How to step down 220v ac/dc transformer from 19v to 12v an other

eugenchivu · 3rd February 2006, 12:43 AM

hey i need a 70w psu for a mini itx project but i only have a transformer left over from a laptop which outputs 19 volts...i need 12 volts of DC.... can i somehow step it down?

or another solution i was thinking is using a halogen light bulb transformer..cuz that takes the 220v to 12v but i think it does it AC/AC and i need AC/DC is there a way to turn the 12v AC to DC?

AndrewT · 3rd February 2006, 04:58 PM

Hi,
is the halogen light bulb converter really a transformer or an AC to AC regulator?

The 19Vac can be rectified and regulated maybe in two stages down to the required 12Vdc.

Similarly the 12 TRANSFORMER can be rectified and regulated.

7812 would do it simply, if the voltage, current and power dissipation suit your load. 317 is adjustable with a couple of extra components.

eugenchivu · 3rd February 2006, 09:48 PM

well i have no clue if the halogen transformer is ac/ac it probably is...

can i find out somehow?

and.."7812 would do it simply, if the voltage, current and power dissipation suit your load. 317 is adjustable with a couple of extra components." i have no clue what this is...what extra components and how exactley

N-Channel · 4th February 2006, 03:57 AM

Eugene-

A few questions: Does the laptop transformer output 19V AC or DC? How much current at 12VDC do you need? Is this for a Book PC-Like CPU? If so, I think you'll need +12V @ 6A and +5V Stdby at, I think, 1.5A.

If fhis is the case, then you can do a simple SMPS buck regulator using a TL-494 PWM chip driving a P-Channel MOSFET to get the regulated 12V out. The reason for using an SMPS is that if you were to regulate the 19V down to 12V using a linear regulator (and the 7812 is good for only 1 Amp), even one rated for 6A, the you would dissipate the difference, (19-12)V x the 6 Amps drawn = 42W dissipated away as waste heat. Not too efficient!

A switcher sidesteps this low-efficiency problem by varying the on-off time of the pass element (in this case, the aforementioned P-Channel MOSFET), instead of its conductance. As for the 5v section, this can be an LM2596 3A PWM SimpleSwitcher chip by National Semiconductor, which is clocked at 150kHz, enabling the use of small magnetics and capacitors.

Hope this helps!

Steve

3rd February 2006, 12:43 AM	#1
eugenchivu diyAudio Member Join Date: Feb 2006 Location: Bucharest	How to step down 220v ac/dc transformer from 19v to 12v an other hey i need a 70w psu for a mini itx project but i only have a transformer left over from a laptop which outputs 19 volts...i need 12 volts of DC.... can i somehow step it down? or another solution i was thinking is using a halogen light bulb transformer..cuz that takes the 220v to 12v but i think it does it AC/AC and i need AC/DC is there a way to turn the 12v AC to DC?

3rd February 2006, 04:58 PM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, is the halogen light bulb converter really a transformer or an AC to AC regulator? The 19Vac can be rectified and regulated maybe in two stages down to the required 12Vdc. Similarly the 12 TRANSFORMER can be rectified and regulated. 7812 would do it simply, if the voltage, current and power dissipation suit your load. 317 is adjustable with a couple of extra components. __________________ regards Andrew T.

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3rd February 2006, 09:48 PM	#3
eugenchivu diyAudio Member Join Date: Feb 2006 Location: Bucharest	well i have no clue if the halogen transformer is ac/ac it probably is... can i find out somehow? and.."7812 would do it simply, if the voltage, current and power dissipation suit your load. 317 is adjustable with a couple of extra components." i have no clue what this is...what extra components and how exactley

4th February 2006, 03:57 AM	#4
N-Channel diyAudio Member Join Date: Mar 2003 Location: Sol System	Eugene- A few questions: Does the laptop transformer output 19V AC or DC? How much current at 12VDC do you need? Is this for a Book PC-Like CPU? If so, I think you'll need +12V @ 6A and +5V Stdby at, I think, 1.5A. If fhis is the case, then you can do a simple SMPS buck regulator using a TL-494 PWM chip driving a P-Channel MOSFET to get the regulated 12V out. The reason for using an SMPS is that if you were to regulate the 19V down to 12V using a linear regulator (and the 7812 is good for only 1 Amp), even one rated for 6A, the you would dissipate the difference, (19-12)V x the 6 Amps drawn = 42W dissipated away as waste heat. Not too efficient! A switcher sidesteps this low-efficiency problem by varying the on-off time of the pass element (in this case, the aforementioned P-Channel MOSFET), instead of its conductance. As for the 5v section, this can be an LM2596 3A PWM SimpleSwitcher chip by National Semiconductor, which is clocked at 150kHz, enabling the use of small magnetics and capacitors. Hope this helps! Steve

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