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Thread: 7.5V composite screen Help

  1. #1
    Maximum Bitrate neon_eddy's Avatar
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    7.5V composite screen Help

    I got a nice Xbox 5" screen for $90 at best buy, open box , it was from a $200 giftcard they gave me for buying my laptop there. Anyway, it's 7.5v and I need to use 12v. I used some ohms law calculators and "guessed" a bit at what I should do.

    I compared amps/watts/ohms/ against 12v and 7.5v each at 1500mA and I got 5 ohms for the 7.5 voltage and 8ohms for the 12v .. so I'm thiunking if I use a 3ohm resistor in series on the 12v line it should work fine?..

    of curse since it runs more like 14v when running should I use 4?.. or will the screen have tolerance?

    I had an old PSone screen a few years ago and ran the straight 12v in and it worked fine for a few hours then fried.. I don't want to repeat this.
    Shawn



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  2. #2
    MySQL Error Scouse Monkey's Avatar
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    i'd get one of those switchable voltage DC-DC converters. They sometimes have 7.5V outputs.

  3. #3
    Maximum Bitrate neon_eddy's Avatar
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    in a perfect world yes.. that would be best.. but I have a box full of various resistors, and they are free.. the adapter is not free... yes I just dropped $90, but I'd rather just use line resistance.. so with that in mind any ideas?..

    I may just try it and if it breaks I'll say "it just stopped working" ..
    Shawn



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  4. #4
    MySQL Error jcdillin's Avatar
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    you dont want to run that through a resistor, all your going to do is create a space heater.

    just pick up a LM317 from radioshack it takes like 3 parts to make a adjustable regulator that is good for 1.5A with a heatsink. The schematic is on the back of the package

    or the other option would be what scouse said, just pick up a variable power supply that is designed to plug into your ciggy lighter and cut it apart
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  5. #5
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    or go to EB games and see if they have a PS1 screen car adaptors, they are native 7.5v

  6. #6
    FLAC evandude's Avatar
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    if you use just a series resistor, you will almost certainly kill it.

    the resistor only drops the proper voltage when it is drawing exactly the right current (1500mA) and it almost never will be. if it draws LESS current at any point, it will see more than 7.5v... which could fry some stuff... and if you draw MORE current for a brief time, it will get less than 7.5v, which will probably cause it to not work properly.

    The problem is that you're assuming that the screen is a resistive load, which is wrong. Almost no electronic devices draw constant current from the power supply, it's constantly changing. It could easily be 1500mA one minute, and 100mA the next (if the screen went into standby or something... not familiar with the particular model, but you get the idea) which would cause it to see almost 12v supply, and probably die in the process.

    Also, while you did get the correct answer for the resistance you should use if your series method were valid, the reasoning behind your method made absolutely no sense. Not a big problem, but it sounds like your electronics knowledge is quite limited. I would listen to what everyone's saying and get a variable regulator or a DC-DC (radio shack variable regs might not supply enough output current, so you'll have to be careful when picking one to buy)

    Like you said with your PSOne screen, that's probably exactly what will happen if you try to use a resistor as a voltage regulator... that's what they make voltage regulators for, after all.
    But don't take it from me! here's a quote from a real, live newbie:
    Quote Originally Posted by Viscouse
    I am learning buttloads just by searching on this forum. I've learned 2 big things so far: 1-it's been done before, and 2-if it hasn't, there is a way to do it.
    eegeek.net

  7. #7
    Maximum Bitrate albysure's Avatar
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    radio shack sells an adapter that plugs into your cigarette lighter that has variable outputs

  8. #8
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    A little trick for you, put your multimeter negative on the red (+5v) wire on your molex and the positive on the yellow (+12) wire on your molex connector...wolla...7 volts. Try it out and see if will power your screen and be bright enough, might be simpler than resistors and VRs.

  9. #9
    Maximum Bitrate starfox's Avatar
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    Nonono, don't do the differential +12V to +5V trick with a LCD!

    That works fine for completely floating devices that have no grounds other than a tie to negative, but there's a few problems with this that will blow up a display: You have to connect a composite cable, or vga cable, or something with a shield to the LCD. This has a ground path. Your supplied ground path is actually +5V, so you may short the +5V to ground. Depending what happens first, your PSU may shut off, or you may fry your LCD.. Also LCDs have an inverter in them to power the backlight and this will introduce differential noise to both the +5V and +12V lines, which is bad for your other components.

    Use a switching DC-DC regulator, or a linear regulator with a shunt transistor across it (preferably the first option).

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