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I seems my TFT monitor is bust. I have a PSU for another TFT which uses the same voltage (12v) but different amps (3.5 v 4.5). Can I try the PSU without risking blowing up a PSU and have two duff TFT's?
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Yes you can, as long as the input voltage (potential) on both units is rated at 12 Volts input as you said yours is.
The amps ( or current draw) will depend on the design of the internal circuits.
And each unit will draw different amps or current ratings depending on circuit design.
The different current draw (amps) is not a problem. This is valid on direct current circuits (DC) and alternating current circuits (AC)
Last edited by ed klein; 8 Sep 2006, 06:08 AM. -
Hi!
The monitor, or anything attached to the PSU, will only draw as much current it needs. You should be able to use another PSU to power the monitor, as long as the PSU is rated higher in current capacity than the load it is attached to. Unless there is something shorted out in the TFT monitor, it should not draw any more current than it is rated at, thus you can safely use a PSU with a higher current capacity.
Common failure on LCD monitors is the backlight. Could be a connection problem, or its internal protection fuse opened up, probably from a voltage surge, or simple fatique of the protection fuse. I have only seen one LCD monitor have a catastrophic failure, but it was due to a hanging plant right over the user's computer desk......when the caretaker of the home watered the plants, the overflow bucket below the plant filled up and some dripped onto the back of the LCD monitor, wiping out the power supply and the drive unit that powers the LCD monitor's backlight, thus no picture on screen.
If you can see a faint light on the entire screen when powering on the LCD monitor, the backlight is ok, and the problem will be within the LCD monitors matrix encoder board that drives all those millions of LCD emitters embedded within the screen.
If you do not see any faint light at power on, the backlight is either bad or its drive circuits are bad or its protection fuse is open, which those are usually soldered onto the pc board, commonly called "pico-fuses". Typical amperage rating on those is about 2 to 2.5 amps.
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If you try them in parallel off of the same PSU it will probably fry the PSU. You can get a cheap Wall Wart at Radio Shack that has variable Voltage to use as a test PSU. If you use the 4.5 PSU on the 3.5 LCD monitor if will probably only draw what it needs so you should be safe. The 4.5 LCD on the 3.5 PSU probably wouldn't power it up but it might (they're usually overpowered for the job they do. Just be extra careful of the polarity. The PSU may also produce a very low ripple voltage so that is another consideration as you may get interference. You should put a voltage meter on the outputs of the PSU'S to see if there is any voltage there. If you don't have a volt/ohm meter don't do any of the above.Comment
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The monitor is receiving the signal ok from the computer. The standby power light flashes on and off when no signal is found. It is on ok. The screen is just black and looks like it is switched off which sounds like the backlight.
It cost me £500 a few years ago.
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Some ofthose older monitor were designed to have the backlight replaced. I would check on that. I would definately try to see if their is voltage present at the headers pins that the backlight connects to but It is really easy to screw up these things and being an electronic technician I have a lot of tools others don't have (You'd have to check it powered up) . They have come down quite a big in price and I would bet a repair might be close to the price of a new one that would probably have higher resolution. It is hard to say not knowing the model LCD you have.Comment
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I agree that it is probably the same price to reapir than buy a new one. I could upgrade to a Acer 19" widescreen which I like for just £140. Another regular l17" can be bought for about £110.Comment
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Try to get one that doesn't weigh too much, I know you Brit's have to pay by the pound! Argh Argh Argh
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Let's see a oz. is 28 grams, kilo means a 1000 so 1000 divided by 28 gives you 35.71 ounces which is a prety good deal, you should be able to get that monitor for a couple of pounds!!!!Comment
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Hi again!
Be extremely carefull if you are going to probe the leads to the backlight to check for voltage. Here is why....
...the backlights are just smaller versions of flourescent lights, which require a ballast to fire. Typical voltages across the backlight in an LCD monitor is around 200vdc, which is produced by the backlight driver board. Make sure your meter is set for 200v or higher to prevent damage to your meter.
The driver board is basically a DC to DC converter, taking the 12vdc and switching it to about 120khz AC, then converts it up to the required drive voltage depending on what it is driving. It is then rectified to the high DC voltage using fast switching diodes and filtered with high tolorance electrolytic capacitors to smooth out the switched DC output. Watch out for stored energy in those filter caps too!!!
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I am bot going to start messing around with it.Comment
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I was hoping you'd ( RFBurns) weigh in on this. I wonder if the how the Caps hold out on these? I think most repairs I have run accross on the forum are beyond the abilities of fellow members (based on what I have read) but I always enjoy reading your comments as they seem to based on real world experience which is great. If there was a Rep system I would definately have thrown you points. I imagine there isn't too much current there but a slip on the probes would be fun to watch!!Comment
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@rf.your electronics advice is good pal keep it up you to cats.Comment
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LOL!!! Indeed. Although there is only a few milliamps of current produced by the ballast driver board, the voltage potential is still something to be carefull about. If your test leads when applied to the backlight connections get too close to each other, you get a small "St. Elmo's Fire" effect, and you can even smell it when it does this...smells like electrical fumes.
The filter caps, which can be identified by their voltage rating, usually between 200vdc and 400vdc, can and will store the voltage produced by the driver board. The reason why they can store that energy even when powered down and unplugged from any source, is because the backlight really is not a load, or resistive load, it is basically an open circuit when probed with a continuity meter. What makes it produce light is the reaction of voltage on each end of the light tube, plus the initial "fire" voltage, which causes the gasses inside the light tube to react, and produce light. The "fire" voltage lead is that 3rd wire connected to a small strap wrapped around the center or near center of the length of the lamp tube.
Be very carefull with that "fire" voltage lead!!!! It has the most voltage potential out of the entire unit. Typical fire voltage is around 300vac or slightly higher.
If you replace the monitor, and if the driver board is bad, you can experiment with the old monitor and maybe install another backlight approach. It might not produce as bright of a pix, but the new super bright LED's, assembled in an array within the reflectors around the old lamp tube can produce some interesting results. If the LED's are placed just right and the picture brightness is good for your uses, the end result is an LCD monitor that draws 1/10th the electrical power than it did with the ballast driven backlight tube.
If you suspect the filter capacitors to still have a charge on them, use a 10 ohm 1/2 watt resistor across the capacitor leads to "slow" discharge. This will prevent what is called a surge discharge, which will preserve the capacitor. If you just short the leads together or use some conductor that has no resistance, it can ruin the filter cap, short it out and when you turn on the unit again, the chances are very good that the power supply or driver board will also go...POOF!!
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