Powering your LCD Monitor

Scouse Monkey · 04-19-2005, 12:36 PM

This thread is for notes, schematics, guidance and considerations for people to use when powering their LCD.

Please do not discuss things and by no means go Off-Topic!

First contribution:

*Powering a 12V rated CCFL inverter off a 12.8V battery could actually cause an output current increase of 5%. This is turn could reduce the life of the CCFL by up to 10%

*A voltage spike on a DC-AC converter can cause flickering and damage to the lamp and inverter

(ref. Design Considerations for LCD Power Supplies, ERG Inc.)

THIS POST IS SUPPOSED TO BE FOR FACTS ONLY TO BE USED AS REFERENCE PLEASE REMOVE ALL QUESTIONS AND PLEASE ELABORATE ON SOLUTIONS IF YOU CAN!

electron · 04-19-2005, 01:20 PM

I took one of my old, defective cpu fans, which came with a pass-through molex connector already, and disconnected the wires going to the fan. I then connected these 2 wires to a female cigarette lighter adapter, which I purchased from Wal*Mart for a few bucks, and I now have an easy way of connecting my LCD without having to cut the original cable which came with the LCD. Since I am using a pass-through molex connector, I also don't have to sacrifice a molex power source.

Warning: Make sure your power supply can handle the LCD before permanently hiding the wiring (I have no problems doing this using my 90W Opus), and make sure you verify that you are tapping the 12+V rail (most cpu fans should be using the 12V rail, yellow and black wire) to avoid any damages.

shotgunefx · 04-19-2005, 02:13 PM

autospeed.com had a nice article on replacing the lilliput ps with a 7812 and a zener. I'll see if I can find it.

UPDATE
I've attached the 7812 based circuit.

First off, I am by no means an electrical expert. Hell, I just know enough to be dangerous. Use at your own risk. You should test the completed circuit with a multimeter before hooking up anything important.

Having said that, I've built this circuit (minus the zener) for other projects and it's always worked fine. Your mileage may vary

Outside of the Zener, everything should be available at Radio Shack.
1) 16v zener diode
1) 7812 regulator
1) 10 uf cap
1) 100 uf cap
1) 5W 1 Ohm resistor
1) Heat sink, bigger is better

Scouse Monkey · 04-19-2005, 04:21 PM

POWERING YOUR SCREEN OFF YOUR PC POWER SUPPLY:

Although certain screens may be quoted as being able to run off 12-24V it can be seen that there is a risk of prematurely killing your CCFL (if not the rest of the components) as the CCFL quite often goes brighter when the screen is connected direct to a car battery (>14V) compared to a 12V supply. This shows there is no or poor regualtion of the input voltage which may cause damage. It shouldn't do as the manufacturers should be underating the inverterif this is the case but regardless your car is not a clean 14V supply and has a lot of spikes and noise - especially during cranking.

It has been widely accepted that the best way to preserve your LCDs health and also a quick and easy way to make sure it turns off and doesnt drain your battery (even in standby it will drain your battery - all be it slowly) is to power it off the 12V line onf your PCs PSU.

On a molex conenctor this is the Yellow wire (12V +ve) and you also want to use the adjacent black wire for -ve.

If you power your monitor in this way you do not have to use the 12V regualtor that comes with it - doing so as well may introduce noise and also cause your screen to be dimmer due to a voltage drop. It is also one more thing to go wrong.

The power of most 250-400nit 7" LCD monitors, and indeed 8" models, is about 10W which is about 1A on your 12V line so make sure you have enough power for it.

If you don't you can use a cheap and simple 1-1.5A 12V DC-DC regulator such as those that come with some screens or ones you find to power portable DVD players. 1200mA models are quite common. These can normally be connected to your ACC line (switched 12V line your HU) as the power is low or you can connect them direct to the battery via a relay that is powered by your PCs PSU or the ACC line. The ACC line or relay and ACC line may be prefered so your screen does not stay on after you have left the car and the Pc is shutting down.

Some of the DC-DC regualtors that come with the screens may be okay to use - you don't have to go out and buy a new one but if possible check the voltages coming from your supply while the engine is running and also check for the screen going brighter when the engine is revved.

davesaudio2 · 04-21-2005, 09:10 PM

Always parallel electrolytics with .1uf or .01uf ceramics on 78xx circuits to prevent oscillations.
The 7812 circuit looks safe enough but when the car is off there is no regulation
-only 2ish volts dropped across the regulator and some across the resistor.
Probably seeing 10-11 volts at your load.
An LDO regulator may be a better choice if you don't want a switching supply

DJSmiley · 04-22-2005, 06:30 PM

Quote: Originally Posted by shotgunefx

autospeed.com had a nice article on replacing the lilliput ps with a 7812 and a zener. I'll see if I can find it.

UPDATE
I've attached the 7812 based circuit.

First off, I am by no means an electrical expert. Hell, I just know enough to be dangerous. Use at your own risk. You should test the completed circuit with a multimeter before hooking up anything important.

Having said that, I've built this circuit (minus the zener) for other projects and it's always worked fine. Your mileage may vary

Outside of the Zener, everything should be available at Radio Shack.
1) 16v zener diode
1) 7812 regulator
1) 10 uf cap
1) 100 uf cap
1) 5W 1 Ohm resistor
1) Heat sink, bigger is better

Some notes:
1. A regular 7812 only allows up to 1A output. For more output, look for a 78S12, that one is able to handle 2A.
2. Heatsink IS requires, bigger = better, ok, but in my case, an old CPU fan does the job perfectly.
3. The capacitor on the input is almost useless, since its directly on the battery. To give it a better use I suggest adding a diode before it, so it will only buffer the 7812, and not your lights/hu/and so on. However, adding a diode causes a decrease of the voltage with about 0.7V.

More important: For good regulation the 7812 requires an input voltage of about 2-3V above the desired output voltage. So its very doubtfull if this will work in the car, since the car voltage IS 12v, and, during drive, about 14V...
Better use a low-drop regulator, since they require less input.

I suggest using 12V from the pc powersupply, or, in my case (i am using an old laptop) connecting the 7812 to the 16-19V output (which my laptop requires)

Or a good switching regulator, saves the heating troubles also

shotgunefx · 04-22-2005, 10:09 PM

Quote: Originally Posted by DJSmiley

Some notes:
1. A regular 7812 only allows up to 1A output. For more output, look for a 78S12, that one is able to handle 2A.
2. Heatsink IS requires, bigger = better, ok, but in my case, an old CPU fan does the job perfectly.
3. The capacitor on the input is almost useless, since its directly on the battery. To give it a better use I suggest adding a diode before it, so it will only buffer the 7812, and not your lights/hu/and so on. However, adding a diode causes a decrease of the voltage with about 0.7V.

More important: For good regulation the 7812 requires an input voltage of about 2-3V above the desired output voltage. So its very doubtfull if this will work in the car, since the car voltage IS 12v, and, during drive, about 14V...
Better use a low-drop regulator, since they require less input.

It does need 2 volts to work optimally, but most LCDs, (Xenarc and Lilliput anyway) say they work with 11v and up so 11.5~12v from the 7812 should be fine. As far as the 78S12, there is anything wrong with using it,, but a 7812 is easy to pick up locally, (ie Radioshack)

It would almost certainly work.I'm not powering my lcd yet with one, but my lighting system uses three of them and I've got a perfect 12v delivering about 1.5 amps through each (I got a 7812 with a slightly higher current capacity).

What would a diode accomplish on the input?

davesaudio2 · 04-22-2005, 10:37 PM

What would a diode accomplish on the input?
It would isolate short voltage dips/spikes.(think mini-tank)
IN5819 is a 1 amp Shottky diode dropping .3v ish. don't have a 3amp part num offhand
D

shotgunefx · 04-23-2005, 05:09 AM

Quote: Originally Posted by davesaudio2

What would a diode accomplish on the input?
It would isolate short voltage dips/spikes.(think mini-tank)
IN5819 is a 1 amp Shottky diode dropping .3v ish. don't have a 3amp part num offhand
D

First off, just want to state again, that I am by no means an expert on these things. But I don't think the diode wouldn't do anything but needlessly drop some voltage and prevent reverse current which should never happen in this particular situation. (I suppose it could if you wired it backwards, but I don't think the regulator would do anything, though wouldn't want to try that theory on my touchscreen)

From my understanding Shottkys are used either for recifying AC due to the quick recovery time or in place of a regular diode where a low drop is desired.

For limiting voltage, Zener diodes are what are normally used. That's what the 16v Zener in the above circuit is for. It provides protection against surges.

I also think that if a diode there would do anything helpful, it would be in one of the circuits in the datasheet or mentioned in the appnotes.

Gobby · 04-23-2005, 05:31 AM

I'm running my LinITX from my ITPS and everything else from the Carnetix, works perfectly, saves using power from my Shutdown controller and both shutdown at the same time. I've also got the LinITX screen to Auto-On as well now using a relay attached to the ignition on, thanks mutilator for the idea.

evandude · 04-23-2005, 08:17 AM

Quote: Originally Posted by shotgunefx

But I don't think the diode wouldn't do anything but needlessly drop some voltage and prevent reverse current which should never happen in this particular situation.

I would just like to note that reverse current CAN flow through a regulator (at least the basic ones, like the 7812, fancier ones might not) I have made that mistake before. I connected a big capacitor AFTER a 5v regulator, thinking that even if the input voltage fell for a short time, the output should stay okay because if no reverse current flowed, the cap would maintain the output voltage for a short time. However, in practice, I discovered that is wrong... the voltage on the output drooped right with the input... I put a diode in series with the regulator input and everything was fine, since no reverse current could flow then.

Also, as people have been saying, find an LDO... you can find 1.5A and 3A LDO's relatively easily, for $1-2 and it will greatly improve your minimum input voltage. Add a schottky diode on the input and you should be good to go...

Juliemac · 05-22-2005, 10:05 PM

A series diode is not needed on the input. Place a diode across the top of the regulator (arrow points to the input) so that the output cap can discharge across that diode when the input drops.
Use some thing like an In4001, a 1a diode. Will work every time.

Juliemac · 05-22-2005, 10:10 PM

Quote: Originally Posted by shotgunefx

autospeed.com had a nice article on replacing the lilliput ps with a 7812 and a zener. I'll see if I can find it.

UPDATE
I've attached the 7812 based circuit.

First off, I am by no means an electrical expert. Hell, I just know enough to be dangerous. Use at your own risk. You should test the completed circuit with a multimeter before hooking up anything important.

Having said that, I've built this circuit (minus the zener) for other projects and it's always worked fine. Your mileage may vary

Outside of the Zener, everything should be available at Radio Shack.
1) 16v zener diode
1) 7812 regulator
1) 10 uf cap
1) 100 uf cap
1) 5W 1 Ohm resistor
1) Heat sink, bigger is better

With the above diagram (minus the resistor and a parallel diode from output to input) use the LM 1084 LDO 12V regulator. Works on my system very well.

Speg · 07-13-2005, 01:52 PM

YOu guys are brilliant!!! My screen was crapping out during the day, so I hooked it to my OPUS, and today, not a single crap (from the screen anyway

)

Thanks once again guys!!!

SPUNOUTKID · 07-21-2005, 02:20 AM

Help me please!!! LOL i finally got my whole system fixed up (year in the making) booted it all and was driving. everything was fine when smoke started rollin out my dash. the power wire to my lilliput touchscreen - the shield was just melting off of it. I am a newb to this type issue - how can i fix this ?? do i need a dedicated circuit from car battery or make a new type of power cable for it or what??? thanx all

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