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Old 02-20-2018, 05:30 AM   #1
lookimback
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Default Samsung UN29F4000 Need help identifying diodes on LED strip.

I've identified the LEDs as SPBWH1332S1BVC1BIB and have them on order. From what I can find online, they are 3v 350ma. The diodes are marked CJ which has 24V on the board next to it. The other is marked CB and has 12V on the board next to it.

I tried something like the third image except I don't have a DC power supply. Instead, I used my variac, a bridge rectifier with a filter circuit, and 2 multimeters. Works OK for low power applications. My results below are the lowest voltages where the 2 meters were no longer the same. The Strips have 8 LEDs each. Do these values seem correct?

CJ = 31.1v
CB = 15.2v
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File Type: jpg Testing-of-Zener-Diode.jpg (37.2 KB, 18 views)
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Old 02-20-2018, 05:56 PM   #2
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

Anyone?
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Old 02-20-2018, 07:19 PM   #3
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

Are they connected in parallel with the LED string of those 8 LEDs whcih are connected in series?
I never see both protection diodes on the same strip, they usually install just one.
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Old 02-20-2018, 07:53 PM   #4
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

Yes, they are. Below is the schematic I just drew of the circuit.
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Old 02-20-2018, 10:22 PM   #5
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

Most LED strips do not have those diodes in place, if the LED in the string go open circuit the the power supply will try to force what ever the constant current is set to to be flowing through the diodes and will probably burn up the diodes, and if some one test the LED strip by applying wrong polarity to the strip, the diodes will be damaged too.
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Old 02-20-2018, 10:37 PM   #6
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

On the bad strip there was one bad LED and both diodes were fried.
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Old 02-20-2018, 10:47 PM   #7
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

Not surprised that the diodes will burn up.
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Old 02-20-2018, 11:17 PM   #8
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

I'm going to try to get a precision power supply before the LEDs get here so I can repeat the tests. If not, maybe I can just add an LM337 and a pot and make something more precise.

Now, if I try a particular diode to replace each, can I verify they are correct by comparing in-circuit voltage across them to the values I get on one of the good strips? That would be bench testing, not in the TV, and both tested at the same voltage.
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Old 02-21-2018, 04:27 AM   #9
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

Ok, I figured it out. I made the circuit in this YouTube video. I had to use four 9v batteries though. I was pretty close yesterday. I think the ripple on my unregulated supply was throwing it off though.

The one marked CJ is 31.5v
The one marked CB is 15v

Maybe I'll pull them from another good strip tomorrow and repeat the test to compare results.

Last edited by lookimback; 02-21-2018 at 04:29 AM..
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Old 02-21-2018, 03:25 PM   #10
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

Are the diodes necessary or can they be bridged as most LED strips dont use them.
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Old 02-21-2018, 03:37 PM   #11
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

Quote:
Originally Posted by sintv View Post
Are the diodes necessary or can they be bridged as most LED strips dont use them.
I think without them, whatever current normally flows through the diodes would instead flow through the LEDs.
They would burn out quickly.
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Old 02-21-2018, 05:10 PM   #12
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

Quote:
Originally Posted by sintv View Post
Are the diodes necessary or can they be bridged as most LED strips dont use them.
If you at the diagram, if you bypass the diodes then you will bypass the whole LED string which will cause the PWM Contolled MOSFET for the constant current control to heat up due to more Vdrops on the MOSFET.
Those two Zener will never go into conduction because the Vf of the LED string is much lower Voltage than the zener diodes.

Last edited by budm; 02-21-2018 at 05:11 PM..
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Old 02-21-2018, 08:49 PM   #13
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

But what if they were left out completely, not bridged but instead left open? Since they never conduct because the vf of the LED string is lower, what is their actual purpose? Also, any idea on their wattages? Is it ok to just use the highest wattages I can find for their replacements?

Last edited by lookimback; 02-21-2018 at 08:53 PM..
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Old 02-21-2018, 09:47 PM   #14
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

I rarely see those diodes in most the LED strips I have to repair or replace.
They are just small protection, if look at most White LED, it has Zener diode (5V is typical) connected in Anti-parallel with the LED already. I believe the LEDs used in your strip do not have built-in protection diode. You can tell if it has the diode or not by measuring the VF of the LED, one way will show about 0.6V Vf which will be the protection diode, and when you reverse the probes it will show the Vf of the LED itself.
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Last edited by budm; 02-21-2018 at 09:48 PM..
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Old 02-22-2018, 06:47 AM   #15
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

In one direction I get nothing, in the other the LED lights.
No reading on the meter at all.
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Old 02-22-2018, 10:26 AM   #16
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

That means it does not have protection Zener like the example I show of the LG LED.
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Old 02-22-2018, 05:35 PM   #17
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

Hence the need for the diodes. I get it now. My plan is to replace all the LEDs and diodes, even on the good strips. I figure if one failed, the others can't be far behind and I don't like pulling panels more than once. Some of them look like they've gotten hot too. The LEDs cost me $4.62 for 100. So to replace all 32 that's $1.30. Maybe $2 for the diodes. Then I use 5 second fix to glue the lenses back on. That's another $5 but probably won't need the whole tube. Under $10 and it should be good for a couple more years. I appreciate your help. I've learned a lot from you over the past few years. I'll let you know how it turns out.
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Old 04-04-2018, 12:20 AM   #18
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Default Re: Samsung UN29F4000 Need help identifying diodes on LED strip.

Fixed! I used DZ2J150M0L for the 15v zeners,and DDZ33BSF-7 for the 31.1v zeners. They are rated 200mw and 500mw, respectively. I wasn't sure about the wattages, so I just got the highest rated for each. The LEDs were SPBWH1332S1BVC1BIB, and I found them on AliExpress for $4.60 shipped. Luckily, I chose to replace all the LEDs and diodes. I say luckily, because I found a shorted diode on one of the working strips. I'd missed that when I tested them before.
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