How could I install a new 12 volt backlight?

Question

I have a 1080p monitor that I really really liked, until its backlight started being a butthead. So, I decided to take out the old backlight, and stick in an aftermarket one. I thought it would be nice and simple, until I realized that the LED driver ran on 12 volts and there was no 12 volt output anywhere on the monitor (the original LED was driven by 80 volts).
I have confirmed that the backlight works by jump-wiring it to a PC power supply, but I need a somewhat space-saving method of driving it within the monitor case using its own power supply so that it turns on and off with the monitor. I would like to just solder a 12 volt regulator directly onto the 80 volt output, but I don't know if there is a small chip capable of providing sufficient power or stepping down that much voltage. Any ideas?

mudz · Accepted Answer

Again, using it wrong,
6th pin is Base, 5th one is collector, 4th one is emitter.
connect 5th pin to 12 v in series with LED lights. and connect emitter to ground using a resistor.

Moreover, as you might have applied much voltage to the base do a quick check if opto-coupler is already damaged/not damaged.

mudz

mcb1 · Answer

An optocoupler (as you discovered when you seardhed it) provides isolation between sides (LED) and the other (Transistor/Tirac/SCR). They come in various forms and some are much more sensitive than others, but the important part is the LED side can be driven by almost any voltage with a few milliamps. When the LED illuminates (internally) it causes the transistor/Triac/SCR to conduct, which can then switch a different voltage which may not be even common to the LED side. The nearest equivalent is a relay, which usually require 5 volts and 50-100mA. So the answer is NO but it is the simpliest and cheapest solution in your application. Mark

mcb1 · Answer

bwarren97

Sorry but work got in the way of element14 community.

Mudz is correct, your connections are incorrect.

12v goes to Pin 5 (collector), enable goes to Pin 4 (emitter).

Generally the base can be left unconnected, but if this causes issues, tying it to the Emitter (pin4) with a 1M or higher generally resolves it.

One of the chips, I have determined, has a broken transistor side considering the resistance between the base and the collector was absolutely zip

With 12v (via LED) applied to the base and a 100 ohm on the emitter it will have drawn 105mA or so, and will therefore provide an output.

There is no spec for the maximum base-emitter current so maybe it is damaged or maybe it isn't.

BTW the LED on the 12v side is unlikely to illuminate if you enable pin only draws 1.4mA.

Mark

mcb1 · Answer

I fixed quite a few LCD monitors. Most of the problem stems from capacitors that have dried out due to age (many are rated for a year or two). Have you tried replacing the capacitors in the original backlight driver board.?? It's usually a few dollars to get something with 2-3 years life expectancy. Most monitors have a AC to DC converter, which then has a DC-DC converter and some regulators for the lower voltages. You'll probably need an external supply, since the DC-DC converter is unlikely to have another 4-5 watts at 12v. You could use an optocoupler to drive the enable pin, which means you don't have to mess with power from different sources. ie connect the LED side through a resistor to the monitor 5v, and the transistor side to the 12v and enable pin. Mark

mcb1 · Answer

bwarren97 Blake A quick check of your connection (well my imagination of it) is 5v through 1000 ohms into the LED on the Optocoupler and then out to a second LED between the Opto and ground. The forward voltage of the LED is 1.2-1.8v so 2x1.8 = 3.6v. With 5v driving these the voltage available is 1.4v and the current flowing through the 1k resistor will be 1.4mA. I suspect this is too low to trigger the Optocoupler. I suggest you change the resistor from 1k down to 150 ohms. (R= E / I = 1.4 / .008 = 175 ohms.) This might give some change on the Output. You will need some form of load so add a 1-2K. Mark

bwarren97 · Answer

Yes, my power supply turns on, and yes, the LED turns on. I have jotted down a crude schematic of what I currently have connected. This is the FOD3184 chip. One interesting thing I noted here is that operating the LED generated slight negative voltage on the ground of one side, but not the other. There was no voltage being output by the sensor side of the coupler at all, as when I disconnected it, I got 0 on the output. Here's my schematic with my voltage and amperage readings, and the link to the full-resolution http://puu.sh/mMi2D/d4665b25e5.jpg

How could I install a new 12 volt backlight?

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