usb Resistive divider

[kodaz]

On my micro hd, i was looking at options for knocking down the voltage to 3 volts to power my transmitter .
one of the sugestions was a voltage divider from wiki .
it shows that r2 needs to be 2 times r1. so i was wondering what combo would not hurt the micro hd to try?
thanks
http://en.wikipedia.org/wiki/Voltage_divider

 

[primestar31]

Say what? You want to use an fta receiver as a power supply to power a transmitter of some sort? Can you elaborate a little bit more so we can understand what you are attempting?

It'll probably go into self-protective shut-down mode, or you'll fry the port. I really wouldn't try it. Used power bricks are all over the place. Find one from an old cellphone and use that.

 

[Pixl]

Your link to wiki is bad. But any voltage divider would have poor regulation and loading it up with some sort of transmitter will cause the 3 V. to drop. Any usb port on most any device is a weak source for 5 v. A cell phone charger is a better choice as suggested. Sprint and Verizon had many models a few years ago that ran on 3 V. well regulated. Ask around, someone probably has one in a drawer.

 

[xtgold]

or check your local goodwill store

 

[Pixl]

or check your local goodwill store

Good idea, our GW store has a whole box full of power adapter.

 

[xtgold]

that is where I got my usb3.0 external hardrive case for $8

 

[updatelee]

a voltage divider is only ok when your looking at very low current or constant current device. If the current fluctuates so will the voltage in the divider.

A simple voltage regulator IC would be much better and really not much more complicated.

How much current does this transmitter use ? a voltage regulator is wastefull so you must be aware of heat.

3v @ 100ma will put 100ma through the 5v regulator, so you;ll end up with 2v x 0.1amp = 0.2watts of heat in waste.

I switching PS is a more efficient method but more complicated and more pricey.

UDL

 

[kodaz]

Thanks for the replies ,
life has been a bit hectic so it took me a bit to see the posts.

http://www.walmart.com/ip/Scosche-FMT4-TFM-Transmitter/16480626

the transmitter is a walmart special located in the automotive dept.

i keep forgetting to turn it off when i stop using the micro. sometimes it is on for days before i remember it.

currently i am using a 5 volt dc 500 mA cellphone transformer and fullwave rectifier on it.

out of the box , they run on 2 AAA batteries.

seems like it could run off the 5 volts from a usb, but i do not wish to hurt my microhd

 

[Anole]

USB port as power source

Since it'll run on two AAA batteries, current drain is quite low.
.
A forward-biased silicon diode or rectifier has around 0.7 volts drop.
If you put two or three in series with your 5 volt source (banded end toward transmitter), then you would have around 3.6 to 2.9 volts.
Just for general good design practice, I would also place a 0.1 ufd and maybe a 10, 33, or 100 ufd electrolytic capacitor between the power and ground terminals of the transmitter (99% chance it'll work anyway).
.
Wouldn't normally take power from a USB port, but this application is obviously under 10 ma, so I wouldn't be concerned.
.

 

[kodaz]

Since it'll run on two AAA batteries, current drain is quite low.
.
A forward-biased silicon diode or rectifier has around 0.7 volts drop.
If you put two or three in series with your 5 volt source (banded end toward transmitter), then you would have around 3.6 to 2.9 volts.
Just for general good design practice, I would also place a 0.1 ufd and maybe a 10, 33, or 100 ufd electrolytic capacitor between the power and ground terminals of the transmitter (99% chance it'll work anyway).
.
Wouldn't normally take power from a USB port, but this application is obviously under 10 ma, so I wouldn't be concerned.
.

Anole, could u please explain the capacitors. I have only used them to filter power sources in cases like this and i am not clear if i need to put one after each diode or how they get included.
thanks

 

[Anole]

I'm on my phone. Let's see if this works:
.

[FONT=arial black][FONT=fixedsys]
  
Transmitter-----------------K--------K--------USB 
                       |        |
                     _|_    _|_  
                     __     __
                       |       |
                       |       |
?                      Ground
  
[/FONT][/FONT]

 

[kodaz]

The antenna comes off the -(neg) side of the transmitter. Shouldn't i isolate it from the usb somehow? On my old system, i have a resister between the neg and power supply. Seemed to work ok but i never tried it without. the Antenna is approx 61 inches long to give a bit better range on my 20 acre property.
Could i turn the diodes around and put them on the ground side?

 

[Anole]

You could.
.
In any respectable product like that, the antenna wire will be coupled through a capacitor to the transmitter, so shorting it to voltage or ground doesn't blow up anything.

 

[xtgold]

I think adding a longer antenna to a low power fm transmitter is a no no.

 

[kodaz]

Even in its current state, it wont quite reach all of the 20 acres, so pumping it up a bit has been working out ok.