Rail Volts?

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PruBowl300

Weaksauce
Joined
Mar 11, 2004
Messages
70
Is there anyway to make a rail like 7 volts? I can't remember, and I would like to use 7. 12 volts is too much and 5 just isn't quite enough. I am modding my 5" LCD so that it will move in and out of my 5.25" bay using an old CD-ROM. I just need to setup the switch and set the voltage. Any ideas? I can post some pics if you don't understand what I am talking about. Let me know.
 
I've read a few times that if you join the 12v and the 5v. It will send 7 v through whatever you have it hooked up to.
 
That's what I was thinking, but I couldn't remember exactly how to do it. I don't want to do it and mess anything up before knowing for sure.
 
Be [H]ardcore and just start hooking wires together randomly until it works.
 
DO NOT use the 7v trick for LCD's... you make your ground line +5 VDC... so as soon as you hook it up to your video card your going to send +5V into its ground lines... = not good
 
Yeah, that definately would be bad. LCDs use the ground with communications, too, so if you change what the ground is then it will mess everything up.
 
The 7 volts will be powering a motor, if I'm not mistaken...

Anyways, you can do this easily with a linear regulator (about 50 cents) and two resistors (only a couple cents each). The LM317 is a popular linear regulator IC and it can easily be adjusted to give a stable 7 V by using the two resistors I mentioned. By using this formula,

V_out = 1.25 (1 + R2/R1)

we find that you need a 4.7 kilo-ohm resistor and a 1 kilo-ohm resistor to get about 7 volts. You would connect R1 (1 k-ohm) to the Adjust leg of the LM317 and to the Out leg. You connect R2 (4.7 k-ohm) to the Adjust leg and to ground (as provided by your power supply). It's okay to substitute the 4.7 k and the 1 k with 470 and 100 ohm resistors, etc. just as long as the two resistors are different by a factor of about 4.7.

Let me know if you have questions.
 
Thanks for paying attention xonik. I will be powering the old CD-ROM motor with the 7 volts. Do you think I should just buy one of those Thermaltake Variable Fan Speed Adjusters? It is really turning out great. I have been tweaking the back of the LCD so that it has an incline. Now it pulls itself back level when the motor is in reverse. I am impressing myself as I go.
 
Yeah, you can use something like that as long as it doesn't use the +5 and +12 V rails together like the 7 volt trick. That's probably a little more user friendly, too :)
 
Oh, that's the trick to get 7 volts by connecting the + lead to +12 V and the - lead to +5 V, yielding an effective 7 V potential difference. Like FLECOM described earlier, it's not good for the rest of the system.
 
Well, I am going to assume that the Thermaltake piece isn't going to do that since it is a retail part. I wouldn't think they would sell something that may cause problems on people's computers.
 
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