Help on basics (transmitter)

[tru_cutru]

Hey I am new here and at this stuff. So i got this transmitter from ramsey electronics and is labeled as "high power 100mW unit transmits...Unit runs on 9v..." but I want the power source to come from a 12v battery. Now I did the math 12v - 9v = 3v So I want 3v to get dissipated across the resistor. Then (this is the part I am not sure of and I want to check my theory) it is said that operates at 100mW So I used

P=I*V
100mW=I * 9V
I = 9V/.1
I = .011Amp , which will be the current at which the transmitter operates good? So If I am right , then,

R = V/I
R = 3V / .011Amp
R = 270 hm:

So 270 hm: would be the resistor I would have to place between the power source and the transmitter? Sorry about the long post.

PS: I saw a post from 2006 that had a similar question, but I want to check my work. Thank you

 

[ericgibbs]

hi,

The 100mW may refer to the RF power output not the power/current required to operate the transmitter.

A small error in your sums.
P=V * I, so I=P/V...... i=0.1/9 = 0.011A [you finished up with correct answer] of 270R.

BUT, if you want to drop the voltage using a resistor, which isn't really the best way, then measure how much current the module requires, when its in standby and in transmit mode WHILE connected to 9Volts..

Let us know and we will suggest ideas.

EricG

 

[dch222]

Or use a 2.7V or 3.1V zener diode in series instead of resistor. Then it won't matter if the current is varying.

 

[tru_cutru]

hi,

The 100mW may refer to the RF power output not the power/current required to operate the transmitter.

A small error in your sums.
P=V * I, so I=P/V...... i=0.1/9 = 0.011A [you finished up with correct answer] of 270R.

BUT, if you want to drop the voltage using a resistor, which isn't really the best way, then measure how much current the module requires, when its in standby and in transmit mode WHILE connected to 9Volts..

Let us know and we will suggest ideas.

EricG

I will do that, ill get back later, i talked to a friend and he siggested the same thing, a "voltage regulator" I will go to his shop in a minute. But why is it not the best way to drop voltage with a resistor? I thought that was the conventional way to do it, although I am a noob...
Thanks ill get back in the afternoon or tonight

 

[dch222]

Because with a resistor, the voltage dropped depends on the current.
If the current varies, the volt-drop varies so the desired final voltage goes up and down. Ohm's Law.

 

[Hero999]

LM7809, LM317, the circuit is on the datasheet.

 

[tru_cutru]

I just wanted to check my work and put a resistor in series, but i guess i will go further than that...Few questions, so the amperage of the transmitter will vary because it will draw more current or less current when I move away or closer to the receptor (antenna w/tv)? which will mean more voltage so more voltage will mean, more resistance to dissipate the greater voltage? am I right? Another challenge is going to be to actually make one, I will need a resistor a transistor and a diode anyone has any pictures or guidelines on how to make a "voltage regulator"? And lastly what is the "LM7809, LM317, the circuit is on the datasheet" that HER999 talks about?
Thank you
-Tru

 

[ericgibbs]

hi,

goto www.datasheetarchive.com and download the datasheet for the LM7809 volt reg.
Look at the application examples on the sheet.

Eric

 

[Gayan Soyza]

tru_cutru use a 9V battery.......are you using this transmitter in a vehicle????

 

[tru_cutru]

tru_cutru use a 9V battery.......are you using this transmitter in a vehicle????

...yes but i want to improve efficiency and cost

 

[chinsoon]

I just wanted to check my work and put a resistor in series, but i guess i will go further than that...Few questions, so the amperage of the transmitter will vary because it will draw more current or less current when I move away or closer to the receptor (antenna w/tv)? which will mean more voltage so more voltage will mean, more resistance to dissipate the greater voltage? am I right? Another challenge is going to be to actually make one, I will need a resistor a transistor and a diode anyone has any pictures or guidelines on how to make a "voltage regulator"? And lastly what is the "LM7809, LM317, the circuit is on the datasheet" that HER999 talks about?
Thank you
-Tru

why would you want to build the regulator when there is a ready made IC available to do exactly what you need? the ic suggested to you will take in input and regulates its output at 9V. isnt that what you want?

by the way, just in case you really want to build a voltage regulator, there are a few things to consider. What type of control element and components you want to use. there are some using transistor, and some using op amps. you can refer to some electronic books. there are circuits, explanation and also calculation given...

 

[Hero999]

If you can't find an LM7809 then use an LM7808 a 9V battery give 8V when slightly run down so it should still work well.

https://www.electro-tech-online.com/custompdfs/2007/03/LM7805.pdf

For the LM317 option:
**broken link removed**

 

[mneary]

You really need to use a regulator; a zener or resistor won't do. The voltage available in a car varies between 12V and about 14.4V.

In most areas (why do so many people omit their country?) a cheap adapter is available that has sort-of regulated outputs, usually at least 3, 4.5, 6, and 9V.

One example in the US is https://www.goldmine-elec-products.com/prodinfo.asp?number=G15988

 

[Hero999]

If you want efficiency use a switching regulator.
https://www.romanblack.com/smps/smps.htm

 

[mneary]

The Roman Black regulator is clever and efficient, but I would expect it to be a little noisy for an audio/RF project. (Needs further filtering).