Powering a LCD car monitor

[BerserkOlaf]

Hello everyone,

Sorry, complete newbie for hardware here, I need advice for powering and connecting a LCD car monitor.
I've got this : display
And I'm trying to power it with this : power supply

There is very little information included with the screen, only basic instructions on how to connect it to your car (well, that's how you're supposed to use it I guess).
The power unit is on 12V setting, since that's apparently what cars supply (is that right?).

I think I have done something wrong, since after a while trying to make it work I found that a fuse (1A 250V) in the screen power cable had blown. What do you think could have caused this? How would you try to fix it?

[joan]

The screen is 12V (according to the specs). I think the centre pole should be positive. Could you have attached the power the wrong way around?

[BerserkOlaf]

That may be the case, actually. Would that be a possible cause?

[PS1981]

I have a similar screen (to be honest I think the same one), which I tested using my laboratory DC power supply. It switches on at around 7.5 volts, and I tested it up to the standard 12v. It seems to imply to me that it's actually running with 5 volts, supplied internally from a voltage regulator like 7805 or similar. If you run with a lower power however, more amperage is taken to supply the power needed. This could cause the Amperage to rise above that which the fuse is made for. IMy screen requires less than 200mA though, which is far from the 1A you stated.

Joan is right (at least in my case) the center pin of the power socket is positive.

[Tasty_PI]

As I recall there was a thread that converted these from 12v to 5v by removing a component. Im sure there will be relevant info in there for you. If I could find the thread :/

[BerserkOlaf]

So I may be able to power both the Pi and the screen directly applying 5V current? That could prove useful indeed.

For now I'm just going to try again and be more careful with polarity. If there is no video input, is there some hint on screen to show it's on, like a "no signal" message?

[joan]

I didn't think mine worked initially as without a signal the screen was blank (different model to yours). Yours may have menu buttons on the back side.

[PS1981]

Mine has AV1 and AV2, which is displayed in green for a few seconds on startup.

[BerserkOlaf]

The model I have has 3 buttons on the back, labeled "menu", "+" and "-".

[PS1981]

I have these too. When the screen is on, if I press the middle one I get a display for Brightness, press again I get Contrast, Press agein I get Language selection etc. The other 2 do + and -. Did you manage to get anything showing on your display? The green writing I get only comes when I've got something plugged into it, camera on yellow plug (AV2) or Pi on white plug (AV1, HDMI must be unplugged otherwise the pi defaults to that).

[BerserkOlaf]

Yes, I have just changed the fuse and checked polarity (center is positive as Joan and you said).
At first I thought it didn't work, but it seems there's just some loose contact problem somewhere with my RCA connection.

I've got the menu with brightness/contrast/colour now, but only with video input from the Pi working. When there is no signal, the screen stays completely black.

Anyway, this works now! Thank you for your help.

[AndyA]

So I may be able to power both the Pi and the screen directly applying 5V current? That could prove useful indeed.
For now I'm just going to try again and be more careful with polarity.

This isn't the thread you're looking for... http://www.raspberrypi.org/phpBB3/viewt ... 64&t=17651

I recently got a 4.3" display that looks much like yours and did the conversion, look right at the (current) end of the thread. However these cheap displays seem to have lots of different versions of internals in otherwise identical housings, they've used whatever was cheapest that week, so your mileage may vary.

The generic solution seems to be:
1) find the switching regulator, normally a small 8 pin part near the power input with lots of inductors and capacitors near it.
2) Use a voltmeter to find the 5V rail, normally easily accessible on a capacitor or inductor near the switching regulator but sometimes there's an even easier to use non-loaded header.
3) remove the regulator IC. Wire from the power input connector to the 5V rail.

But there may be some board variations where that isn't the case, it's hard to be sure.

I've build a few of these up now and have started removing F3 from the Pi (normally green, on the back of the Pi under the power input socket) and replacing it with a wire. Removing the input fuse isn't something I'd recommend everyone do but it does allow me to then power the display from the 5V and GND pins of the expansion header without worrying about going over the input current limit.
On one unit I've even soldered the display input cable directly to the Pi power, ground and video out signals, no need for the bulky video out plug that way and you end up with a very compact Pi and Display unit.

If there is no video input, is there some hint on screen to show it's on, like a "no signal" message?

Nope, that would be far too useful. You may get the back light flash on and off or a brief white flicker when first powered up but that's all and probably depends on the exact version of the board you have.