Delete Delete Delete - Page 1
Forum section: electronics / beginners

author: The_Almighty_Bacon_Lord
author_rank: Contributor
timestamp: January 30, 2017, 07:39:28 pm
content: Delete Delete Delete

author: capt bullshot
author_rank: Super Contributor
timestamp: January 30, 2017, 08:11:00 pm
content: I don't know what test equipment you have, but using a smartphone app as a function generator sounds ridiculous to me. This may work for audio tests, but not to drive a MOSFET (you'll need higher voltages and faster rise/fall times than the phone can provide).Either way, driving the gate of the power MOSFET by an external function generator to test a SMPS might be possible if you designed the circuit, but I'd not recommended this for a test on an unknown circuit. Your power supply runs off the AC line, messing with the MOSFET drive signal is dangerous for you (even if you use an isolation transformer) and may quickly result in a nice bang (exploding MOSFET and/or PCB traces).You may want to reverse engineer the circuit to find the on/off control signal and tie this to the "on" state to see if the backlight supply is working. For an TV I'd expect a PWM brightness signal and maybe a on/off signal which is coupled into the regulation loop of the control IC.If your TV has LED backlight, you might be able to test the backlight with a suitable lab power supply. Starting at a low current limit (20mA), observing the right polarity and increasing the voltage, at some point the LEDs should light up (rather dim at the low current). Warning: depending on the particular backlight, you may need some 10 to 100 volts to start the LEDs. Look at the rated voltages of the caps to get an idea of the required voltage.

author: rstofer
author_rank: Super Contributor
timestamp: January 30, 2017, 11:43:27 pm
content: I would assume the MOSFET is controlled by PWM so hitting the gate with something else might not work. The problem is knowing how the signal should look and keeping other components from wiping it out. Personally, I wouldn't go there unless I could find a "how to fix it" specific to the TV somewhere on the web. You just might...If you want to play, you have to pay! At the very minimum you need a scope. Depending on the size of the TV, the scope will cost more than a new TV. Yes, it's good to have a scope but you still need to have all the documentation in order to know where to probe and what to look for. Many decades ago, this information was for sale from a company whose name I forget. I could send them money and they would send me the schematics. These schematics were pretty good because that had voltage and waveforms all over the place. But that was a long time ago... We used to have tube testers in the drug stores...

author: Cubdriver
author_rank: Unknown
timestamp: January 31, 2017, 03:20:11 am
content: Quote from: The_Almighty_Bacon_Lord on January 31, 2017, 02:06:44 amBased on this statement, I'd strongly advise you not to mess around with the backlight circuitry - you might cause further damage. The voltages shown on the capacitors are the maximum voltages that they may be safely operated at, and may or may not give any indication of the actual voltage in the circuit. It is quite possible to use a higher voltage capacitor in a lower voltage circuit, and it may be done by manufacturers for reducing the number of different parts that must be stocked and for economies of scale. The circuits may be operating at voltages considerably less than those of the caps, and inadvertently driving them with excessive voltage could cause failure.Do you have a schematic of the television and/or the backlight circuit?-Pat