I know that they are used to only allow a certain amount of voltage to go through but say you install a diode that only allows 7.5v to go through and you feed it 15v. Will this trip something in the diode and the system stop working or will it work same as ever dispite the 15v going into it. Heres what I'm thinking for my portable:
I can only charge the system while it's off so I'm going to install a port that allows me to run the system off of wall power while the system is off and charging. However what if I forget the disconnect the power cord before I turn the system on, then I'm feeding the system the voltage from the battery and the power cord which is 15v, will the system keep working dispite the 15v going into the diode.
First, a diode allows voltage to only go in one direction, and generally doesn't care what the voltage is. Amperage, maybe, but not voltage. They are generally used as circuit protection, not voltage regulation.
A voltage regulator is used to cut down supply voltage to a desired voltage. For instance, the TI regulator commonly used for N64 portables takes input voltage down to the necessary 3.3v. Get one of those.
The screen will be a little more difficult... use a 7808, that'll take your 15v down to 8v. The screen itself is good up to 9v, if I remember correctly, so the 7808 should do fine. You can also feed the 12v line of the N64 - it goes to a 5v regulator anyway.
Finally, I would use a 3-way switch for charging - one way for on, other way for charging, middle for off. Radioshack has them.
