# LM317T or LM7812

Discussion in 'General Electronics Chat' started by nerdegutta, May 25, 2013.

1. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
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Hi.

What would you prefer, the LM317T or the LM7812?

I have an input voltage of 24vdc and need it regulated down to 12vdc. The 12vdc is for a 600w inverter. (PM-A-0600AH-12).

The inverter will be used to charge a laptop, and a cell phone.

Apr 5, 2008
18,706
3,643
Hello,

Why not use the 24 Volts model of the inverter?
When you go down from 24 Volts to 12 Volts using a linear regulator, you will throw away half the used energy.

Bertus

3. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
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I have the 12v in my hand.

4. ### t06afre AAC Fanatic!

May 11, 2009
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Have you considered the current that inverter will draw Your regulator will for sure be overloaded. Most PC vendors offer supply solutions for automotive use

5. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
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Um... Doing the math again, I see I might be a little off. My new numbers are:

Laptop draw appx 198W (220v * 0.9 = 198W)

198W / 24V = 8.25A

85% efficient inverter

8.25 / 0.85 = 9.7A

I'm putting the LM317T and LM7812 aside, how about a proper heatsinked LM338K. (TO-3 case)
It's rated 5A so it will be overloaded, but not as much as the LM317T.

Perhaps a voltage divider would be best?

6. ### t06afre AAC Fanatic!

May 11, 2009
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What you could do. Is to discharge your lap top to a low battery status. Then hook it up to the inverter, and measure the current needed on the 12 volt side of the inverter. Also I would highly recommend using a switchmode approach in this case. Unless you also want your setup to double as heater

7. ### tracecom AAC Fanatic!

Apr 16, 2010
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I may be missing something.

Why use the inverter at all? Why not regulate the 24VDC down to whatever voltage the laptop expects as a charging voltage, and do the same for the cell phone?

It seems unnecessary to regulate down from 24 VDC to 12 VDC, then invert to mains AC, then rectify and regulate down to two different DC voltages.

8. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
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I see your point, a good one, but I figure it will be more versatile with an inverter.

What about a voltage divider? Been fiddling with Bill Bowdens Voltage Divider Calculator. Could this work?

9. ### t06afre AAC Fanatic!

May 11, 2009
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A modern laptop will not work in worst case or refuse to charge in best case. If it is connected to some selfmade power supply. I have used a powered USB hub as a charging station for cell phones in the field.

10. ### tracecom AAC Fanatic!

Apr 16, 2010
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I think a voltage divider would be very inefficient, and at the currents you are working with, would take some huge resistors.

11. ### tubeguy Well-Known Member

Nov 3, 2012
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I may be way off base, but I'm wondering if the laptop itself doesn't need less current. (198 watts is a lot of heat).
I looked at the power supply for my older laptop. It is rated at 100-240 VAC in
at 1500ma. But output is 19VDC at 4740 ma. Which equals about 90 watts.

12. ### t06afre AAC Fanatic!

May 11, 2009
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It is also my impression that all laptop charger are oversized regarding current output

13. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
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So...

I got an old IBM AC adapter which output is 19VCD, 4.5A. (this is not the laptop I will be using.)

Would this mean that I can use the LM338K.
19VDC x 4.5A = 85.5W

85.5W / 24VDC ≈ 3.6A

3.6A / 0.85 ≈ 4.2 A

Correct or not?

14. ### t06afre AAC Fanatic!

May 11, 2009
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I think you have to look at the laptop charger input current.

15. ### tubeguy Well-Known Member

Nov 3, 2012
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The regulators dissipation is the current through it multiplied by the voltage drop across it. So something like (24-19)volts *4.5 amps
= ~ 22.5 watts.

16. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
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OK, now I'm confused.

If I were to make one for my laptop, an old IBM:

Input 100 - 240VAC 50/60Hz 1.5A - 0.9A
Output 19VDC, 4.5A

Looking at the input current I get 240 * 0.9 = 216W
Looking at the output current i get (24-19) * 4.5A = 22.5W

If I remember correct, the charger didn't get so hot, so I'm thinking the 22.5W is right.

17. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
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What happened to GopherTs post about paralleling?

Nov 3, 2012
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19. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
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I cannot see the paralleling in the datasheet...

File size:
3.8 MB
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20. ### tubeguy Well-Known Member

Nov 3, 2012
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Oops, I was thinking of the LM338 datasheet.

Edit: Check out page 24 on the 317 sheet.

Last edited: May 25, 2013