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.
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.