Convert 19V PSU to USB safe voltage

[EDPR]

My situation:
1) LG computer monitor powered by a external 19V 1.7A PSU. 
2) Raspberry Pi mini computer, powered via standard microUSB cable on a 5V 2A PSU (but actually only needs a big 1A)

My question:
1) Is it possible to use the LG power adaptor to power both? If it matters, the screen's brightness is on about 50%
2) If possible & safe, how to convert the 19V to 5V 1 or 2A?  I've read on some forums a higher Amps might damage the computer.  Hard to believe, but I don't want to test that out.


*** side info
1) I live in a quite remote area but we have a local electronics shop who stocks common components as long as it isn't too exotic.
2) I've tried to power the computer via the screen's media USB port, it does works but it's no solution (because of the screen's eco-mode etc)

Thanks for any valuable input!
EDPR

[Simon]

You need a DC/DC switchmode converter (SMPS) of the buck type. You can buy off the shelf controller IC's or as you have limited parts a 555 timer running a fixed PWM (as your input is a stable 19V) into the usual buck configuration LC lowpass filter and diode will work. You will need a power transistor as the 555 is not powerful enough and may not work on 19V so a voltage regulator as well.

Another way is to use a comparator oscillator to produce the required drive PWM.

[EDPR]

Hi Simon,

Thanks for your help!

They do stock things like the LM2596.  Would this be the most important component?  What else is on the shopping list?  Is there simple schematics available to build this?  I'm ok with a soldering iron, but there my experience in electronics stops I'm afraid.

[Simon]

http://www.ti.com/product/lm2596 has everything you need, should be a link to the datasheet on there and further down the page there is the basic schematic for a 5V 3A output.

[EDPR]

Thanks so much!  That doesn't seem to be too difficult to build, even for me 

Don't know if related, but the component shopping list is indeed about a buck.

I'm only a bit scared about the rumors that 3A would be too much to handle for the Raspberry computer.  What are your thoughts on that?

[Simon]

You only need to worry about voltage. Any circuit will use as much amperage as it needs, no matter how powerful the supply, you could make 100A available and will only use 1A under normal conditions, the important thing is to be at 5V or less (or whatever voltage you are using).

[langwadt]

doesn't get much simpler than something like this: http://www.ebay.co.uk/itm/121676434723

[EDPR]

You're absolutely right!  If only I could get this one here...

The Indonesian Postal system is so bad that, if you don't send it by registered letter, it just won't arrive.  Registered post could take a month easy.
Having it couriered is also no guarantee, I had packages delivered and signed off to completely different named people in a complete different city.  The only thing what was correct, is that they delivered it to the right house number.  Took a month to solve this.
International deliveries are very painful as well, as they will be blocked at customs, again, for months.

But apart from that, it looks like it's exactly what I'm looking for!  And so cheap!
However, I'll have to assemble it by myself.   I'll spend 10-fold on toll roads and parking fees only but it is what it is

[langwadt]

You're absolutely right!  If only I could get this one here...

The Indonesian Postal system is so bad that, if you don't send it by registered letter, it just won't arrive.  Registered post could take a month easy.
Having it couriered is also no guarantee, I had packages delivered and signed off to completely different named people in a complete different city.  The only thing what was correct, is that they delivered it to the right house number.  Took a month to solve this.
International deliveries are very painful as well, as they will be blocked at customs, again, for months.

But apart from that, it looks like it's exactly what I'm looking for!  And so cheap!
However, I'll have to assemble it by myself.   I'll spend 10-fold on toll roads and parking fees only but it is what it is

if you can find a cheap car charger with USB it would probably work just fine on 19V

[Simon]

Good point there are plenty available that do 12-24V and output 1A or more, thought something tells me they will have lots of ripple unless you get a good one.

[EDPR]

Langwadt: that's genius!  These things are literally available in every store here.

Simon: what do you mean with ripple?  And how does 'rippled' compare to 'a good one'?

I found some, all between the $1 and $2 price range:
single port ones with 1A
dual port ones with 1A / 2A (or 1A / 2.1A for what it's worth)
some say 12V, others 12-18V, others 12-24V

Is there any particular specs which say do (not) buy?
None of them say 3A, so does this mean it doesn't use the LM2596? 

[Simon]

The will be using cheap chinese control chips. A SMPS works on the principle of turning the input on and off making it PWM power, that is then put through and LC filter so that if for example you have 50% duty 12v is converted to 6V (ignoring losses) because your using PWM the output can be noisy, all SMPS have ripple which is a low amplitude oscilation on the output, usually specified in mV 50mVpp ripple is typical that means that for example 5V with 50mVpp ripple could be continually fluctuating from 4.975V to 5.025V which is fine for digital stuff, not the sort of thing you want in analogue stuff there noise is a problem. To get rid of ripple you can have larger output capacitors or better have two LC filters in series, but of course that takes space and in the compactness of a cigarette lighter socket you could get some pretty horrendouse stuff, things like satnavs will probably re regulate down to 3.3V so don't care.

[Psi]

be careful with those dirt cheap china LM2596 PCB modules. The one i got was noisy as #^!@ 
100mV ripple