Testing PCB Components

Rowan88

Rowan88

Rowan88

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Hi,

I was sorting some LED 5050 strips for a PC im building. Unfortunatley I think a solder point was touching another and the lights came on then turned off straight away. They won't power at all now and looking at the PCB board I don't see any obvious damage to the components. Is there a way I can test with a multimeter or something each part to see which one is the problem?

Thanks
 
You can sometimes test LEDs with a multimeter in diode mode. If you probe either side of the led it should light faintly. Failing that you might notice a voltage drop across it one way and no continuity the other.

If the positive and negative rails are shorted then nothing will light as all current will flow across the short.

It is unlikely youve blown all LEDs as you'd need to short across all the resistors individually, or maybe you threw too many volts through it.
 
Thanks,

I doubt that is the case also but its just I have had 2 IR sensors blow from the same strips so I have resoldered all the points and made sure none are touching.

I will google checking the LEDs to see if they are shorted but surely even if they were that wouldn't cause the IR sensor to blow, it must have been the wiring correct?

I have another (last) IR sensor coming today so I don't want to connect it for it to blow again if it turns out it is the strips that are causing it.
 
Sorry for the double post but I got a plain strip of LED and my multimeter. I set the multimeter to 2000 OHMs.

When I put one point onto the +12V copper connector and then the other end to the next +12V copper connector I get a reading that says that it comes up around 400-600 for about half a second then drops to 003. I have tried this on all the +12V pads.

Am I doing something wrong?
 
Yes.

See in the attached image there is a setting opposite the selected one that looks a bit like this. ->|- This is the diode setting.

16igrcj.jpg


On my multimeter if I probe an LED it will light up a little as the multimeter provides a small voltage. I have a few multimeters and I'm sure it works/worked on all, even the now broken chinese cheap ones I bought and have broken. It'll only work in one direction.

Setting it to 2000 Ohms is a resistance setting and applies a reference resistor and an even smaller voltage across the probes so won't work. It may detect the resistor in line with the LEDs if you were to probe from the resistor to the last LED in the chain.

Does your LED strip look like this, but with just the single colour LEDs?

P1040263.jpg


If so you should be able to get the LEDs to light by probing the solder around the 5050 LED packages. Check for a short across the power rails with the continuity checker if your DMM has one. This will look a bit like a wifi signal strength meter and often doubles up with the diode selection.

What's this IR sensor and where is it located. Maybe you have some kind of different LED strip to what I've seen and used. The only IR sensor on my LED strips was attached to the PWM controller for my RGB LED strips and there's nothing much one could do to break that unless the controller was faulty, or if too a high a voltage was applied.
 
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Yes I have those LED's but unfortunatley on my multimeter it doesn't have the WIFI symbol.

I tested the LED's and they light up the 3 colours fine so its not them. I just want to check that my soldering voltage is okay so how would I do that (or do I need the WIFI symbol)?

The IR sensor is the standard ones you see on eBay. I had one in my PC which was working perfect. I then put it into my friends to test and it blew one of the small black capacitors?! next to the R G B 12V. I bought another and put it in thinking it was the IR sensor that caused it and that one blew too. I then looked at the capacitor looked burned so I swapped one of them with another working one and it worked fine again in my PC.

However being an idiot I tested an LED strip and it lit up for around 5 seconds before making a pop and stopping all together (I think when I was holding it I may have touched two of the cables that were loose together by mistake).

So I have resoldered the led strips to the best of my ability, no wires touching anywhere and the solder points look solid. But this is why I wanted to check it with a multimeter first so I don't put my 3rd sensor on it and it goes bang also :(

My setup is basically a molex adapter with only the yellow and black wire connecting into the end of a 12V AC adapter. Then I connect that into my IR Sensor and the IR sensor with a 4 pin male connector to the LED strip. Then at the corners of each LED strip when I am turning it I solder 4 small wires to allow it to bend.
 
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I have it all soldered properly (as far as I can tell).. I have checked all the LEDs, the 151 and 331 capacitors which are all showing the proper readings. I have taped around the soldered points with electrical tape to keep it nice and neat.

So the only thing I haven't done is supplied 12V power to it which I will do tonight/tomorrow when the IR Sensor arrives and if it blows again I will go nuts.

:)
 
Just so you know the 151 and 331 components are resistors. 150 Ohm and 330 Ohm.

You can also check continuity with the meter set to a fairly low Ohm scale, 2000 Ohms should be ok. Anything approaching 0 Ohms is a dead short.

In my RGB strip image you should see 0 Ohms across the 12 points left to right, the 3 colours should also show a similar reading as they are the associated ground lines.

There should be no continuity, a 0L reading or something like that between any of the 4 points 12v, G, R, B. If you have one probe on G and one on R for example and your DMM shows something like 0.1 -1 Ohm then you have a short. Check all 12-G, G-R, R-B and other combinations in case there is an issue with the PCB.

When you say IR sensor do you mean a box something like this on the right? If so that's the PWM controller/IR receiver box and I can't imagine why one of these would be blowing your LED strips as they should be fairly robust.

2ymizcw.jpg
 
Yeah that's the thing I have bought another of...

I plugged my Molex to AC adapter in and checked and its giving a reading of 12.06v so I assume that is okay..

I done what you said regarding checking each of the points.. I am a little confused:

1) Do I have to have the LED strips powered to a 12v to check them first or can this be done without power?

2) When checking each of them, do I put one prong on say G and then the second prong on RED (but not in the same line of 4, in different bits)?

:confused:
 
12.06v is fine, provided it is polarised correctly.

1) All continuity/ohm tests are done with no voltage present.

2) You would probe each of them in the vertical line. 12 to G, G-R, R-B etc as I already said. If there is a short anywhere along the PCB it would show a low resistance. Resistance even across a set of LEDs or two should be an Ohm or less so it would make little difference exactly where horizontally you took the meaurement.

The IR sensor is the standard ones you see on eBay. I had one in my PC which was working perfect. I then put it into my friends to test and it blew one of the small black capacitors?! next to the R G B 12V. I bought another and put it in thinking it was the IR sensor that caused it and that one blew too. I then looked at the capacitor looked burned so I swapped one of them with another working one and it worked fine again in my PC.
Click to expand...

I'm not altogether sure what you've swapped here.


If you mean resistor blew then it does sound like you have/had a short straight from 12v to one of the grounds, or possible through maybe 1 LED. Which resistor blew? Which number was on it? On my LED strip I have 151 , 201 and 331 used on the Blue, Green and Red circuits respectively. The worst one for a short would be the 151 and would be burning 0.96W if the short was after the resistor and before the LEDs.
 
I set the multimeter to 2000 Ohms and then put one prong onto 12v and the other onto the GREEN right next to it but the multimeter just shows 1 which is what it shows when I don't even put the prongs on anything...

The only way I can get the multimeter to give a reading from 2000 Ohms on the 12/G/R/B is if I put one prong onto 12V say, and the other onto the 12V next to it (or G to G, B to B or R to R).

The reading then shows around 003-006.


As for the resistors on the LED strip they all look fine and when tested they show the correct voltage. The numbers are 151 and 311. When I say something blew I don't mean on the LED strip itself, it was inside the IR Receiver.

A small black square with 3 prongs coming out of it. There was 3 in total, near the 12V/G/R/B solder points.
 
The top photo.

One of them blew, I replaced it with a working one from another and then it blew which I think was because I touched two wires together by mistake.

Does that mean on a brand new IR receiver, the only way it would blow one of them is if on the LED strip I had the wiring wrong or touching?

If everything is correct and soldered neatly it should work no problem?
 
That little rectangle is the output transistor for the LEDs. It is probably a MOSFET and what you probably did is short it out.

Provided your strip of LEDs is fine then everything should work OK.
 
My friends IR sensor came today, plugged it in and all is working perfectly, must have been a connection touching or bad soldering on my part before.

As for mine I received a new IR sensor and mine is back to working order also.

Thanks for all the support and help as always,

Cheers
 
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