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Simple 12 VDC to 6 VDC high Current draw circuit HELP!!!

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Khronos1

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Hello. I am new to this forum and need some help.

I am trying to create a simple Linear Regulator that will convert 12 VDC(13.8 VDC from car) to about 8 VDC.

I am attaching circuit schematic below.

I am using an LM1084 to do the regulating and will be using a heatsink to dissipate.

Here is the issue. Current draw. The lights I am trying to dimly light are normally run at 12 volt 27 watts (2.5 A). When I convert to 8 volts, this increases to 4 Amps. I have to drive 2 of them so I need current to be min 8 amps, preferably 15 amps to be safe. (I hope my math is correct in all this above)

I am somewhat new to circuits so I don't know how to accomplish this. Can someone please give me some guidance. Have thought about using transistor to boost amperage but not sure where to start.

Thanks for any help on this.

Franklin
 

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4pyros

Well-Known Member
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When I convert to 8 volts, this increases to 4 Amps
The current will not increase it will decrease.
27w divided by 12v = 2.25a
12v divided by 2.25a = 5.33ohms
8v divided by 5.33ohms = 1.5amp
1.5aamp X 2=3amp
The LM1084 is a 5amp regulater you should be fine.
Andy
 
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ronv

Well-Known Member
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Maybe not. Lamps have lower resistance when cold. Inrush is often 10X running current.
Could you perhaps put the bulbs in series?
Even if you can't it might be a good idea to get the current at the lower voltage to have a good target to shoot for.
 

ronv

Well-Known Member
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A PWM dimmer might be a better choice. A little more complicated, but less heat.
 

moffy

Member
Power @ 12v = 27w
Power @ 8v = 32w

There is something wrong with these figures as power should drop with voltage, for a simple filament lamp. But if they have some kind of self regulating supply it would make sense.
 

Reloadron

Well-Known Member
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Something is wrong. If I reduce the voltage to an everyday incandescent lamp the current will drop as was covered. Two lamps (automotive) designed I would guess for about 12 to 14 volts that are incandescent should show a drop in current at 8 volts.

27 watts / 12 volts = 2.25 amps each. The same bulbs running at 8 volts should draw about 2/3 their normal current so 2.25 * .6666 = 1.499 amps or so per lamp or 3 amps total for two lamps at 8 volts. In a constant on state I can't see where 4 amps comes from? It sort of would be defying the rules. Something else has to be wrong.

Ron
 
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