Step down of 19v to 12v

Thread Starter

Rogue

Joined Apr 23, 2007
6
hi,

i have a power supply unit, the one from a laptop.its a 19v 2.3A dc output. im planning to use it to power my fans in my desktop.the reason i want to use it this power supply unit is because i dont want to burden my desktop's power unit. i've got three fans having 0.8A+0.15+0.20=1.15A in total.all 12V fans.

my problem is i dont have any idea on how to make a circuit using the 7812 IC chip.can anyone guide me please?
 

John Luciani

Joined Apr 3, 2007
477
There are schematics in the 7812 datasheet.

You need to be careful of power dissipation in the 7812 and use the appropriate
heatsink. If you have a 7V drop (19-12) and require 1A you will dissipate
7W.

Look in the datasheet for a maximum power dissipation curve to determine
the size of a heatsink. For the National LM7812 in a TO220 package you would
need a heatsink that as a thermal resistance of less than 10 DegC/W.
To not run at the maximum operating temperature I would get a heatsink in the
5-7 DegC/W range.

(* jcl *)
---
www.luciani.org
 

Thread Starter

Rogue

Joined Apr 23, 2007
6
john,

i was told by a friend of mine that i would have to use three of the 7812's.do i have to do that?

about the heatsink, they have heatsink that fit the ic's with a screw and thermal paste in between.is that the one's that your saying?
 

thingmaker3

Joined May 16, 2005
5,084
The Fairchild LM7812 will only source one Amp. The ST brand L7812 will source 1.2 Amps.

Rather than wiring multiple 7812s in parallel (risking thermal shutdown due to current inequities), why not use a single KA7812? It will source 2.5 Amps.
 

John Luciani

Joined Apr 3, 2007
477
i was told by a friend of mine that i would have to use three of the 7812's.do i have to do that?
The 7812 is rated for 1A. There are other versions the 7812 that are rated for lower currents. 78L12 is 0.1A the 78M12 is 0.5A. Check the datasheet for the part that
you have. Check out www.onsemi.com.

about the heatsink, they have heatsink that fit the ic's with a screw and thermal paste in between.is that the one's that your saying?
The 7812 comes in a TO220 or TO3 package. You need to find a heatsink that
matches your package. It is important to get a heatsink with a low enough
thermal resistance to cool your part.

You want to use a *thin* layer of thermal grease between the component tab (TO220)
and the heatsink. You are only trying to fill in small air gaps with the paste.

(* jcl *)
---
www.luciani.org
 

nomurphy

Joined Aug 8, 2005
567
I think the suggestion for three regulators was not to place them in parallel, but to have a regulator for each fan.

This would make the regulators for the 0.15A and 0.2A fans rather easy, and the 0.8A fan somewhat easier. Also, if a problem occured you would most likely not lose all three fans at once.

Note that you could also do it with two regulators; one for the two lower amp fans combined, and one for the larger.

Placing a series power resistor in front of the regulator(s) will also drop some voltage so that the regulator device will not have to dissipate all the power.
 

John Luciani

Joined Apr 3, 2007
477
All of those suggestions work. It may even be cheaper to spread the heat the out
than to do one device with a heatsink.

I would probably go with an LM317T and an Aavid 529802B02500 heatsink.
The LM317T is $0.48 and the heatsink is $1.49 from Digikey. The LM317
is 1.5A and adjustable. You can stock one part and use it for output voltages
from 1.2V to 37V.

(* jcl *)
---
www.luciani.org
 

Thread Starter

Rogue

Joined Apr 23, 2007
6
What do u guys think of this?

7812CT.JPG


Will it work well? the heatsink will be a good one, with thermal paste.

Or should i just use one?
 

Thread Starter

Rogue

Joined Apr 23, 2007
6
the diagram above had all the ic's in parallel with only one output. since im planning to use three fans, do u think i should use three outputs and then use the three fans or merge it all into one?
 

John Luciani

Joined Apr 3, 2007
477
the diagram above had all the ic's in parallel with only one output. since im planning to use three fans, do u think i should use three outputs and then use the three fans or merge it all into one?
You can't parallel the outputs. Unless you have the 7812s you would better off
with an LM317.

The LM317T is $0.48 and an Aavid 529802B02500 heatsink (3.7DegC/W) is $1.49 from Digikey. The LM317is 1.5A and adjustable. You can stock one part and use it for output voltages from 1.2V to 37V.

(* jcl *)
 

Thread Starter

Rogue

Joined Apr 23, 2007
6
ok. with an lm317t and a 19v power supply with 3 fans and one empty slot for future use, what will the connection diagram be? i cant do it myself. please help~
 

Thread Starter

Rogue

Joined Apr 23, 2007
6
the 'adjust in the data sheet means variable resistor isnt it? if so how many Kohms should be used(10K)? my loads are 1.15Ain total and the supply is 2.3A in total.
 

John Luciani

Joined Apr 3, 2007
477
the 'adjust in the data sheet means variable resistor isnt it? if so how many Kohms should be used(10K)? my loads are 1.15Ain total and the supply is 2.3A in total.
The "adjust" is the resistance that is used to change the output voltage. It can be
a variable resistor but since you need a fixed output voltage it should be a fixed
resistor. Use the Vout equation to calculate the resistance.

(* jcl *)
 

windy

Joined Apr 19, 2007
26
In most cases fans are run by an unregulated power supply. The laptop power unit is a switching power supply, and this is OK to use but I would be using a voltage divider to run my fans not a voltage regulator. Calculate the resistance required for the two resistors solder them in and vola 12v tap to run your fan
 

thingmaker3

Joined May 16, 2005
5,084
Vout = 1.25(1+R2/R1), so R2/R1 = 12/1.25 - 1 = 8.6

R2 must be roughly 8.6 times as large as R1. 43K and 5.1K would be close. Or 18K & 2K. Or 160K over 18K.
 

nomurphy

Joined Aug 8, 2005
567
Most such devices have a minimum load current, it is the minimum current required to maintain regulation. Normally, the current in the resistor divider for setting the output voltage is selected to meet the minimum load current requirement -- typically ~10mA for standard regulators.

Micro-regulators in the 100mA region may require only ~1mA for load regulation.

Note that minimum load regulation current tends to be about 1% of the max output current (but, it is always best practice to consult the datasheet).
 
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