How much power can I take from a desktop monitor power supply board? - Page 1
Forum section: electronics / projects

author: seanspotatobusiness
author_rank: Regular Contributor
timestamp: July 13, 2017, 11:51:28 am
content: (Illustrations below)I'm using a collection of broken laptop and desktop monitors I got free from a repair workshop to assemble a floodlight/spotlight from the CCFLs and their inverters. The desktop monitor power supplies take mains voltage (240 V) and power the CCFLs but also seem to provide a 5 V and 12 V supply to the electronics that control the LCD. The laptop inverters themselves accept either 5 V in some cases or 12 V in others so naturally I want to run the laptop inverters off the 12 V and 5 V supply because otherwise it's wasted. The problem is that I don't know how much power I can draw from the 5 V and 12 V of the power supply.I've had a look at the back of a monitor that says it draws 0.7 A at 240 V. What translates to 168 W which is more than I was expecting. Assuming that the four built-in CCFLs use 8 W each, that would still leave 136 W. Assuming the power supply is only 60% efficient that leaves (168 W x 60%) - (4 x 8 W) = ~68 W. I've no way to tell how that's distributed across the 12 V and 5 V but if it's 34 W each then I can draw 2.8 A from 12 V and 6.8 A from 5 V?I figure (guess) I can probably, maybe attach eight laptop inverters to each desktop PSU (four on 12 V (1.6 A) and four on 5 V (4 A) with only 5% risk of fire and death?My illustrations may help this make more sense.

author: alm
author_rank: Super Contributor
timestamp: July 13, 2017, 12:08:26 pm
content: 0.7 A is probably a peak value, and the power factor may be far from 1 (which means not all current gets converted to usable power). I think 68W is still high. Depending on the size, I would expect the monitors to draw maybe 40-50W? Then multiply by efficiency and subtract inverter power draw. No idea how this would be distributed between 5 V and 12 V. If some in working condition, then you can use one of those cheap kill-a-watt type power meters to estimate AC power draw.

author: seanspotatobusiness
author_rank: Regular Contributor
timestamp: July 14, 2017, 11:14:15 pm
content: I figure I can take at least two amps from 5 V from the monitor with four USB sockets. If I measure the current drawn through each of the 12 V and 5 V wires whilst switched on I can add this to the 2 A and I'll know I can at least take that much.

author: mariush
author_rank: Super Contributor
timestamp: July 15, 2017, 05:15:30 am
content: My two 24" monitors (Samsung T240 and 2494HM or something that sounds like that) with plain ccfl lightning use around 40-50w in total ..using maybe 75% brightnessMy guess is most have around 10w for 5v and 3.3v for the tiny board with the actual logic. Some have 12v at something like 10-15w for audio amplifier for monitors with speakers.I wouldn't give more than 20-30w for backlight circuit (usually either around 14-15v or around 20v on most boards i fixed in the past)You could tap into the voltage for the backlight by soldering wires to the leads of one of those two capacitors by the high frequency transformers. For safety you could remove those two transformers and use the psu as dc power supply to power LEDs (make spotlights or whatever)You'd still need led driver or at least a current limiting resistor for the leds but that's easy