Probing LED-Stripes from a TV without dismantling?

[TheGenesis]

Hi,

i already tried to measure the resistance of a TV with a failed backlight. The Backlight consists of 2 string 110v each (is written on the driver board).
My Multimeter shows OpenLine ... I dont want to open the panel right now because i have a strong feeling that the led driver part of the power supply is faulty I dont believe that both strings and the supply have gone at the same time.

Is there any way to test this with a multimeter?

thx in advance
Thom

[amyk]

No, a string of series LEDs that need 110V will show as open on a multimeter because it supplies nowhere near enough voltage to get them to pass any current. You really need a current-limited high voltage supply to test those in circuit.

[TheGenesis]

Thanks for the clarification ... I think I'm not willing to buy such a supply for a one off ... Mhmmm ... on the other side ... just saw these china things for 20 bucks ... are they good enough for the job?

I'll tried to measure the boosting circuit on the supply and found an output pin for (S)hort(C)ircuit on the driver chip which is pulled up to 5V ... according to the datasheet this indicates "output short circuit condition or DC/DC loop disconnection condition". Did they mean disconneted load?
If so, how could I distinguish between an open circuit on the circuit side or on the led side?

This is the power supply (the led boost circuit is on the last page):

http://genesis-web.de/updown/darfon_b191-001_4hb1910001d1_psu.pdf

... and here is the driver ic:

http://genesis-web.de/updown/OB3372-On-Bright.pdf

[TheGenesis]

If someone has the same issue. My was a little bit weired.

Finally I had to replace only 2 LEDs. There are 2 strings and only one LED in each string failed.
Altough both LEDs felt short, the driver circuit is briefly testing both strings (when switching the TV on) and immediately decides to switch them off.
I think this is planned obsolence because each string has 30 LEDs and therefore enough room for compensating ... if they wanted ...

[james_s]

You can build a current limited supply sufficient for testing like this for very little. All you need is a bridge rectifier and a power resistor sufficient to limit the current, you only need to push maybe 1mA though the LED strip to get them to glow and also get an idea of the voltage across the strip. If you rectify 240V mains you'll get around 340VDC, if the LED drops 110V then that leaves 240V to drop across the resistor. Something around 180k-220k 1/2W ought to do the trick. Obviously you need to be extremely careful with this sort of non-isolated source.

[TheGenesis]

Thanks for the advice. But i have already bought a cheap chinese Tester for about 20 bucks. It is horrible constructed, has the wrong outlet, only chinese symbols and a totally useless manual.
The probes provided wont even fit into the banana jacks ... but ... it works with one, two and even 30 leds

[Chris56000]

Hi!

A safer means of testing high voltage strings of LED backlight s is one of these, which can be operated from a 9V battery:–

https://www.ebay.com/itm/261169220982

Chris Williams