how to drop current

So i have a FM transmitter that takes 2 AAA batteries and we are going through them like crazy. I want to hard wire it into my car. How would i do that that? I was thinking of using resisters inline. What are your guys thoughts on this.

Spec:
3.2v
0.012amps

Are you using and after market Radio? If so what amp do you have coming off the Remote wire?

yes its an after market radio and im sure it 5 amp

Probably not resistors - they only suit constant loads..
You need a voltage converter or regulator.

Since 3.2V@0.012A is only 40mW, a simple 3.3V Zener regulator should do...

With your transmitter = R2 in the above link = 267R (3.2/.012), I reckon a 1/2W 680R (680 Ohm) resistor (though a 1/4W resistor will do) and a 400mW or 1W 3.3V zener.

That should cover from under 12V to over 15V input.
Even though Simple Zener Regulator regulation isn't that good, it should be ok.


Otherwise use an LM317 voltage regulator (any size - it's passing under 20mA) with its usual R1 = 120R and R2 = 180R (both 1/8W or higher) - eg: see Martin Meserve's LM-317 Voltage Regulator Designer. Caps might not be required...


BTW - you want a voltage reducer - not a current reducer.
Like most loads, the FM Tx requires a certain voltage; the current then looks after itself.

Hey man thanks for your help. To be honest im a little confused on what to do, maybe it because im just a rookie.

Paraphrasing, you need a voltage regulator.

Probably the easiest is using the LM317.
The smallest version will do - it looks like a small transistor.
It should cost $1-$3, plus two $0.05c resistors from most electronics shops. (The 317 is a very common device.)
Then solder it up - as you would/should have with resistors anyhow.

Other than the 2 LM317 links I gave, you can search for more info.

FYI - instead of using 180 Ohm for R2, you could use a 200R or 500R trimpot (variable resistor) to vary the voltage if you have a multimeter.
A 120R resistor is usually used for R1 because that provides the minimum load of 10mA that is required by the LM317.


Personally, I prefer radios etc with the correct inputs, else I'll hack one. Though the FM modulators are a neat idea - especially for portability - always thought they compromised sound.

"FYI - instead of using 180 Ohm for R2, you could use a 200R or 500R trimpot (variable resistor) to vary the voltage if you have a multimeter"

what is R2 and 200R or 500R mean and yes i have a flux 82 multimeter. Do i heed a heat sink? Thank you for explaining this to me it very educational to me.

Aha! So you didn't use my link to LM317!! Geez, why do I bother? (LOL! It's cool - just jesting...)

I use "R" instead of "Ω" for Ohms, hence 200R & 500R mean 200Ω & 500Ω.

The R1 & R2 refer the the following pic in the LM317 link:

(And most refer to the same R1 & R2, but not always... (eg, my other "design" link earlier).)

Heatsink?
Assume the 317 drops from a max of 15V to 3V - henc e a 12V drop.
Current is its own 10mA plus your 0.012 = 12mA, hence 22mA total.
Power = Volts x Amps; P=VI (Watts, Volts, Amps)
So P = 12V x 22mA = 264mW = .264W. (IE a bit over 1/4 Watt.)   

So probably in general, under 1/4W heat dissipation from the LM117.
(reality or Santity check - if typical 14.4V charging & 3.2V out, then 14.4-3.2 = 11.1V x 22mA = 244mW ~250mW.... cool! Or warm?)

That is fine without a heatsink for the "smallest common" package - the LM317H in its TO-39 metal case.
WARNING - usually the cases on the LM317 are (also) the output, so do not short to +12V or ground!!

That's all, but read on at you peril.
Or to check for my mistakes...
Though the LM/LM317.pdf is a good document - good design stuff - use the LM317 for any voltage up to nearly 40V, or as a current limiter....

More info?
Thanks to you (Grrr!) I downloaded the latest
LM/LM317.pdf from National (hey man - they now come in a 20-pin chip-carrier leadless package too! That's in addition to the 3-pin SMD TO-263 version..... Wow!)... where was I?   

Yeah ....to check heatsinking. It's all there... after deciphering.
(In short, TO-39 = "H" package = 186°C/W without heatsink).
186°C/W x .25W = 47°C temp rise.
The LM317 cannot exceed 125°C junction temp, hence outside (ambient) cannot exceed 125°C - 47°C = 78°C = 172°F.
So keep you cabin below about 70°C (~160°F) and you should be okay without a heatsink even with the worst heat-dissipating package, the TO-39.

Hey man THANKS a bunch!!!! I did click the link and i just didnt put the two together. So for R1 i am going to run 180R and for R2 im going to uses a 500R trimpot. Does the arrow in R2 in diagram represent Variable? and what does the C1 and C2 mean?

NO! R1 is 120 Ohm. 180 is too high - that was for R2.
(Forget R1 = 240 as in that pic - that's for the 117 which is a different spec than the 317.)

The Cs are capacitors. You could add C1 (input cap(s) but do not add C2 (output caps) without a protection diode - see the pdf.

And yes - the arrow means variable.
Already you are getting the gist of circuit diagrams.

laziebun wrote:

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im a little confused... maybe it because im just a rookie.

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Yeah, right!
(That's supposed to be a compliment...)

120R is what i meant. PDF link isnt working. Why does the C2 look different than the C1 on the diagram?

Because the caps are different types (mainly due to their size).
Wiki caps etc.

Sorry about the bad link.... LM317.pdf