Puzzling Power Problem

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theinfamousbob

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(sorry for the bad title, but I'm a bit tired.)

Today I picked up a LCD screen for an overhead projector -- it's transmissive so light can pass through projected on a wall/screen/etc. Basically a poor man's digital projector. Did some research; it's capable of 800x600 @ 32 bit color. Pretty decent. Then I got an idea -- use it inside my computer case, where I can see the display while having a backdrop of computer components. Sat down, ripped the LCD and driver board out of the case, tested -- worked fine on supplied adapter. Fitted the LCD to the side of my case + glass, tested LCD with supplied adapter -- worked fine. Solder a +12V line and GND from my computer's PSU connector onto the board in place of the supplied +12V adapter -- no luck. There's a 7805 on the driver that overheats and goes into thermal protect. I can't figure out why the LCD just doesn't work off of my computer supply opposed to the supplied PSU. Both read within a hundredth of a volt (12.24V). Is it that the PSU brick that came with the LCD drops voltage when something is connected more so than my computer's supply? This one's got me stumped. Any ideas?

Thanks!
 
Hi,
Supplied adapter appears cheaper and the dropped voltage helps 7805 to have in-out differential less then when you use computer PSU. the 12V remains so, the the 5v Regulator heats up. because of almost 7V across input and output of the regulator. perhaps if you pre regulate to 9V and then supply to LCD it might work


One way it is better that you save the power of computer by using external adopter for the LCD part.
 

That's kinda what I was thinking, but I thought that 7805s were good up past 20v. I'll look in to regulating power beforehand, but the LCD might need a 12v supply to work properly.

And as for power coming from my computer, that's not a problem. I invested in a system that can put out 20A of current on two 12V lines (40A total 12V).

Thanks, and I'll see if I can post some pics of the board to make things a little more clear.
 
theinfamousbob said:
That's kinda what I was thinking, but I thought that 7805s were good up past 20v. I'll look in to regulating power beforehand, but the LCD might need a 12v supply to work properly.
Click to expand...

They are, with an adequate heat sink--that's the small print on that feature. If you know the voltage being dropped over the 7805 and the amps being pulled, you can work out the number of watts of power being dissipated, which will let you work out the temperature rise. I forget the formulas at the moment but you can google them easily.

And as for power coming from my computer, that's not a problem. I invested in a system that can put out 20A of current on two 12V lines (40A total 12V).

Thanks, and I'll see if I can post some pics of the board to make things a little more clear.
Click to expand...

It sounds to me like sarma is on the right track (note: I'm just a hobbyist; not a pro like many around here). The supplied adaptor probably doesn't supply a full 12 V under the load presented by the LCD panel. Maybe it sags to say 9V--then the 7805 only has to drop 4V. The computer supply is nice and beefy ("stiff"), which means it doesn't sag under that same load, and so the 7805 gets all 12 V under load, and needs to drop the full 7 V. The related heat dissipation could be tripping the 7805's thermal shutdown.

I think your computer supply should have a regulated +5V lead you could use and skip the 7805 entirely, but get some confirmation on that from someone without so much lager in them before hooking it up and cooking something.


Sounds like a neat project. Good luck!

Torben
 
theinfamousbob said:
(sorry for the bad title, but I'm a bit tired.)

<snip> ..... tested LCD with supplied adapter -- worked fine. Solder a +12V line and GND from my computer's PSU connector onto the board in place of the supplied +12V adapter -- no luck. ....<snip>
Click to expand...


Using the display's own power supply, measure the voltage between the PC's ground and the display's adapter's negative lead. If you get more than a few milivolts you can't connect them together.

The display may use an split supply, and so the display's power "-v" is not ground.

In that case you can't use the PC's supply to power the display, unless you add an insulating DC/DC converter to "float" the display.
 

When I read your post I did a "D'oh!" and thought about how some LCDs need a V- supply. No luck, though. Difference between the two GND lines was between .4 - 1 mV. I'm guessing it's just the voltage drop of the circuit.

EDIT: Just checked the voltage under load, it drops to 7.22 volts. I can use the computer's +12 line as the +V supply and the 5v line as ground. It should work out to 7.2 volts because my computer +12 supply is a hair over 12v.

EDIT 2: Turns out the board has a switching regulator on it to provide 3.3 and 5v, but not much 5v. I've never worked with a switching power supply before, so where does that put me?
 
Last edited:
Hi,

Just a thought. If the lcd uses a 7805 to drop the incoming power to 5v, can't you just power it with a +5v supply from your pc's psu? the feed to floppy, HDD and CDROM all have a +5v feed.
 

i have a doubt, whether the poer supply that came along with LCD display is not low voltage AC instead of DC and it is managed inside the LCD panel.-- perhaps your LCD also warrents -ve voltage? and when you think of extending the 12DC there, it may not abe able too produce _ve voltages.--
this case may perhaps be comparable to external modems of yesterdays, with used to come with a step down transformer.

Having said all this may i ask for make and model number of the LCD display device that you bought?

Sarma
 
I knew someone who did this using an old laptop and overhead projector he purchased from eBay. It worked surprisingly well too.
 

I know it's a Sharp LCD, but I'll have to open my computer (now a daunting task) to grab the model number. I know it's a DC supply, and it reads 12v. I pulled the LCD out of an old overhead projector adapter to make it a digital computer projector. It came from an nView Viewframe Spectra Pro.

The LCD has no built-in backlight or backing, so I had to create one (see pics in previous post). My guess is that the 12V computer line is just too much for the switching regulator or something similar.

I could try wiring in the 5V line in place of the 7805, but I'm still a little nervous. I'll have to test it in an older computer instead of my nice one.

Thanks!
 
I understand your problem. but i don't find any obvious reason that the LCD SHARP model accepts 12V from a wall adopter and NOT from a 12 V battery of laptop or its adopter? genrally lap tops working of around 18V battery and when adopter is parallely connected, the voltage may be around 18+2 =20V.
the LCD panel may have a high voltage sense, and thus perhaps refusing to accept the external supply from the laptop. this is of course wild imagination - so that you may analyze and act on situation
regards
Sarma
 
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Thanks for all your help! I'm going to see if I can dismantle and poke around the transformer. Hopefully that'll shed some light on this.

Thanks again!
 
Fixed it. Don't ask me why, but when I cut a power cord that fit the jack, stripped it and used a computer +12 supply, the thing worked magically. I'm guessing that there was something wrong with just soldering to the board; maybe a missed PCB layer or something.

Plus, I salvaged a backlight almost the same size out of an old laptop!


Thanks for your help everyone, will post pics as soon as it's all done!

-Infamous
 
Hai, CONGRATS for the success

that is nice and you are now able to feed from laptop bty.

all the best

Sarma
 
Thanks for all your help. I ended up having a weird ground loop feeding thru the LCD, but I fixed it by ripping everything apart and insulating it from my case.

The screen still needs color calibration, but it's ok for right now:

https://hs.facebook.com/album.php?aid=2001162&l=354a6&id=1436340228

After redoing it all, I plugged it in and nothing happened -- the MultiWatt 5 (case) switching power IC had snapped off of itself, so I had to resolder it, works fine now.

All in all, it was quite fun, but a bit challenging. Next time I'd just buy a normal LCD and a bigger case.
 
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