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Author Topic: Led lighted aracde buttons issue - dud leds?  (Read 790 times)

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swamprat96

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Led lighted aracde buttons issue - dud leds?
« on: July 22, 2010, 07:46:43 pm »
I'm scratching my head on this one. Building a mame cabinet that a mate picked up as an unfinished project. There are 12 buttons used- all led driven illuminated

Most of the leds were "out"- when I hooked up the 5vdc rail only about 4 lit up. Most unnusual for leds. Under individual testing most showed dead. So I've ordered more complete assemblies as they are so cheap- BUT

Today I pulled apart one button and removed the led. It has a resistor on the + side obviously to allow 5vdc to drive it reliably. When I quickly applied 5vdc above the resistor (so that the LED got 5vdc neat) it worked. OK I think - blown resistor. Then I metered it - 560 ohms- about right. I then pulled a known working one and the resistor measured identically.

WTF? I'm thinking that the leds used may have different draw down voltages - or the quality used is so crap that the tolerances vary a lot. Any other suggestions?

RandyT

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Re: Led lighted aracde buttons issue - dud leds?
« Reply #1 on: July 22, 2010, 08:38:24 pm »

You might want to show a photo of the parts in question, or at least mention the source.  It's impossible to guess what the issue might be without providing a clue as to which parts you are complaining about.

RandyT

swamprat96

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Re: Led lighted aracde buttons issue - dud leds?
« Reply #2 on: July 22, 2010, 09:18:13 pm »
OK here's a photo of the led then one of the switch assy. I don't know the source as this was an inherited project. I "guessed" they were 5vdc but more worked on 12vdc. However I still have four that work when the resistor is bypassed but not on either 12 or 5vdc. The 5vdc is directly from a computer power supply and verified at 5.08vdc and 12.15vdc respectively

« Last Edit: July 22, 2010, 09:23:14 pm by swamprat96 »

RandyT

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Re: Led lighted aracde buttons issue - dud leds?
« Reply #3 on: July 22, 2010, 09:47:11 pm »

Ok, since you "inherited" the project, anything is possible at this point.

560ohms is a lot of resistance for a "superbright" LED meant for 5v.  So I would venture a guess that these are 12v units, not 5v.  Make sure the ones which lit at 5v don't have a different resistor.  As you can see, it's not rocket science to replace the resistor so they work at 5v. 

As for why some lit after bypassing the resistor, that's anyone's guess.  From the looks of the poor solder job, I would guess a cold solder joint.  If you have a multi-meter, check for continuity with the solder joint between the probes.  Then measure the resistance of the resistor.  If you do both of those things, you will be pretty certain where the problem lies.

If you don't have a meter, get one.  It's  the only real way to troubleshoot this kind of thing.

RandyT

swamprat96

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Re: Led lighted aracde buttons issue - dud leds?
« Reply #4 on: July 23, 2010, 12:27:31 am »
Resoldered the joint and metered out both the resistor (550 ohms) and checked the joint. No change. Weird.

Spent enought time at it. At $3.00 per button I'm just going to replace them with a 5v series of buttons

RandyT

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Re: Led lighted aracde buttons issue - dud leds?
« Reply #5 on: July 23, 2010, 02:01:32 am »
Resoldered the joint and metered out both the resistor (550 ohms) and checked the joint. No change. Weird.

It's possible that someone had already mistreated these with too much current and they are on their last legs.    I agree....weird.




zafdor

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Re: Led lighted aracde buttons issue - dud leds?
« Reply #6 on: July 23, 2010, 08:24:10 am »
I agree that this looks like a hack job on the connections.   You can test the LED element with multimeter, some even have a diode (which is the 'D' in LED) setting.  In one direction the LED will probably read 1.5V or so on the diode setting and something other then open circuit on ohms. 

If it is connection related, start with the red meter lead on the + wire and meter set on diode.  Move to each of the connections towards towards the - lead, when you hit a point where you read open ciruit, that is where the connection problem is.