backlight tester malfunction

[stinger]

I purchased a DLV-300 LED Backlight Tester a short time ago and tried it on a couple of led strips and it seemed to work fine. It would start at 300v and when testing the led's, it would adjust to the voltage needed to light them.
I then went to use it while diagnosing problems with a tv set I have and at first the tester worked properly and lit up the first led strip and then lit up half of the second strip. But when I proceeded to find which leds were bad I noticed the tester wasn't working right. It only reads 17v then slowly goes up to 20v max. Does anyone have experience with these testers or have have any ideas what could have happened to it?

Thanks

[redwire]

Re: backlight tester malfunction

I sounds like the HV boost converter has stopped working.
If you can post clear pictures of the inside and both sides of the PCB, can figure it out.

[stinger]

Re: backlight tester malfunction

I will take some pics tomorrow, thanks

[cheeky2]

Re: backlight tester malfunction

The high voltage is normally present when you don't have any leads connected to any leds. Once you have leds connected then the voltage drops down low and gradually ramps up to a level where the leds will turn on and gradually get brighter. I suspect nothing is wrong wih it but your expectations of how it works?
Led's strips can be paralled giving you a lower voltage than you expect. I suggest you take pictures of your led's testing as I would suspect misoperation. Who knows I maybe wrong?

[R_J]

Re: backlight tester malfunction

The high voltage is normally present when you don't have any leads connected to any leds. Once you have leds connected then the voltage drops down low and gradually ramps up to a level where the leds will turn on and gradually get brighter. I suspect nothing is wrong wih it but your expectations of how it works?
Led's strips can be paralled giving you a lower voltage than you expect. I suggest you take pictures of your led's testing as I would suspect misoperation. Who knows I maybe wrong?

I don't think you read the whole post, He said it WAS working and supplying 300v when testing the strips and NOW only shows 17v~20v
I suspect the unit has a poor solder connection inside and that has stopped it from working, but he has not provided any pictures, so we can't help him until he does.

[cheeky2]

Re: backlight tester malfunction

You should only get a 300V output when either the leds are open circuit or there is no continuity with the leds ie a brake in the connector. I know of no other leds that will give you this output from a led tester with good leds inside the TV. It would require over 100 3v leds or over 50 6v leds. I know of no TV with this ammount of leds so that's why I came to the conclusion I did. My led backlight tester operates only at 320V when open circuit or reversed leads. It should never start off at 320V with leds connected for obvious reasons.

[petehall347]

Re: backlight tester malfunction

switch it off and back on ..it fixed mine once .

[R_J]

Re: backlight tester malfunction

You should only get a 300V output when either the leds are open circuit or there is no continuity with the leds ie a brake in the connector. I know of no other leds that will give you this output from a led tester with good leds inside the TV. It would require over 100 3v leds or over 50 6v leds. I know of no TV with this ammount of leds so that's why I came to the conclusion I did. My led backlight tester operates only at 320V when open circuit or reversed leads. It should never start off at 320V with leds connected for obvious reasons.

So what do you do? connect the tester to the led string THEN turn it on? The testers I have used, "display" a high voltage, then as soon as the string is connected the "voltage display" drops to the voltage to deliver about 30ma of current to turn on the string. and displays the output voltage.
Here is the quote from the original post

I purchased a DLV-300 LED Backlight Tester a short time ago and tried it on a couple of led strips and it seemed to work fine. It would start at 300v and when testing the led's, it would adjust to the voltage needed to light them.

Then the tester developed a problem

But when I proceeded to find which leds were bad I noticed the tester wasn't working right. It only reads 17v then slowly goes up to 20v max

He likely has a problem with the boost circuit inside the tester.

[redwire]

Re: backlight tester malfunction

It seems 325VDC is available with open-load, which normally would damage an LED if connected backwards? It starts at low current and slowly ramps it up.
https://youtu.be/IuHZBE2NP7c?t=274
He takes it apart, and backwards on a little 0603 it reads ~145V. Hmmm.

[R_J]

Re: backlight tester malfunction

When the leads were backwards there was very little current flowing so the voltage dropped to 145v, when it was the right way around, the voltage dropped to 2.7v because there was current flowing (30ma?) Nobody measured the voltage at the test leads.

[stinger]

Re: backlight tester malfunction

I'm sorry, Due to family med emergency I have been unable to get back here. To confirm, when I received the tester it would power up to 300v with test leads open and adjust accordingly when load was applied. Now it powers up to 17-20v max when turned on with no load at test probes. I took pics of circuit board and will post them.

[stinger]

Re: backlight tester malfunction

Pictures are not too clear.

[stinger]

Re: backlight tester malfunction

Hope these are clearer

[R_J]

Re: backlight tester malfunction

I would check the mosfet? near the red output plug, then check the diodes in that area. Also check for any poor solder connections.
If you only had the tester for a short time, contact the supplier and tell them, maybe they will replace it for you.

[stinger]

Re: backlight tester malfunction

The mosfet seems to be bad(open) and one diode seems shorted, but I will unsolder it and check out of circuit. I will order and replace those parts. Thank you. The supplier wasn't too cooperative even though the tester is only about a month old and only used twice.

[R_J]

Re: backlight tester malfunction

If the diode checks shorted, It could be all that is wrong, the mosfet might be ok. Can you show which diode is shorted?

[stinger]

Re: backlight tester malfunction

It is the one with yellow arrow pointing to it, next to D3. There is no value marked on it.

[R_J]

Re: backlight tester malfunction

I can see that there IS a marking on that component, but I can't make it out in your picture, Could be a 1 amp schottky diode OR something else, What is the board designation? You can clean the top of the component with alcohol to see the number better.

[stinger]

Re: backlight tester malfunction

I looked closer with loupes and see it says US1M. All 4 in that area are the same.

[R_J]

Re: backlight tester malfunction

I looked closer with loupes and see it says US1M. All 4 in that area are the same.

1.0A 1000v SURFACE MOUNT ULTRA-FAST RECTIFIER