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19v to 12v without current drop? Answered

i have laptop charger of 19V 3.42A(65W).i want to step down that 19v to 12v without any decrease in current.i need 12v 3.42+A 
what is the simple way?
can i use resistors to radius voltage without current drop?
can i use LM7812C to get 12v 3.42+A
there is ant way to get 12v 5A(65W)



2 years ago

Here is a circuit that can reduce 19v at 3.42A down to 12v at 3.42A, in fact it can supply up to 6A if the source can deliver 6A.

Keep in mind that most loads that draw 3.42A at 19v

will draw less current at a lower voltage like 12 volts..



Can you please explain the value of 47,000 uf? Do we need to use 47,000uf for this circuit? are there any alternatives for this capacitor?

thank you



2 years ago

You can use smaller regulators like the 7812, with the help of a power transistor on the output. You can find schematics like that online. A large enough linear regulator will drop the surplus voltage away, but the maximum current will be about the same as the supply if the voltage regulator is not the bottleneck. Just remember that linear regulators absolutely cannot produce more output current that what the supply can deliver. If that is fine, then also make sure you use a big enough heatsink, because the power dissipation would be about (19V-12V)*3.42A = 23.94W!

A DC to DC buck converter will allow the current to be boosted while lowering the voltage. Building up a Switchmode controllers is a bit more of an 'involved' process, it is not as simple as linear in practice... Or in theory for that matter. But first thing to pop up on amazon is this: http://www.amazon.com/DROK-Converter-Numerical-Adj...

thank you for your reply

but LM7812 can handle 3+A?Peak Output Current of LM78xxis 2.4 or less

No. 1.5A maximum for the 7812. It is stated on the datasheet, on the 4th page. Peak output current is NOT a rating to go by, as it is only capable of that current for a short amount of time, and only if the regulator is kept cold. Usually though datasheets will specify how long a peek anything is.


thanks for your kind suggestion

but it can only give up to 5v output.i need 12v

It will go High as 15V

Check out Pg 13, Section 8.2.1

Oh and the LM388 can do up to 25V

Pg 9 top right corrner

??? If you open the datasheet, it is actually for the all the 78xx series. Including the 7812 regulator. That does output 12V.

Like I said, if you scroll down to the third and fourth page, you can see the ratings and parameters for 7805, 7812, and 7815 series IC's. The forth page specifies maximum current.

No, as Wired Mist, and the datasheet SAYS its ADJUSTABLE


i think its a good idea.each regulator can handling up to 1 Amp current, so 5 7812 in parallel can give 12V 5a max

There are a few "gotchas" with doing that, but yes, you can indeed do that. And like I said before, you can also supplement the regulator with another transistor to increase power/current handling capability. Here is a good read about that: http://www.linear.com/solutions/5759

Only if the 7812 are getting 5 amps will it give you 12 volts and 5 amps.

The words "radius" and "reduce" have totally different meanings. That's just FYI.

Right. You want to know the simple way, perhaps the most simple way, to change a 19 VDC power brick into a 12VDC power brick.

Well, this might sound dumb, but the most simple way depends on what resources you have available. I'm going to try to focus on two approaches other answerers have not mention. First approach: try to find a used power brick that actually matches what you want. Second approach: take apart and rewire your power brick in a way that commands its regulator circuit to give you 12 V instead of 19.

First approach: finding the brick you want. Suppose you knew someone with a big mountain-like pile of power bricks, of all shapes and sizes. Moreover, suppose this person was willing to trade with you, accepting your 19 VDC laptop charger power brick, in exchange for a power brick that was just the spec you wanted, in this case 12 VDC at 3.4 A, or 5A, or whatever. Truly, that would be the easiest way to "change" your power brick.

This might sound like a crazy fantasy, but I claim the power brick you want is out there, and it is in the hands of someone who thinks it is almost worthless.

As an example, someone on this forum, on the same day you asked your question, was asking about what to do with two broken XBox(r) 360s, uh, here,


and it just so happens the power supply for those is a very beefy 12 VDC power supply. I think its like 12 VDC, with max current draw of like 16 amperes!

So what should he with his broken XBox(r)s? I dunno. Maybe he give one of the power supplies to you? Right?

I know that's probably not practical, especially if you and he live on different sides of the planet (I noticed your profile says you're in India.), so the cost for shipping would probably be outrageous. However, what I am saying is: there are people out there, with used power supplies, who don't know what to do with them, and to whom those power supplies are perceived as worthless, essentially garbage that they don't know what to do with.

And no doubt, a lot of Xbox(r) power bricks, and others, literally get tossed into the garbage. In my home country (the Former United States) there exist these places called "thrift stores" which are a kind of a market for used junk, and it turns out these places are a good place to find used power supplies.


The song (by Macklemore and Ryan Lewis) is amusing too, if you have not seen it before.

I have no idea what the marketplace for used junk looks like in your town, but if you can discover where people take their broken electronics (What do they do with it? Do they throw it in the trash? Is there some central place that recycles it? Do they take it to a person who repairs broken electronics?) I think this knowledge could be valuable to you.

That might seem strange, but this is basically my answer to the most simple way to get a 12VDC power supply, is to go out and find one. This approach might take time and money, but so will this idea of using a linear 12 volt regulator (like the 7812 or similar), plus heat sink.

You really need that heat sink, with the voltage drop (19-12=7) times current (3.4 A) you plan to push through it, meaning power dissipation of circa 7*3.4=23.8 watts.

Another approach, which I honestly think is beyond your current skill level, and possibly mine too, would be to open up your 19 VDC power brick, and discover the place where a signal corresponding to output voltage is fed back, and change this signal as to give an output that is proportionally lower, changing the "target" voltage from 19 volts to 12 volts.

Of course this approach requires some deep understanding of how your power brick actually works, but that information may be available, especially if it is a single IC (integrated circuit) doing the voltage regulation, and you can identify that IC on the board; i.e. the numbers on it are readable. I mean if you can figure out which IC is doing this regulation function, the data sheet for that IC is out there, and the words in that data sheet (should) explain how the IC does what it does.

It is close but yes with a DC to DC converter, you only have a leeway of 5 watts out of your 65 watts input so it depends on the efficiency of your DC to DC converter.