You don't want to use a resistor, because if you turn up the volume, the change in current drawn as the sound gets louder and softer will "modulate" the voltage.
You need a properly regulated supply to step the voltage down to 9V. I don't know who makes one, especially of the capacity needed for a boombox.
I plan to use a ''boombox" in my vehicle because the existing sound device is defective and next to impossible to remove. My boombox uses 6 AA batteries...would it be safe to just run a plug from the 12v cig lighter to a power plug on the boombox? I imagine I will need some sort of voltage reducer. Anyone got any ideas on how one could build such a reducer. I am fearing such a device may induce noise. . . . . .I am of an understanding that 1.5v batterys are acutally 1.2v. Is this correct? If so, I could run the 12v car voltage to 10 AA batterys and then tap off six batterys.... Else, if the AA's are truly 1.5 volts then run the 12v car power to 8 of the AA's and tap off 6. any help would be greatly appreciated.. . . chas
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AA batteries (regular carbon or nickel metal hydride) are 1.4 to
1.5V, NICAD AA batteries are 1.2V. So you need to drop 14V to 9V. 14V because that is the actual voltage of a car system whenever the engine is running,12V is the nominal battery voltage when the engine is not running.You need to determine the current drain of the device in order to figure out what value of resistor you need to put in series with the
14V to drop it to 9V. You need to drop 5V, the value of the resistor needed is R=3DV/I where V=3D9, I=3Dcurrent drain in amps.
Well, yes there are plugs that would be safe. They're commonly used for cellphones and such, and they contain a fuse and suitable stepdown power supply. First, you need the power ratings of the boombox (probably 9V at 1A will suffice to fill a small car with tunes), and of the lighter socket.
The car power may have spikes up to 50V, so the conversion does offer some challenges. If the 'boom box' has AC power option, an off-the-shelf inverter to make 120VAC will be the easiest option.
If memory serves me (and it doesn't always), at the specified specific gravity of a lead/acid cell for automotive use is the nominal voltage is2.2 volts.
Thus a fully charged 6 cell battery is nominally 13.2 volts.
But, of course, in the automotive world the battery is only used for starting unless the engine is off. When engine is running the voltage available to the vehicle's electrical system is determined by the alternator and its associated regulation system.
A decent voltmeter is a handy way to check your own particular situation.
Charlie
AA batteries (regular carbon or nickel metal hydride) are 1.4 to
1.5V, NICAD AA batteries are 1.2V. So you need to drop 14V to 9V. 14V because that is the actual voltage of a car system whenever the engine is running,12V is the nominal battery voltage when the engine is not running.You need to determine the current drain of the device in order to figure out what value of resistor you need to put in series with the
14V to drop it to 9V. You need to drop 5V, the value of the resistor needed is R=V/I where V=9, I=current drain in amps.
Just search for "three terminal regulator" until you find a circuit for the 9volt one. This is a very simple circuit and easy to build, I have had such in use for years in my vehicles. You can build it in a small diecast box which would be strong and act as a heat sink for the regulator.
Rheilly P
Don't do that. It's wrong for a lot of reasons. You need a regulator. If you intend building something, make sure you understand this:
Safest thing is to buy a car adapter.
Umm, looking at the OP's message that might be a tad over the top for his level ?? Sure there can be lots of transients but I reckon for domestic applications it's not necessary to go beyond some stiff caps or a resistor and zener or even a choke where the job is to step down ?
Rheilly P
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Anything's better than trying to use a battery string as a voltage regulator. The worst that could happen using your idea is that he would have to buy a new boom box. Second worst is that he would have to buy a new three-terminal regulator.
Yes, that is what I always see about auto battery voltages....13+vdc. Chas
Would you please share your schematic for the voltage regulator? I would be most appreciative. . . . .Also, thanks for the guidance. chas. L7912CV-Negative
Here's another possibility...
Buy an inverter that converts 12V DC to 120V AC. Then use an AC power supply ("wall wart") suitable for the boombox. It might even have its own built-in AC adapter.
I find that UseNet groups almost always suggest the most-complex and most-difficult-to-implement solutions one can imagine.
There are three simple solutions...
1) Use rechargeable batteries. This won't be cheap, but the OP can operate the boombox anywhere. This is an essentially fool-proof solution.2) Find a 9V auto cord.
WHY?
It requires no energy -- it's plug 'n play.
2-Pcs-3-Terminals-1-5A-12V-L7912CV-Negative-Voltage-Regulator-/280803733057?pt=LH_DefaultDomain_0&hash=item416134ee41
Targus and Igo make universal car chargers. Can be had for $5 or so shipped on ebay. The problem is that they require special tips to configure the voltage and the cheap ones come without tips. But if you can design a regulator, you can open 'em up and add the required voltage setting resistor and put the right plug on the wire.
2-Pcs-3-Terminals-1-5A-12V-L7912CV-Negative-Voltage-Regulator-/280803733057?pt=LH_DefaultDomain_0&hash=item416134ee41 .
hi, you must know how many amperes your boombox will draw from the battery ... because if it draws 1 amp, and you must step down the voltage from 12v to 9v you will have to dissipate 3W from the 78xx regulator also a 7912 will output 12v, negative, and a 7809 will really outpout
9v positive what you need... if it is too hot, you must then use a switching regulator then it's cheaper generally to buy a ready made from ebay or dealextreme hope this helps.-- ---------- Kripton the ESR Repository @ http://kripton2035.free.fr/esr-repository.html the Geiger Repository @ http://kripton2035.free.fr/geiger-repositor.html
total waste of energy ...
-- ---------- Kripton the ESR Repository @ http://kripton2035.free.fr/esr-repository.html the Geiger Repository @ http://kripton2035.free.fr/geiger-repositor.html
I don't blame you for wanting to be green, but the energy involved is tiny, compared to the energy the engine uses. For example, 20HP is about 15kW. Ten watts (at worst) to run a boom box is nothing.
I'm not talking of human energy, but electricity energy ... each time you convert voltage (12v->120V) then 120V->9V you have power losses, that generate heat. that means the energy if transformed into heat, useless and poor solution for the planet ... instead use a smps converter from 12vdc to 9vdc they exists they are cheap, and the power losses are reducted to a minimum
-- ---------- Kripton the ESR Repository @ http://kripton2035.free.fr/esr-repository.html the Geiger Repository @ http://kripton2035.free.fr/geiger-repositor.html
2-Pcs-3-Terminals-1-5A-12V-L7912CV-Negative-Voltage-Regulator-/280803733057?pt=LH_DefaultDomain_0&hash=item416134ee41
Try this, just use a 7809 regulator instead.
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