Want to convert 12v to 5v in my car.

[Dussy69]

I want to hard wire my Sirius radio into my car. The power plug right now says that it is 5v 1.2A. Anyone know how I may acheve this without tearing apart the power plug now? I want to keep it so I can move the radio to my wife's vehicle on long trips.

 

[Hero999]

The LM7805 will probably do.

 

[Dussy69]

Here is some info about the radio...

Specifications
Power Requirements . . . . . . . . . . . . . . . . . 5.2 Volts, Negative Ground, DC

Cigarette Lighter Adapter Fuse Requirement . . . . . . . . . . . . . . 2A Slow Blow


If someone could help me out by maybe a drawing on how to wire in whatever I would need would be great...

 

[iflymyhelishigh]

Uhm. LM7805. I'll draw you something later.

 

[kripton2035]

first, lm7805 is 1A, 1.5A max so it will not be able to draw 2A
second, the total power dissipated will be : (12-5)*2 = 14W
which is enormous for this device, if even you faind one that can draw 2A
you will need a heat sink that is the size of the car...
the better way is to use a dc-dc converter device. maxim has lot of them, texas too, or national semi lm2585
look at their web site, they have search engine with voltage input, output and current required, that lead you to schematics and proper devices.

also more informations here about the mecanism of a smps:
Official Home Page of Roman Black

 

[crutschow]

The power plug has a rating 1.2A, which would be the maximum current, so the maximum power dissipated by the regulator (generator charging) would be about (14 - 5) x 1.2 = 10.8W. That can be handled by a moderate sized finned heat sink.

 

[colin55]

If you want to divide the heat-loss, use a 9v (7809) and 5v (7805) in series.

 

[crutschow]

You can also reduce the regulator power dissipation by adding a power resistor in series with the line from the battery to the regulator. A 3 ohm, 5W resistor would dissipate a little over 4W and still leave over 9V to the regulator.

 

[colin55]

Adding a power resistor to the front-end is the best idea.


Colin

 

[kripton2035]

smps is more efficient,smaller, less energy consuming, than linear regulator
especially in this kind of application.
as you want, brute force, or smart thing.
price will be the same.

 

[colin55]

Switch-mode will be a lot more expensive.
Wasted power is of little concern in this application.
A 5w resistor and a voltage regulator will cost less than $4.00
A heatsink can be made from any piece of 2mm - 5mm aluminium sheet about 10cm x 10cm

 

[colin55]

The only other thing is to get a phone charger from a junk shop for $5.00 and convert it to 5v out. These chargers will deliver 1amp.

Look in the re-cycling bins of shops that take your old phone.
Or junk shops that sell chargers for obsolete phones. I bought one for $5.00 and converted it. It uses a buck regulator and heafty choke. You just need to change the voltage-dividers.

 

[kripton2035]

these phone chargers rarely deliver more than 1A.. we need 2A here...
phone chargers that deliver 2A are smps based !!!!
why ?

 

[colin55]

The request was for 1.2A not 2A

"The power plug right now says that it is 5v 1.2A"


Rarely would the radio need more than 1A.

 

[fsmyth]

If you use something like:
Analog Power Supply Schematics - page 6
you will not have heat or current issues.
<als>

 

[kripton2035]

if you still prefer to use a linear regulator, better use a lm317 than a 78xx
you will draw more power with less heat.
but I continue to say it will be better with a smps for such current...

 

[audioguru]

if you still prefer to use a linear regulator, better use a lm317 than a 78xx
you will draw more power with less heat.
but I continue to say it will be better with a smps for such current...

The heating will be exactly the same if the voltages and currents are the same.

 

[mexzy]

hello pls help

hello pls i wud like to build a small solar panel which wud give me an output of 12vdc.i dont really know much about it.pls any1 who can help pls.its a project.u know how lecturers can be.i wud be gratefull.also any1 know how to build an inverter 12vdc to 220v-240vac 50-60hz 2000kva.and pls tell me how to increase the power.need a circuit diagram fully understandable.my email is lilcoonlyfe@yahoo.com.pls help a friend in need,thanks

 

[Hero999]

**broken link removed**
**broken link removed**

 

[kripton2035]

@hero999:
the site you give has a heat problem because he inputed alternate voltage on the lm78xx
the problem stopped when he put continus voltage...
but as we dont know how much current he needs, we can assume our will not heat too much.
again.
the 78xx devices are good to deliver 1A when the voltage on input is near the nominal voltage of the device. from 6v to 5v for example
they are really not adapted for 12v to 5v with 1A. too much heat.
I wonder if the people here saying it works have tried it really...
I tried it really, and the best thing I found to solve it was using a smps...