Does this laptop psu work(?) and alternative use.(4 questions) - Page 1
Forum section: electronics / repair

author: fagouma
author_rank: Contributor
timestamp: November 28, 2017, 08:19:25 am
content: Hello to the community.I have 4 questions regarding a laptop power supply found in my uni's electronics dumpster where i make everyday dives.On its label it says that it's rated at 19.5V and 3.34A .1. Please watch the shared video from my Dropbox, showing you, the output voltage measured from my cheap multimeter.Is it working properly? I have no prior experience with laptop psu. It's not a stable output voltage but ratherjumping around the rated output voltage.https://www.dropbox.com/s/urbh0la70cagx3w/video-1511783244.mp4?dl=02. The power supply has a small smd capacitor between the output terminals. What is the purpose of that? Smoothing the output voltage ?Can i change it, maybe, with a THT or smd of another value with better results?3. My girlfriend moved to a new house last month and the previous guy had left an LG LED pc monitor laying around witha broken base. But without its power supply. So i want to use the above power supply for that purpose. The monitor is rated for 19Vand 1.2A . If the power supply is working properly how i can lower the output voltage to 19V and use it to test the monitor? Is itpossible to do it with resistors and what circuit should i use?4. Is it possible to find schematics for such power supply units? There is a plethora of them.Thank you in advance and waiting for you suggestions.

author: Armadillo
author_rank: Super Contributor
timestamp: November 28, 2017, 08:55:04 am
content: I don't think you conveniently have a DC load bank,so we will use the repairman technique, quick and simple,you use the car back lamp or the brake lamp, you know the ~20W incandescent light bulb,then you hook it to the output as load,then if the light bulb is lighted up brightly, you measure that output voltage,it should not be jumping around but may drop to somewhere 18.5~19V.If so, it is a working psu and you can safely plug to the monitor without any harm.hope it helps;

author: fagouma
author_rank: Contributor
timestamp: November 28, 2017, 10:00:21 am
content: Thank you for your answer.QuoteIf i understand this right, you mean that i don't have sth connected to the psu. Yeah, that's right. I was measuring the voltage without connecting the charger somewhere.Should i connect the lamp parallel or in series in the positive/negative lead and then close the circuit? I know this is a stupid question but i am a noob and self teaching. I don't study electronics.Isn't it easier to do it with a resistor? Or the heat would be too much? If yes, how i should connected it?Edit: The car lights work at 12volts. Won't the brake lamp burn out or be too hot ? Lets say the monitor works and i want a permanent solution. I can't leave a flaming lamp behind the monitor.

author: Armadillo
author_rank: Super Contributor
timestamp: November 28, 2017, 10:20:39 am
content: The idea is load testing the psu as what any commercial repairmen will do.You can test with a resistor. But it must be large enough to draw the amps.Say from 10 to 30 ohms and from 36 watts to 12 watts respectively.or something reasonable you have on hands. At most burn one cheap resistor.You wire and measure in parallel.+ve lead to one end, and negative lead to one end and also measure the voltage in parallel.However, How do you know that the monitor is working properly in the first place.?If you know it is working, then I think it is safe to plug the psu in to test it directly, the most is the monitor don't display anything.

author: fagouma
author_rank: Contributor
timestamp: November 28, 2017, 10:35:55 am
content: QuoteI don't know if it's working. That's why i need to lower the voltage of the psu to use it with the monitor. The psu outputs 19.5 volts and the monitor accepts 19V. I didn't do it earlier cuz i am afraid not to burn something in monitor's board.Or the above is nonsense? Maybe what i should have asked is, if it's ok to plug a 19.5volt psu to a 19volt monitor. Is it ok?

author: Armadillo
author_rank: Super Contributor
timestamp: November 28, 2017, 10:49:12 am
content: Quote from: fagouma on November 28, 2017, 10:35:55 amWhen the psu is not loaded, the voltage will be a bit higher.When you connect to a load, it will regulate its voltage to ~19V.So yes it is OK to connect that psu to the monitor [but the monitor cannot be 10V, it's a typo yes?]But if the monitor is shorted or defective, then it will not be ok to connect the psu in.To verify the monitor is not shorted, you can measure the ohms from the power input connection of the monitor, just make sure it is high ohms.Regarding the test bulb lamp, it's only for testing the psu. After that, you dismantle the test lamp from the psu.Edit: regarding the lamp burn out, No the tungsten filament lamp is able to withstand the short transient voltage with no problem. But remember the lamp is for testing, not a permanent fixture to your monitor.

author: fagouma
author_rank: Contributor
timestamp: November 28, 2017, 10:59:17 am
content: Thank for your answers @Armadillo.QuoteIt's a typo. The monitor's input is 19volts.(the psu's output is 19.5V)QuoteSo i will measure the input. If i don't get a big reading of Ohms, i will see what i will do with it.

author: Armadillo
author_rank: Super Contributor
timestamp: November 28, 2017, 11:06:17 am
content: Quote from: fagouma on November 28, 2017, 10:59:17 amIf there is ON/OFF switch, make sure its on before you measure the ohms.And also consider the video signal to see something on the screen.

author: soubitos
author_rank: Frequent Contributor
timestamp: November 28, 2017, 11:12:09 am
content: Makes excellent power supply for a TS100 soldering iron!

author: capt bullshot
author_rank: Super Contributor
timestamp: November 28, 2017, 11:38:14 am
content: Quote from: fagouma on November 28, 2017, 10:35:55 amYou can connect the 19.5V rated supply to the 19V rated monitor with no risk of damaging the monitor, this is well within the usual tolerances. If the PSU outputs a voltage near or jumping around it's rated voltage, I'd suppose it's working (if it would be borked, there's usually 0V at its output). Just see if the monitor works and enjoy your score.

author: fagouma
author_rank: Contributor
timestamp: November 28, 2017, 01:08:05 pm
content: QuoteCan you help me determine the positive and negative pin on the dc in connector on the board?Below is a photo of the connector from the front and a photo with the pins of the connector which is the board, flipped 180 degrees to the left.Diode modeon multimeter:When positive of multimeter toPin 2and negative toPin 1-> 640 on the screen.When i do the reversed -> nothing on the screen.Resistance mode with 2M choiceon the "wheel:When positive of multimeter toPin 2and negative toPin 1-> 1.122Mohms on the screen.When i do the reversed -> 499Kohms on the screen.Is Pin 2 the positive and Pin 1 the Ground of the board from the above readings?

author: capt bullshot
author_rank: Super Contributor
timestamp: November 28, 2017, 01:18:28 pm
content: Ground is on Pin 2, positive DC input on Pin 1.With (almost) all equipment like your monitor, the large filled areas on the PCB are circuit GND (and "-" DC input). So the other pin of the connector is "+". No need for guessing with a Diode / Ohm meter.The connector on your picture normally is "-" on the outer shell and "+" inside. There are exceptions, but not expected with computers and accessories.Edit: your multimeter diode reading is correct, "640" in diode mode means a forward biased diode, which means in this case "false polarity". High impedance reading points towards correct polarity.

author: fagouma
author_rank: Contributor
timestamp: November 28, 2017, 01:24:34 pm
content: Quote from: capt bullshot on November 28, 2017, 01:18:28 pmI was thinking the same but when i wanted to read resistance to see if there was a short circuit before connecting the psu, i was confused by the numbers. If you are sure that pin 1 is DC input then the "inner shell" is "+" cuz it's directly above Pin 1. So makes sense.