Need some help from the electrical Gurus!

MonsterBash

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So I have a standard laptop power brick at 19V 3.42A DC.

I need to drop the voltage to 12V and leave the amperage the same.

I went through a chart online here http://www.blackfiveservices.co.uk/fanspeedcalc.php?Voltage=19&Current=4&Target=12

which stated to use a 2.7 OHM resister but...that just doesn't seem right to me.

Can someone with more knowledge on this chime in if they think it'll work?

Thanks!!!
 
That resistor seems a bit high to me, you would get a drop of 8.38 volts and that minus the 19vdc would yield 10.62 volts and that is not close enough to 12 to work. Now I don't know what the 12 volts is for but that would work for a sound amp probably.

Note:

Don't use a 1/4 watt resistor for this also.

EDIT:

I didn't see the fan resistance that would actually give you about 12.2 volts out of r1 and that will work but you need to use about a 20watt resistor.
 
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That resistor seems a bit high to me, you would get a drop of 8.38 volts and that minus the 19vdc would yield 10.62 volts and that is not close enough to 12 to work. Now I don't know what the 12 volts is for but that would work for a sound amp probably.

Note:

Don't use a 1/4 watt resistor for this also.

EDIT:

I didn't see the fan resistance that would actually give you about 12.2 volts out of r1 and that will work but you need to use about a 20watt resistor.
He dosnt specify what he is trying to run on the power supply. Im guessing the fan calculation page was just a random calculator he found.

And if he is running this thing at full load, he probably wants a higher wattage resistor than 20. It calcs to 65w, but id go 70 to be safe.
 
He dosnt specify what he is trying to run on the power supply. Im guessing the fan calculation page was just a random calculator he found.

And if he is running this thing at full load, he probably wants a higher wattage resistor than 20. It calcs to 65w, but id go 70 to be safe.

I fully agree and we need to know what the load resistance for this project is. I suspect it is for a charger of some type and doubt it is for an arcade game.

I think I would just use a car battery that can produce all kinds of current with no problem at 13.8 volts.
 
If your application doesn't exceed 3A you could build a regulator using a LM7812 or
LM2576-12V. 12V 5A switchers are so cheap now, I don't see why you would bother
converting your old PSU.
 
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