that shows it uses a standard USB interface. But the card runs off 3.3V. IS it possible to convert the 5V line that comes with the USB interface into 3.3V that this card can use? Thanks in advance.

 

[phisrow]

For quite low power applications like this it should be pretty easy. You can either use resistors or diodes to drop the voltage. Somewhat more elegant; but larger and more expensive, would be a DC-DC converter. That is probably overkill for something like this, though.

 

[MrDudeMan]

i dont know if i would arbitrarily throw some resistors in series with it. you dont know the equivalent resistance of the card and the voltage dropped across it is directly dependent on the series resistance in the circuit.

 

[phisrow]

Might be best to use diodes, then, those offer pretty stable voltage drops over a reasonable range of currents, and you aren't going to manage anything unreasonable from a USB port.

 

[Pneumothorax]

What diode would you use for the voltage drop from 5V to 3.3V? Maybe something that drops 1.5V? I think 3.5V would be within tolerance.

 

[MrDudeMan]

Originally posted by: Pneumothorax
What diode would you use for the voltage drop from 5V to 3.3V? Maybe something that drops 1.5V? I think 3.5V would be within tolerance.

you will need to find one that has that spec...

the type of diode i would use is a zener if you can find one with the right parameters. there are several kinds that will work though...

 

[Bassyhead]

Originally posted by: Bigsm00th

Originally posted by: Pneumothorax
What diode would you use for the voltage drop from 5V to 3.3V? Maybe something that drops 1.5V? I think 3.5V would be within tolerance.

you will need to find one that has that spec...

the type of diode i would use is a zener if you can find one with the right parameters. there are several kinds that will work though...

Also you might need to use more than one, you can use several in series to get the drop you need

 

[pm]

My choice for this sort of stuff is an LM317. Buy them at Radio Shack for about a buck. Then just use two resistors to dial the voltage. For what you want, two identically sized resistors would probably put you close enough - so 2 1kohm resistors would get you 2.5V which would probably be close enough. If not, go to something like R1=1000, R2=1600 - without looking to see if there is a 1.6kohm resistor commonly made.

But anyway, they are cheap, they are fun, they are easy, they are easy to find - Radio Shack carries them.

Here's a tutorial from Casemods.com
http://casemods.pointofnoreturn.org/vregtut/tutorial-full.html

 

[bobsmith1492]

A 317 probably wouldn't be stable with that low of a dropout voltage, and it's overkill. Just put a trio of 1n4001/2/3/4 in series with it. They drop ~0.6 volts, so 5-1.8 = 3.2 volts. If that's too low for your device, then use two.

EDIT: Yeah, you'd only want two. Check out http://www.fairchildsemi.com/ds/1N/1N4001.pdf, the top-right graph on the second page, and check how much current your device draws. If it's around 200mA, two diodes would be perfect.

 

[pm]

I've used them in this specific application - driving 5V down to 2.4V to replace two alkaline cells with a regulator (because I had a 5V supply handy and weight was an issue). I've never seen any issues with instability. I have 2 devices (a Sony U20 camera and a mini-camcorder) set up like this. Overkill... perhaps. I just like LM317's though. Handy little IC's.

 

[bobsmith1492]

Yeah, I guess I wasn't really thinking stability as much as just actual regulation; it wouldn't have much margin of voltage to run on. Depending on the current draw, it drops from 1 to 2.5 volts minimum - 1.7 volts drop (5 to 3.3) doesn't leave much margin.

 

[SamurAchzar]

Take into account you can draw no more than 500mA from the 5V of the USB per the specification - if that's a consideration.