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DC 12v to DC 15v

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zorbzz

Member
Im trying to learn how to use and program Pic16f628a microcontrollers and the amount of Text is huge.

Ive got an old laptop that I want to take to work so i can read the PDF's while Im having lunch in my car but the Laptop I'm using takes a 15volt DC input.

The batteries only last for about 5 mins and when plugged into the cars lighter socket at 12v the Laptop lasts for about 15 minutes then just turns off.

Ive used a 15 volt DC transformer at home and the thing runs all day.

Is there a way to get 15 volts out of the cars 12v socket that isnt to complicated or should I just buy an adaptor from china off ebay.

Thanks alot.
 
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bountyhunter

Well-Known Member
12 - 15 conversion is a pain because I recall a boost won't work right there, I think it needs a SEPIC converter which is a pain to do. Boosts are great as long as Vout/Vin ratio is greater than about 1.5.
 

Hero999

Banned
Actually, I think a boost will work.

The laptop probably works down to 13V or so which is why it works at first then shuts down.

Even if the input voltage exceeds 15V plus the Schottky loss the converter will turn off leaving an unregulated output and as long as the converter IC and laptop can handle it, no harm will be done.

The only thing I'd recommend is adding is some sort of transient suppressor to protect the laptop from high voltage spikes created by the ignition system, assuming it's petrol, if it's a desil then I wouldn't bother.
 

bountyhunter

Well-Known Member
Actually, I think a boost will work..
Maybe so, I just remember that the boosts get unhappy at low boost ratios and the compensation gets squirrely. A good way to find out for free might be to use National Semiconductor's Simple Switcher software with one of their simple switcher chips. The software lets you put in your Vin, Vout, and load ratings and it selects a part and runs a design for you. If the design is feasible with boost, it will do a boost design. The software used to be free on their website.
 

MikeMl

Well-Known Member
Most Helpful Member
The other possibility is just go ahead and boost to >15V, say 18V. Chances are that the existing laptop power supply is a regulated switcher, and it will tolerate a higher input voltage. That way, less heat is wasted compared to a linear regulator...
 

Mr RB

Well-Known Member
Make a stacker regulator. Generate an isolated 3v DC supply (ie a forward converter), and stack it on top of the 12v supply. Run the regulation control from the combined output to maintain 15v out.

It's just like putting a 3v battery in series with a 12v one, and is very efficient as the 12v portion fo the supply is delivered at 100% efficiency and only a small percentage of the supply is done by the SMPS. Total efficiencies can easily be 95%+.

And these days I don't think anyone still does stacker designs like this except me. :(
 

Hero999

Banned
Maybe so, I just remember that the boosts get unhappy at low boost ratios and the compensation gets squirrely. A good way to find out for free might be to use National Semiconductor's Simple Switcher software with one of their simple switcher chips. The software lets you put in your Vin, Vout, and load ratings and it selects a part and runs a design for you. If the design is feasible with boost, it will do a boost design. The software used to be free on their website.
It depends on the controller.

A simple home made blocking oscillator or comparator boost regulator will certainly work, although it won't be very efficient and will be quite noisy.
 

bountyhunter

Well-Known Member
Make a stacker regulator. Generate an isolated 3v DC supply (ie a forward converter), and stack it on top of the 12v supply. Run the regulation control from the combined output to maintain 15v out.

It's just like putting a 3v battery in series with a 12v one, and is very efficient as the 12v portion fo the supply is delivered at 100% efficiency and only a small percentage of the supply is done by the SMPS. Total efficiencies can easily be 95%+.

And these days I don't think anyone still does stacker designs like this except me. :(
That is definitely possible. I believe it would require using a transformer design to get a floating output winding (as opposed to a simpler inductor only design in a boost converter), but it's definitely possible.
 

bountyhunter

Well-Known Member
The other possibility is just go ahead and boost to >15V, say 18V. Chances are that the existing laptop power supply is a regulated switcher, and it will tolerate a higher input voltage. That way, less heat is wasted compared to a linear regulator...
Good point: how about an actual spec on what the VIN (and current) have to be to run this laptop? That always helps when designing a power supply.....:p
 

Hero999

Banned
Inside the cigarette lighter charger is a chip that can be converted to produce any voltage. Look for the voltage divider resistors and change them so that the chip produces 15v. Most of these chargers deliver 1 amp at 3v. They can be converted to produce 15v at 1 amp.
I doubt it, if it's step down, it'll be buck, if it's step upt it'll be boost.

Or you can stack the 3v output on top of the 12v from the car to get 15v. It couldn't be simpler.
That will only work if the output is isolated from the input; not likely.
 

Mr RB

Well-Known Member
I said that! :) An isolated 3v DC supply, using a forward converter. Because of the turns ratio about 4:1 they are easy to make, just get a prewound bobbin style inductor like 100uH etc and wind about 10 turns on top of it. 3 transistors tops. ;)
 

homik

New Member
This might be incredibly inefficient, but you could get a 12vDC to 120vAC car inverter and use the regular laptop power supply.

Again, i'm not sure this would be the most efficient method, but it would save a lot of time.
 

Ghosty_Ghoul

New Member
That wouldn't be regulated though would it?
 

mneary

New Member
This might be incredibly inefficient, but you could get a 12vDC to 120vAC car inverter and use the regular laptop power supply.

Again, i'm not sure this would be the most efficient method, but it would save a lot of time.
It's not the most efficient, but it's the way I do it. I keep a 75W "modified sine wave" inverter in the glove box; it's handy for all sorts of things.
 

bountyhunter

Well-Known Member
That wouldn't be regulated though would it?
It is possible to make a "floating regulator" that rides on another regulated output if they are powered from independent and isolated sources. It's the same thing we do when we "stack" two lab power supplies by connecting the pos output of one to the neg output of the other to raise that one farther above ground. If they are independent, they will regulate independently.
 
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