Powering FM transmitter via USB?

[Dazza]

I would like to connect my FM stereo transmitter via the USB, so that when the computer is shutdown so will the transmitter, the transmitter requires 3VDC, so I will need to drop the voltage down from 5V to 3V that's simple enough ;D, but is it just a simple matter of taking the + and - supply from the USB to power my transmitter, or should there be something else in between for example, to protect the PC from damage, or maybe from interference from the transmitter.


Thanks for any help ,

 

[audioguru2]

Hi Dazza,
Your transmitter probably has a filter cap across its power pins so nothing will feed past it into your computer.
How are you going to drop 5V down to 3V?

 

[Dazza]

Hi audioguru, and thanks for your reply .

I was thinking of a whole heap of diodes in series ;D, and I haven't thought much further than that yet.

If you have some suggestions on how to drop the voltage, I am all ears ;D.

 

[Dazza]

Hang on I can just use a LM317T ;D, I have a couple of them and it's dropout voltage is 3v .

 

[audioguru2]

Hi Dazza,
If the dropout votage is 3V then you need 5.9V input to get a "dropped-out" 2.9V output when it is designed for a 3V output. It is dropped-out when it is regulating poorly. You might be lucky and find one with a lower dropout voltage, or just use a two-resistor voltage divider feeding an emitter-follower.

 

[GreekPIC]

If it's just a matter of going off with the PC why don't you just drive a relay from the USB?

 

[Dazza]

Thanks for your help Audioguru , I should be able to sort something out, I would prefer to try to use a voltage regulator if I can, and include a bridge rectifier which will be bypassed when powered from the USB, that way it could be powered by many different power sources such as my car. I have quite a few recycled adjustable voltage regulators so maybe one of them will be nice and work reliable on this low voltage.

Hi GreekPIC , yes I did think of that first up, but then I realised why do that when I can power the transmitter directly from the USB, I'm killing two birds with one stone ;D, because at the moment the transmitter is power via batteries, so now I don't have to worry about using a transformer to power the transmitter.

 

[Alun]

I would recommend a small inductor with a ferite core followed by a 1uf then a 100nf and a 1nf in parallel.