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Noob making a power supply

giser3546

May 13, 2015
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Hello everyone, Noob here looking for guidance with my first project as well as the hobby in general. I have a long standing interest in electronics and as a result have a sizable collection of outdated computers and audio equipment that I'm thinking I could do something with. I have a working knowledge of electricity, work with software and computers for a living, and have done some basic electrical installation and rewiring work in the past.

For my first project I'm hoping to build a power supply for a laptop mother board I have. Its a Toshiba Satellite requiring 19v and 3.42 amps. I do have a desktop power supply I'm hoping I could re purpose for this. I have seen many walk throughs outlining a similar procedure but as of yet I'm at a loss on how I would get precisely the voltage I need. I have a multi-meter an old oscilloscope and willing to purchase any other basic tools or equipment I may need.

Thanks everyone.
 

Gryd3

Jun 25, 2014
4,098
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Jun 25, 2014
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Hello everyone, Noob here looking for guidance with my first project as well as the hobby in general. I have a long standing interest in electronics and as a result have a sizable collection of outdated computers and audio equipment that I'm thinking I could do something with. I have a working knowledge of electricity, work with software and computers for a living, and have done some basic electrical installation and rewiring work in the past.

For my first project I'm hoping to build a power supply for a laptop mother board I have. Its a Toshiba Satellite requiring 19v and 3.42 amps. I do have a desktop power supply I'm hoping I could re purpose for this. I have seen many walk throughs outlining a similar procedure but as of yet I'm at a loss on how I would get precisely the voltage I need. I have a multi-meter an old oscilloscope and willing to purchase any other basic tools or equipment I may need.

Thanks everyone.
The tricky part will be getting the precise voltage in a Do-It-Yourself manner.
Desktop power supplies have multiple different voltage rails such as +12, +5, +3.3, GND, -5, and -12 Volts DC.
You cannot use a combination of those rails to get 19V.

That said, you have two options here:
-Use the 12V rail (As it usually supports at least 20A and is closer to the target) and connect it to a variable 'boost' converter to step up the voltage from 12V to 19V. (Note that this will result in about 5.5A drawn from the 12V wire due to the conversion.) You could build it, or buy it : http://www.dx.com/p/120w-dc-dc-10-3...verter-charger-module-red-156701#.VVOfcPlVhBc
-Use the +12V and -12V rail to get 24V, and drop it down to 19V with a regulator. This would be easier to build if you build a 'linear' regulator, but will give off some additional heat. *Note that some power supplies may not be designed to tie the + and - rails together.
 

giser3546

May 13, 2015
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May 13, 2015
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Appreciate it Gryd3. Some idiot questions...
- I'm confused about the distinction between the + and - rails. I am of course familiar with + and - but am not used to - having voltage ratings.
- I'm familiar with the purpose of a voltage regulator stepping the voltage down from 24 to 19, but how would the amperage be controlled?
 

Gryd3

Jun 25, 2014
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Appreciate it Gryd3. Some idiot questions...
- I'm confused about the distinction between the + and - rails. I am of course familiar with + and - but am not used to - having voltage ratings.
- I'm familiar with the purpose of a voltage regulator stepping the voltage down from 24 to 19, but how would the amperage be controlled?
I've attached a picture that may help you understand the negative voltages from a power supply.
In reality, the circuit is much more complex in a desktop power supply, but you can test and play around with this type of multiple output power supply with batteries.
Untitled.png
The middle is Ground, so the black probe on the multimeter you have will stay there.
You will get the indicated readings as you move the red probe to each output.
If you want to get fancy and use 24 V, you simply move the black probe from the Ground to the -12V point, and put the red probe on the +12V point. The total difference in voltage here is 24V! :)

Now to answer your question about controlling current I will give you some detail (most likely too much, but bear with me)
Voltage, Current, and Resistance are all directly proportional to each other. This formula accounts for it:
V = I * R

To be broken down, it simply means that if two values are known, the third can be calculated. This also means that if one is known, you can't control the other two and make them anything you want ;)

So in this case, your laptop uses 19V, and up to 3.42A.
This means that we can pretend your laptop is a big resistor. *Please keep in mind this is over-simplifying it... the laptop will vary how much current it takes, meaning it's resistance will change! This is why you need a 'regulator' to adjust the circuit to keep the voltage at 19V.
R = V / I <> 19V / 3.42A = 5.5Ω
So, when the laptop's resistance drops down to 5.5Ω the regulator will put out 3.42A @ 19V.
If the laptop's resistance climbs to 11Ω (Say... you paused the movie and closed the lid) then the regulator will again, will put out 1.7A @19V
Many people have tried to use a resistor to do this... but because the laptop's resistance changes you end up with the voltage changing at the laptop as well which is a bad thing!
You personally, don't actually need to do anything special, you simply need a voltage regulator that you can set to 19V ;) It does everything else behind the scenes. The laptop will only take as much current as it needs at the time, it could be anything at or below 3.42A

Now... some more detail, hopefully you're still with me.
A linear regulator is simple, but inefficient. It simply wastes the extra energy as heat. So in this case a linear regulator would simply waste 5V if you wanted to take 19V from a 24V power supply. But it's REALLY easy to do. In this case the laptop can pull up to 3.42A from the regulator, which will pull the same from the power supply.
The next one is a 'switch-mode' regulator. This is what they use in computers. They are a little more complicated, and run much more efficiently. They operate by 'converting' the voltage from one value to another. (This has a side-effect of converting Current in the opposite direction... remember that formula above? The other one is for Power. Voltage * Current = Power. When you 'convert' a voltage, the power that comes out is never higher than the power that goes in.) So, if you convert 24V to 19V, you are using roughly 80% of the voltage. So to keep the power the same, the current goes down a little.
So there would be about 2.75A drawn from the 24V supply, which is converted to 19V and 3.42A.

I hope this helps. Let me know if you want me to clarify further.
 

giser3546

May 13, 2015
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Yes, that makes perfect sense. I can get the 24 V from the power supply and use the regulator to lower that to 19v. And from what you've said as long as the multi-meter reads this connection as having less than 3.42 amps the laptop mother board will be able to get what it needs without exploding or catching on fire or something. Would I need to do something to prevent more than 3.42 amps being carried to the laptop?

... on second thought I think I'm understanding why that wouldn't be an issue.
 

Gryd3

Jun 25, 2014
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Yes, that makes perfect sense. I can get the 24 V from the power supply and use the regulator to lower that to 19v. And from what you've said as long as the multi-meter reads this connection as having less than 3.42 amps the laptop mother board will be able to get what it needs without exploding or catching on fire or something. Would I need to do something to prevent more than 3.42 amps being carried to the laptop?
It's backwards actually. The power supply needs to provide at least up to 3.42 amps. It's usually safe to go a little higher.. Would you feel safe driving on a highway with tyres rated for 100kph ? (100kph is the limit for many Canadian Highways. running 'at' the limit can be ... not good) You could have a power supply rated for 1000A, and the laptop would simply draw what it needed. No more. Current is 'drawn' from things, it's not pushed into things.*
You can put a fuse in line with the power supply so that if the laptop were to 'pull' more than it should.


*Most devices work with a specific 'voltage', but there are exceptions to every rule.
LED's are driven by 'current', and the voltage is not nearly as important. In this case it's possible to provide too much current. (Just like it's possible to supply too much voltage to you're laptop.)
 

giser3546

May 13, 2015
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Current is 'drawn' from things, it's not pushed into things
Very helpful and makes even more sense. I'm sure to further review this as I tinker this evening. Thanks again for your help. Its always difficult to dive into things like this and have to start with the stupid questions.
 

Gryd3

Jun 25, 2014
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Very helpful and makes even more sense. I'm sure to further review this as I tinker this evening. Thanks again for your help. Its always difficult to dive into things like this and have to start with the stupid questions.
I'd much rather you ask stupid questions based on lack of knowledge than assumptions formed from fragments of knowledge you pick up.
Everyone has to start somewhere.
 

Fish4Fun

So long, and Thanks for all the Fish!
Aug 27, 2013
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Aug 27, 2013
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giser3546,

At the risk of sounding lazy.....why wouldn't you just order a power supply designed for it? I am as hard-headed and cheap as they come, but for less than $11 you can get a brand new one delivered.....Not sure what the import duty to Ca would add, but even if it is 100% you are still looking @ less than $25.....for "brand new // known working"....with a warranty... ( http://www.ebay.com/itm/19V-3-95A-7...122?pt=LH_DefaultDomain_0&hash=item3f28078cf2 )...And that isn't even the cheapest one....just the first when sorted by "best match"....Just saying, by the time you order anything to complete your power supply you will likely have more in it than the price of a new one....

Fish
 

giser3546

May 13, 2015
12
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May 13, 2015
Messages
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Fish4Fun, A valid point but I'm doing this largely to get a good understanding of it with the hopes of expanding this into other projects. Infact I may get the charger for this at some point in the future and convert this project into a bench power supply for other projects.
 

Kiwi

Jan 28, 2013
471
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Jan 28, 2013
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The -12v rail on ATX power supplies is usually limited to under 1A, so wouldn't be suitable for your application.
 

Fish4Fun

So long, and Thanks for all the Fish!
Aug 27, 2013
481
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Aug 27, 2013
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giser3546,

I am all for learning oriented projects.....I constantly strive to invent one type of wheel or another.....You might consider learning about power supplies by starting with a simple constant-current Buck Converter to drive LEDs....Lot less collateral damage when something goes wrong and there is a lot to learn....when you get feeling froggy you can give a forward converter a go.....Again, plenty to learn and the cost of collateral damage is still low. You might consider a "chopper" circuit for speed control of a small DC motor....tons of ways to pursue learning about power supplies w/o trying to build something to power something as sensitive as a computer....But I certainly understand being motivated on a particular project,...and to be fair the laptop MOBO should do most of the hard stuff...LOL, so carry on :)

Have Fun!

Fish
 
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