usb Resistive divider

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kodaz

SatelliteGuys Pro
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Pub Member / Supporter
Aug 22, 2010
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North East PA
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

 
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.
 
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.
 
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
 
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
 
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.
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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
 
I'm on my phone. Let's see if this works:
.
Code:
[FONT=arial black][FONT=fixedsys]
  
Transmitter-----------------K--------K--------USB 
                       |        |
                     _|_    _|_  
                     __     __
                       |       |
                       |       |
?                      Ground
  
[/FONT][/FONT]
 
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?
 
You could.
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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.
 
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.
 
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