Hello. i need a small help

This is about three LED indicators on a DVR. they are named as POWER, NET and HDD.



At the moment the POWER LED is not working. the NET and HDD LEDs are working fine.

so first i got the voltage readings.

i think the readings are ok.

then i did a diode test for all three LEDs and these are the results i got.


as you can see the working LEDs show OL in both direction. can someone elaborate this. Thank you so much.

 

It would appear the POWER LED is faulty.

 

I concur. To check, measure the voltages indicated by the red and green arrows below., when power is on.

 

Doing a diode test on an LED in-circuit is largely meaningless. You don't know what the impact of the rest of the circuit that is around it.

The right-hand solder connection on the POWER LED looks a bit suspicious and might be a cold solder joint.

Did the POWER LED ever work?]

Are these all the same color of LED? 1.46 V is probably too low to turn on a visible LED, so either the LED has failed as a resistive short (pretty rare, but not impossible) or something else in the circuit is either clamping the voltage or otherwise preventing it from developing full voltage.

It should be a simple matter to desolder the LED from the board. That will let you test the LED directly to confirm whether it is good or bad, and also let you test the board without the LED load to see if it is generating enough voltage and current to drive the LED.

 

I use an old school continuity checker to check LEDs and transistor junctions in-circuit and out:

 

Hello. i have a small update. today while i was getting the readings, the POWER LED began to work as usual. at the same time i took the voltage reading across its terminals and it was 1.711V. it stayed on for a few minutes and then turned off itself. after that i got its reading and it showed 5.22V.

System works fine. other two LEDs work fine. hard drive, network access and all are working as usual. even the USB port works well which is on the same PCB board just under these LEDs.

@crutschow thank you.

@Irving thank you. i tested them and got these readings



@WBahn thank you. yea the LED had been working for last couple years. i noticed this fault a few days ago. POWER LED is a red color one. the other two are green/yellow LEDs.

@dl324 thank you. it's quite handy.

 

Hello. i have a small update. today while i was getting the readings, the POWER LED began to work as usual. at the same time i took the voltage reading across its terminals and it was 1.711V. it stayed on for a few minutes and then turned off itself. after that i got its reading and it showed 5.22V.

That would be quite consistent with there being a bad solder joint. The one at point B in the diagram looks pretty suspect. You might just touch it up with an iron and maybe a bit of flux and see if that fixes the problem.

 

That would be quite consistent with there being a bad solder joint. The one at point B in the diagram looks pretty suspect. You might just touch it up with an iron and maybe a bit of flux and see if that fixes the problem.

thank you so much. will do it and let you know. do you have any idea why the POWER LED gets 5.2V ?

 

thank you so much. will do it and let you know. do you have any idea why the POWER LED gets 5.2V ?

If the problem is a bad solder joint that is disconnecting the LED lead, what you are measuring is the open-circuit voltage which means that there is no drop across the current limiting resistor. Assuming the resistor R2 is the current limiting resistor, and assuming I'm making out the print on it correctly and it says 101, making it a 100 Ω resistor, and assuming that the 5.2 V is the voltage on the other side of that resistor when everything is working fine, and assuming that the voltage across the LED is 1.7 V when it is working, based on your measured results, (note how many assumptions are going into this) then the normal current in the LED would be about 35 mA, which is quite a bit more than I would expect (I would expect about a third to a half of that). But that probably just means that one of my assumptions is wrong.

 

thank you so much. will do it and let you know. do you have any idea why the POWER LED gets 5.2V ?

5v +/- 0.25v is a standard power rail in almost every piece of electronics. If the LED is working correctly the voltage across it should be 1.7 - 2v typically, but if it is open-circuit, either internally or due to a bad solder joint externally then no current flows through it or the resistor (R2 nearby) so the voltage that appears across the LED is the full supply voltage.

 

Id R2 is marked 102 (still looks a bit more like a 1 to me, but it is far from clear and my eyes are far from good), that would make the nominal current around 3 mA. Lot's of LEDs today can work fine at that, so that is not unreasonable.