How to build a DC/DC  12v/4,5v converter

Hi all, i’m interesting on trasform my power line (12v, with a maximum of 4A) into a line of 4.5V @ (a minimum) of 5A

how is possible? i know there is a lot of ready to use AC/DC converter that turn excatly 220v alternated into the 4,5v that i need but usually they can handle only 1A, and the idea of using 5 in parallels sounds not efficients.

any idea?
tnx a lot ;D

DC to DC converters are hard to build, this is because they require a proper PCB layout to keep them stable. It is not the sort of thing you can just throw together and hop will work.

However, there are lots of ready built modules about just search your local electronics supplier, unfortunately they are not as cheap as you think they might be.

Oh :smiley:
tnx for the answer :slight_smile:

just to know, wich is the better solution out there to turn a 220v 50hz AC into a 4,5v DC (with 5A need)?

i thought on the dc/dc converter since i know only 12v ac/dc converter that can handle such power. but probably there are other build in solution that i don’t know.

the final idea is to power up 15 of this:

http://www.dealextreme.com/details.dx/sku.3160

and off course i need to power up some high power rgb led (with 2.9 and 3.9 voltage dropout)
since the current sink works with a 4,5v input i thought to first convert the ac/dc and then turn down the voltage from 12 to 4,5 but probably there is something simply out there

let me know :smiley:

If you don’t care about efficiency (operating off mains instead of battery), there are shematics out there like this one that describe a power supply using an LM723 as an adjustable regulator and one or more 3055 transistors as current amplifiers.

You can probably simplify the circuit to use a 7805 instead of the 723 for the reference voltage.

Alternative: modify a computer power supply so that it will run without being plugged in to a motherboard, and tap the 5V supply from it. There should be plenty of current.

Another alternative: find a surplus unit like this one and use it.

-j

Doesn’t look that terrible to do this,

check out this guy:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=AP1501A-50T5L-UDI-ND

3 External components and a heatsink.

Since your are switching down from 12V, visit your local computer recyclers and grab a heatsink/fan combo like they used to use on the Pentium Era PCs.

If you’ve got a few bucks sitting around, your local DIY computer shop should have options as well. One close to me has THIS for $20.

wow, lots of new things to know, tnx to both :smiley:
well since that componenet i mentioned ( http://www.dealextreme.com/details.dx/sku.3160 ) works with a maximum Voltage of 4.5 the 5v are too higher, i don’t know what can happen with that 0.5 v in additional /maybe only a bit warmer …

what do you suggest?
keep the line power to 5v instead of 4.5 (that it will be also usefull to powerup arduino without stressing the onboard voltageregulator)
or find another solution for have an exactly 4.5 v line ?

Well 2 things pop into my head on that note:

  1. Use a normal everyday diode such as 1N4007 to drop the 5V to 4.5V.

  2. Check for a data sheet. They mention that 3 AA’s can be used, I measured 1.65V on some new ones here which gives 4.95V…

Then again, the first thing I would do on that note is use the diode again anyways.

I vote diode. ;D

Just run the Arduino before the Diode to give it the full 5V. (Have tested, Arduino does run fine on 4.5V as well)

very good! and what about the power dissipation over the diode … ? i’m not sure how to calculate the current throu the diode, but
if each of the led current sink generate 350mA current i can easily image that the total current drown is around 5A that mean a 2,5W over the diode? right ?
it can handle this ?

diode such as 1N4007

Those diodes are only rated at 1A and he want’s 5A so you better get a bigger diode.

and what about the power dissipation over the diode

Same as anything else, current times voltage. So 5A at 0.7v = 3.5 watts

my apologies…

5A diode:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-5231-1-ND

umm 3.5W over the diode, since i don’t know how the electronic stuff are usually made, is it a reasonable solution ? i have the nightmare of power dissipation (and the less efficience). and 3.5W on a 15W systems, looks like a lot for me…

am i too much paranoid?

Well… This 5A diode seems to have a higher voltage drop of 1.3V at 5A, which makes it useless to drop the 5V down.

I am a total noob on this but can’t 5x 1A diodes in parallel do the job?

I am a total noob on this but can’t 5x 1A diodes in parallel do the job

No not really they won’t share evenly.

Given that this is a

4.5V @ (a minimum) of 5A

It is a 22.5W system not a 15W system. The power has to go somewhere and with a liner regulator be it transistor or diode that power is burnt in heat. As I said at the start a switching regulator is the answer but they are difficult to layout. It is all well and good in finding a switching regulator as per the links above BUT layout is very critical to the stability of such a system.

Interesting thread. Are you trying to do some kind of LED lighting solution off the mains. I am currently looking at something like that for my own lighting.

Its not that difficult to build DC DC convertors as it used to be. Like was mentioned earlier things can go unstable very quickly.

AS someone suggested you could use a linear regulator but what you really want is a switching regulator because this gives you the best efficieny.

Research buck convertors and look at Maxims range of controller IC’s.

i’m trying to drive 5rgb leds, all with 3(each color) differnt pwm signal, so at all, 15pwm signal.

the point is:
Blue and green component have voltage drop of 3.95, and redish one have 2.95 (all deaw 350mA).

the first solution that comes in my mind was to use a VCC + CURRENT LIMITER RESISTOR + LED.
So i must go down to the nearest value of voltage to feed the led.

4.5 is a good value, since is a common transformer value, it’s not far from both the voltage drop, (and i waste a maximum of 500mW over the red-linked resistence).

unfortunatly i must consider a huge amount of current (each color draw 350mA, so each RGB led draw 1050, that mean 5A in the worst case of all light in white)

unfortunatly it’s hard to find over 2A AC to DC (4,5v) transformer.

So here i’am… probably i used the wrong idea at the begin when i try to use a common resistence-limiter method, when something more efficient is possible out there :smiley:

So here i’am… probably i used the wrong idea

Yep you can’t really get away with that using high power LEDs. You need a constant current drive for each colour. Ready built modules are available.