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Transforming 12v to 9v

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maxxoholic420

New Member
I was wondering how i could change 12v DC (car cigarette lighter voltage) to 9v 100mah. I have a mini headphone amp(made from the circuit board of computer speaker) and it uses a 9v 100 mah adapter but i wanted to make it portable so i am using 2 9v batteries and it works just fine. I also wanted to be able to plug it into a cigarette lighter(as not to waste batteries). I have very little knowledge in electronics, so please...speak ENGLISH. Just tell me how to connect the capacitors, transistors, or whatever i need to use. dont show me one of those diagrams as i have no idea how to read it.

i would also appreciate it if you could show me a key of what the parts in the diagrams mean and how to read it soo i could use it for future reference.

Thanx and sorry for my ignorance
 

nettron1000

New Member
That would require a circuit involving a 9 volt voltage regulator such as a 7809. Tho your prolly just starting out in electronics thats a good circuit for a beginner to build and learn from.
There aint all that many parts and its not all that complicated.

Im assuming you have atleast some basic understanding of electronics?
 

nettron1000

New Member
A 7809 voltage regulator is like it says, its an electronic component that regulates an input voltage such as the voltage from your car and outputs 9 volts.
There are other types such as a 7805, 7806,7808,7812 etc. The number after "78" tells you what voltage it regulates or outputs . Also "78" means its a positive voltage regulator, if the number were "79" that woulfd mean its a negative regulator such as 7905,7906,etc.

Well before i confuse you even further...... heres a link to a site that explains how to build a 9 volt voltage regulator that you can use to convert your car battery power to operate a 9 volt CD player .

http://www.aaroncake.net/circuits/cdsupply.htm
 

Roff

Well-Known Member
nettron1000 said:
<snip>Also "78" means its a negative voltage regulator, if the number were "79" that woulfd mean its a positive regulator such as 7905,7906,etc.<snip>
You got those backward. 7800 series are positive regulators. 7900 series are negative regulators.
 

nettron1000

New Member
Ack! yes your right Ron, thanx for catching that, hadnt noticed.

Correction made.
 

Chippie

Member
there's probably insufficient headroom for a 7809 reg to work properly....
 

Roff

Well-Known Member
Chippie said:
there's probably insufficient headroom for a 7809 reg to work properly....
Yeah, I was concerned about that too, but if the engine is running, the battery voltage should be above 12v. You should even have headroom for a Schottky diode between the battery and the regulator/capacitor, to eliminate negative spikes.
 

nettron1000

New Member
Yeah, I was concerned about that too, but if the engine is running, the battery voltage should be above 12v

Yeah i beleive its 13.8 Volts or there abouts. Im still not absolutly sure if thats sufficient "headroom" for the 7809 regulator to operate properly, i'll have to look it up, but i guess the person who designed and built that circuit would make us beleive it does.
 

Roff

Well-Known Member
I checked a lot of datasheets, and all quoted typical dropout voltage=2V. Only ST Microelectronics quoted a maximum (2.5V), but they didn't have the 9 volt regulator (they had 5, 6, 8, 12, etc). I have always used 3V as a rule of thumb. Given the tolerance of 5%, and adding 0.3V for a series Schottky diode, the 7809 should work with Vbatt > 12.8 volts worst case, with typical being more like 11.3V. BTW, National apparently no longer makes LM7809, but there are a lot of other Mfrs who still make it, but with a different prefix (KA, MC, etc.)
We still haven't solved maxxoholic420's problem. He wants a verbal description because he can't read schematics. Do you suppose a Spice netlist would suffice? :twisted:
Seriously, someone with more patience than I have should tackle this.
 

nettron1000

New Member
Thanx for the info Ron, good dig.

Dang! Ron i was hoping you would tackle this one, ive got an headache already and im flat out of asprin !

A verbal discription ? Hmmm...letsee so we would have to use simple unconventional terminology.

Like this maybe: connect the thingymabob to one of those do-hickeys and watch that the magic smoke doesnt escape ! :lol:

Just kidding!
 

Roff

Well-Known Member
OK, you shamed me into it. Get the 7809 datasheet. Get an LM7809, MC7809, whatever. Get a 0.33uF ceramic capacitor and a 0.1uF ceramic capacitor. You are going to be constructing the circuit in Fig. 8 of the datasheet. The pinout for the part is also in the datasheet.
If you enclose the circuit, put it in a metal box with the 7809 tab connected to the box by a machine screw, lock washer, and nut. This will dissipate the heat (about 1/2 watt max) to the ambient air. Make sure pins 1 and 3 do not touch the metal box. You will want to mount the 7809 before you make the solder connections.
I hope you know how to solder. If not, see Basic Soldering & Desoldering Guide. Solder one lead of the 0.33uF cap to pin 1 of the 7809, and the other lead of this cap to pin 2. Solder one lead of the 0.1uF cap to pin 3 and the other lead of this cap to pin 2. Keep the leads on the caps as short as possible. Solder the +12v wire from your cigarette lighter adapter to pin 1 of the 7809. Connect the minus wire to pin 2 of the 7809. The lengths of these wires is not critical.
Pin 3 of the 7809 is the +9v output to your headphone amp, and pin 2 is the other power connection.
Don't be confused by the rake-looking symbol (ground) connected to pin 2 in the schematic. This indicates the circuit common (GND) node, which will be taken care of when you hook all this stuff together as above.

I was going to tell you to put a fuse in line with the battery, but the lighter socket should already be fused in your fuse panel.

Let us know if it works. :)
 

undergraduate

New Member
7809 circuit

Hi there guys. You seem to know your stuff. I have a similar question. I want to do the same thing in my car, however i want to connect a 9V 300mA camera. Would I have to use different capacitors or resistors in my setup? Please help me out!
 

crutschow

Well-Known Member
Most Helpful Member
there's probably insufficient headroom for a 7809 reg to work properly....
There probably is sufficient headroom when the battery is being charged. And even when the engine is off the regulator output voltage will probably only drop slightly as the battery voltage drops, so the amp will likely still work.
 
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undergraduate

New Member
7809

There probably is sufficient headroom when the battery is being charged. And even when the engine is off the regulator output voltage will probably only drop slightly as the battery voltage drops, so the amp will likely still work.
Thanks. And what about the rest of the circuit components? I guess I don't know why capacitors would be required for this. ?
 

undergraduate

New Member
polarity

Do you guys know how to tell what the polarity is on the wires of an AC wall adapter? There's two wires bound together coming from the adapter, one has white lines drawn across it and the other is solid black. Which one is the positive wire and which one is the negative wire? Thanks.
 

Mikebits

Well-Known Member
Since AC alternates between positive and negative there is no positive or negative wire. There is however, a Hot lead and neutral. If I recall right, black is hot, white is not or neutral. Since this color code may vary from country to country, you would need to look up your local electrical codes. It is always a good idea to verify this by measuring from earth ground to each wire.

One more thing, you should have asked this question in a new thread.
 
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Torben

Well-Known Member
Undergraduate, do you mean "AC-DC wall adaptor"? If so, then the line with the white stripes on it is probably the 0V and the unmarked one is positive. You should really test that with a meter though.

Hasn't this thread been dead for 6.5 years? Perhaps a new thread is in order instead of hijacking this unrelated one.


Cheers,

Torben
 

Diver300

Well-Known Member
Most Helpful Member
Do you guys know how to tell what the polarity is on the wires of an AC wall adapter? There's two wires bound together coming from the adapter, one has white lines drawn across it and the other is solid black. Which one is the positive wire and which one is the negative wire? Thanks.
Coming out of an AC adapter, there if often a polarity. The wire with the white line if often +ve, but you should check with a voltmeter.
 

Mikebits

Well-Known Member
Ack, I totally read the question wrong. I thought he was asking about AC power. Gadzooks.
 
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