Hisense LED driver output way too high! - Page 1
Forum section: electronics / repair

author: gojunnyo
author_rank: Contributor
timestamp: April 26, 2020, 07:44:37 am
content: I have this TV power board named Hisense rsag7.820.5535 I can't find the schematics anywhere so I am confused where to start, the specifications said that the LED driver output should be 140V at 400mA. As I tested the output rails, it shows 232V at the output!, that's why the backlight of the said unit, was burnt into pieces.I am looking for the TL431, but I can't find it anywhere due to the lack of schematic as a reference. Maybe it is on SMD package (Will try to look for it again) But is my suspicion correct?Any tips where to start? Enlighten me with your knowledge and experience

author: Chalcogenide
author_rank: Regular Contributor
timestamp: April 26, 2020, 08:03:58 am
content: The LED driver is current-controlled, so if the LED strip is broken (i.e. behaves as an open circuit) then the voltage will shoot MUCH higher than the nominal voltage, and is only stopped by over-voltage "open circuit" protection, so measuring 200+ V on a nominal 140 V output is relatively normal. I don't expect such a circuit to use a TL431, but a dedicated IC with current sensing.

author: gojunnyo
author_rank: Contributor
timestamp: April 26, 2020, 08:22:23 am
content: Quote from: Chalcogenide on April 26, 2020, 08:03:58 amThank you for such wonderful insight! and for the new knowledge. So the thing I should do is just replace this burnt parts and not expect it to happen again?

author: Zero999
author_rank: Super Contributor
timestamp: April 26, 2020, 10:52:26 am
content: How are the LEDs connected up? If they're all in series, simply replacing the bad ones will do. The problem is, if some of them are in parallel, then forward voltage matching will be requited for good current sharing, otherwise you'll end up with a patch of brighter or dimmer LEDs, with the other carrying more or less current.

author: gojunnyo
author_rank: Contributor
timestamp: April 26, 2020, 11:53:25 am
content: The LED's are connected in series, majority of them are shorted

author: Tom45
author_rank: Frequent Contributor
timestamp: April 26, 2020, 04:58:49 pm
content: As already stated, the LED driver is a current source and the open circuit voltage tells you nothing.However, the LED driver could still be faulty and the culprit. If it is, that could explain why some of the LEDs are bad.Try connecting it to an incandescent light bulb of at least 60 watts. Also, put a 1 ohm resistor in series with the bulb. Then turn on the power and measure the voltage across the 1 ohm resistor. You should see around 0.4 volts across the resistor if the driver is OK.

author: TheMG
author_rank: Frequent Contributor
timestamp: April 27, 2020, 12:25:36 am
content: Most LED drivers in TVs will turn off after a very short amount of time if an open circuit is detected (voltage rises above a certain threshold without reaching the target current).The reason for this is precisely to avoid the kind of carnage that happened - things burning up is an LED goes open-circuit and begins to arc!So it's somewhat odd that this TV's driver doesn't have such a feature. What is the part number of the LED driver IC (it will be near the boost regulator for the LED output)?If you have a 150 ohm 25W resistor (or something close) handy, hook that up and see what happens. The current should not exceed 400mA if the circuit is working correctly. If it exceeds it by a significant amount then there's a fault in the driver circuit and it's not current regulating like it should.

author: gojunnyo
author_rank: Contributor
timestamp: April 27, 2020, 04:45:06 am
content: Quote from: TheMG on April 27, 2020, 12:25:36 amThe LED driver chip is selc2010m, I do not know where in the particular datasheet would mention such featurehere's the link btwhttps://www.datasheetq.com/datasheet-download/882166/1/Magnachip/SELC2010M

author: gojunnyo
author_rank: Contributor
timestamp: April 27, 2020, 05:08:17 am
content: Btw, how can I avoid such problems to happen again? as I was told the problem happened after a power surge due to lightning. But I was complete surprised that only the backlight was damaged, (I do not know yet on the LED driver) but the rest is working fine.

author: gojunnyo
author_rank: Contributor
timestamp: April 27, 2020, 10:10:58 am
content: UPDATE: I connected the replaced LED strips to the LED driver, I am confused, when verifying on the multimeter. The output of the driver should be on DC right?Scenario 1: Connecting the + multimeter lead to + rail and - multimeter lead to - railthe output is more or less 30VDCScenario 2: Connecting the - multimeterlead to + rail and + multimeter lead to - rail or in simpler terms connecting in reversethe output is neagtive more or less 120VDCScenario 3: AC mode and Connecting the + multimeter lead to + rail and - multimeter lead to - railthe output is more or less 110VACis it my multimeter bricked or the led power board?

author: Chalcogenide
author_rank: Regular Contributor
timestamp: April 27, 2020, 10:58:07 am
content: Probably PWM is used to set the LED brightness. If you have the chance, set the range on your multimeter to something higher than 200 V and try again. If your multimeter is not true RMS, the values could be all over the place, as you are measuring now. You could also try to measure the voltage after manually setting the highest possible backlight value on your TV, which should increase PWM duty cycle to 100% or quite close to that value.

author: gojunnyo
author_rank: Contributor
timestamp: April 27, 2020, 11:28:12 am
content: Quote from: Chalcogenide on April 27, 2020, 10:58:07 amHow can I manually change the pwm without having the hassle to mount the LCD panel again and illuminate it with a flashlight.

author: gojunnyo
author_rank: Contributor
timestamp: April 27, 2020, 02:54:48 pm
content: UPDATE:The LED strips now lights up but are quite dim, and for split seconds it would light up bright and dim again. More or less a fluctuation of voltages? Any thoughts are highly appreciated