Power Question

[AidenE]

i am building my first portable using a retro duo (for just the NES portion) with an unmodded PSOne Screen and my battery is a Radio Shack 3600mAh battery (one of the RC car batteries). Well my question is that my portable only runs for 51 minutes before the screen shuts off. I have the power from my battery to a 7805 to give me my five volts. Any ideas how to see where all my power is going? Cause i should at least get two hours of play time...

[hailrazer]

I would guess your battery is almost at it's end. They don't last forever you know

[Hobbes]

Hailraizer is right, maybe it's nearing the end of it's lifetime and there is the possibility that it can't handle the load. Plus the 7805 isn't 100% efficient, so that's going to mess up your figures.

[timmeh87]

Theoretical play time decreases if you draw very large amounts of current (a good sign of this is if your batteries get warm when you are using them)

Also to get all of the theoretical power you have to be able to operate all the way down to about 1v, maybe 1.1v per cell (for NiMh), or around 6 - 6.6v in your case.

Also I happen to know offhand that a 7805 usually needs at least 7V to run (it varies) .

[Hobbes]

Also I happen to know offhand that a 7805 usually needs at least 7V to run (it varies) .

I don't mean to start a fight, but I've never had one that needed more than 6.5v I have like 30 of them and tested them all when I was just learning. They're all pretty much the radio shack ones. 5.9v - 6.5v was the range I had.

[timmeh87]

Meh, before posting I looked up two datasheets for generic 7805s and got 7.5 and 7v. That is the "guaranteed to work" range, there's probably a little margin on that to account for temperature and process variations or whatever else. Loading is probably a huge factor.

There exist LDO (Low Drop-Out) regulators that will definitely work down to 6.something volts.

[AidenE]

thanks for all the help guys.. i have another battery i am gonna use it's one of those portable DVD player batteries..the only problem i have is that it is 9 volts and the PSone screen needs like 7.5 or 8 volts..what is the best way to get 7.5 or 8volts? should i use a 7808 or is there a more efficent way.

[timmeh87]

You will have the same problem, with the 7808 requiring "at least 10 volts" to run. You're better off looking for a 7805 that works better. Or maybe get a switching regulator, or LED mod your screen.