markland556

Just like the topic says. It seems soo simple, i have 12v from a car (actually it changes from 12-14v) and i need a steady 7.8v. How should i do this. I need to finish this by tomarrow and im in a bind. I need the easiest way to do this. Its for a small screen in a car, the power jack runs on 120VAC and outputs 7.5VDC. So i need the strait 7.8v DC in the car..

EDIT::: Sorry, it should be 7.5V DC instead of 7.8

audioguru

We can't help you if you don't say how much current. An LM317 can supply up to 1.5A but will need a heatsink at such a high current. Just 2 resistors set its output voltage.

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Roff

How much current does your small screen draw?

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Ron

markland556

The power supply puts out 1500ma... but im not sure how to test how much the screen draws..

grim

one thing to watch out for though is noise - if the equipment was designed to run off 50/60hz derived smoothed dc, then it may not like the noise that you usually see on the 12v supply of a car

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HiTech

Leave your vehicle headlights on overnight. Tomorrow morning you'll have 7.5 volts at the cigarette lighter socket.

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markland556

With the crude circuitry on the board i dont think it will be too much of an issue. Is it as easy as adding in a resistor to the power line?

on1aag

Hi Markland556,

You could use a simple 3 pin regulator like the L78S75CV, two small
capacitors and a (large) heatsink.

http://uk.farnell.com/jsp/search2/br...questid=769486

You could also add an extra 1,5 hm: ;5 watt resistor to connect the
circuit to the battery to take away some of the dissipation of the regulator.

on1aag.

markland556

hey, quick question. I got those regulators. Now im a little confused because the data sheet for it is kinda weird. But all i need to do is hook +12v into the input and the ground then the last with will have +7.5v??

philba

my guess is that the screen actually runs on 5V and the 7.5 V requirement is to allow the regulator to do it's thing. Does the unit get hot when you run it from your wall wart? If not then I'd check the current (dmm set on current in series with the wall wart). If it's fairly low, you may actually be able to just use 12V. Even if it's too high, you should know the current to determine whether you will need a heatsink.

tunedwolf

here's something I knocked up for ya to use...the pass transistor (3055) doesn't have to be a 3055, but anything with similar gain and current handling will do, just make sure it's insulated and on a good heatsink, because it will put out around 7W of heat...the zener diodes are pretty slack too, any combination that makes up 8.6 - 9v will do you...

I hope this helps ya out

Well...duh!! me

here's the corrected schematic...

Attached Images

7.5 V 1.5A Regulator.jpg (17.1 KB, 12 views)