LM339 comparator will not output on channel 3, all other channels working. - Page 1
Forum section: electronics / beginners

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: January 01, 2014, 04:31:35 pm
content: I am having the strangest issue with the lm339 quad comparator. All four channels are wired the exact same, yet channel 3 refuses to output anything!I am using a pot set at .6v for the reference on all the inverting pins, on the + pins, I have the output of some IR sensors im using for quadrature encoders. Every output is pulled up with a 10k resistor. Now every single output is behaving as expected when viewed on a scope, except output 3.. it just stays hi. I have tested it by reading the input to channel 3, it looked as it should. I put channel 3s inputs on channel 1, they worked fine! Next I took the inputs from a know working channel and put them on channel 3, still nothing outputting on channel 3. I swapped channel 3s pull up with a working pull up from channel 1, still nothing. I switched chips still no output! I also tested the continuity of the breadboard, everything is fine there. Lastly I tried putting a 1MOhm resistor between the channel 3 negative feedback and the output, still nothing.Anyone have any ideas?

author: JoeO
author_rank: Frequent Contributor
timestamp: January 01, 2014, 07:18:47 pm
content: Can you move the 339 to another position on the breadboard?Bad run (questionable) chipsBreadboard is bad.You have a mistake in your pinouts?Post a picture. That will tell all.

author: deth502
author_rank: Frequent Contributor
timestamp: January 01, 2014, 08:38:47 pm
content: ive sen them fail like this before.i once bought a large lot of quad op-amps/comparators (forget what pn) that were SUPPOSED to be a certain part, but had some companies proprietary marking on it.i got the 100 ic's, and immediately took one out to test it, to assure myself it was what it was supposed to be. i hooked up the power, and a simple circuit on one channel. nothing. tried another chanel on that ic. nothing. so at this point, im contacting the seller saying, this aint what its supposed to be, they arent working. got a response back assuring me they were, and included a data sheet which supported his claim.i immediately went back to my testing, grabbed another ic. hmm, that one worked. went through 10-15 more, all worked fine. went back to that original one, and tried the other 2 channels i didnt try at first, they both worked.turns out, out of 100 ic's, (well, 80, i still have about 20 left, somewhere) i had that one with 2 bad channels, one with 3 bad channels, and 3 with one bad channel, but NONE of them were completely fried.eta: they were NOS, not recycled/recovered. were shipped in the anti static plastic tubes.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: January 01, 2014, 09:06:50 pm
content: Well I was going to take a picture, so I rewired the IC much neater for the picture and in the process I moved the comparator to a different position on the breadboard, and channel 3 started working. What is really strange is I tested the continuity of the breadboard and everything is fine. I must have just had a bad connection somewhere

author: JoeO
author_rank: Frequent Contributor
timestamp: January 02, 2014, 01:55:32 am
content: You changed everything else so the only thing left was the breadboard.Congrats!

author: plesa
author_rank: Frequent Contributor
timestamp: January 02, 2014, 02:00:54 am
content: Many times I have to repair some equipment the LM339 was faulty. It is quite ESD sensitive - TI datasheet and maximal rating is 600V.