PSOne Screen flickering/won't turn on with 9.6V NiCd battery

[gage]

I have a rechargeable 9.6V NiMh 1000mAh battery going through a 7808 regulator, to a power switch, then out to my PSOne screen. When I flip the switch, the screen occasionally turns on, but appears to flicker. It's as if there isn't a constant stream of juice going to the screen. Some other times, the screen does not turn on at all.

I hooked it up to wall power with the screen's standard power jack, and it works perfectly. Seems to be the battery, but I wanted to see if anybody out there could help troubleshoot.

FYI, when I tested the battery using a multimeter, it shows 8.7ish volts coming off the battery.

Any ideas?

[XCVG]

You don't have enough voltage to the regulator, so it randomly cuts out. Either that or the voltage is two uneven. In either case, try some caps.

[tom61]

FYI, when I tested the battery using a multimeter, it shows 8.7ish volts coming off the battery.

Sounds like the battery isn't charged all the way, or is dead. It should measure close to 9.6 when charged and good.

[gage]

The battery is actually a 9.6V NiCd battery. I previously wrote NiMh. Not sure it makes a difference unless you're charing it. I charged it overnight (it automatically shuts off when it's "charged") and when I tested it this morning, it came in at 10.65 Volts. That slowly dropped (about .01 Volts per second) until it seemed to even out around 8-9 volts. That's when I hooked it to my setup and got nothing.

What size of a capacitor would you guys recommend? Where/how do I hook that up to the battery/regulator/switch/etc? (I apologize, I'm pretty new to the modding scene...love Ben's book, though!)

Appreciate all the help. You guys are awesome.

[Skyone]

-A 7808 requires at least 10.2v under most situations.
-A 7808 only provides 1.0A anyway, which is pushing it (unless the screen is LED modded)

You should have gotten a 7.2v battery. You can find a switching regulator that will work with your battery and the screen, though.

[Kurt_]

I'm surprised nobody's asked yet: Did you measure 8V with the system on, or the battery disconnected? I assume you are measuring it discconected from the system, or with the system off. If that is the case, the voltage under load will be less. It might drop a good 1-4 volts, which is plenty to cause the problems you're describing.

[gage]

Yep. Testing the disconnected battery with a multimeter. Somebody suggested I use a 7808 to drop the 9.6V battery down to 8V output for my PSOne screen. Evidently, that's the wrong route, eh?

Would a modified 7505 regulator be okay? I think there's a way to get a 7505 to put out more than 5V using a few resistors or something...read it on instructibles.com, I believe.

I thought about using a 7.2v battery, but I knew the screen takes 7.5v to fire up. Ben's book suggests you want more battery than what you need, so it lasts longer.

[Kurt_]

Test the voltage with it connected. Testing it with power off it not the voltage you're looking to find.

[Skyone]

Would a modified 7505 regulator be okay? I think there's a way to get a 7505 to put out more than 5V using a few resistors or something...read it on instructibles.com, I believe.

I thought about using a 7.2v battery, but I knew the screen takes 7.5v to fire up. Ben's book suggests you want more battery than what you need, so it lasts longer.

The rule for any 78xx is that the input voltage must be ~2.2v over the output voltage. Even if you change the output of a 7805, it will still require Vout + 2.2v.

A 7.2v NiMH/NiCD battery pack would range from 6-9 volts (completely dead to completely charged), which is almost perfect for a PSOne screen with no need for regulation. However, any fluctuation due to the lack of a regulator would perhaps mess up the screen...

I still agree with myself when I suggest a switching regulator.

[lelejk1986]

I'm gonna suggest something that isn't the best path to follow (quick and dirty ), but which can work (I've done it in the past, but never tried it with something using 1A)...
Why don't you just wire 3 diodes in series between the screen and the battery?
The voltage drop would be about 0.7V each, for about 2.1V total, so you'd feed the screen 9.6-2.1=7.5V, which should be ok.
Make sure that the current rating of the diode supports 1A.
It's probably not very energy efficient, but it's gonna be better than a 7808 and easier than a DC-DC converter.

[Kurt_]

Dudes!

8V not under load will be around 7V under load. With Ni-Cad, maybe lower. Any lower than 7 volts, and the 7805 cuts off.

[lelejk1986]

Well, you COULD always use 2 diodes instead of 3 or test it a little before...

[Skyone]

That is not regulating voltage, that's limiting it. It's also a poor method at that, for it has to give all of that voltage off as heat.

[lelejk1986]

I never said it was efficient!
I don't see much difference with using a 7808 though, apart from the fact that a 7808 always gives out 8V (if the input is in the correct range), while this just takes away a certain amount of volts from the input. They are both getting rid of the excess voltage, turning it into heat, and we all know that a 78xx with no heat sink is good only for small currents.
I'm not saying it's a good solution, I'm just saying that it might work, and it's definitely easier than a DC-DC converter (not that it is hard to do, but harder).
That said, I wouldn't use it myself in a portable, but I've used this method when I needed a couple of volts less than my power supply could give and had no time/desire to hack something better up.

[Mario]

-A 7808 requires at least 10.2v under most situations.

This.