What methods do you use to test LM239 comparators? - Page 1
Forum section: electronics / repair

author: Postal2
author_rank: Frequent Contributor
timestamp: January 05, 2025, 02:41:32 am
content: I encountered difficulties when testing LM239 comparators in a circuit. Not only do leaks appear at the inputs, which disrupt the external voltage divider and, as a result, change the response thresholds, but also cross leaks occur between 4 comparators, which are extremely difficult to discover.At the moment, I use the method of complete replacement of all comparators. With this approach, I get one out of 6 comparators that is faulty.Accordingly, I want to have (buy) a device that tests these comparators. First of all, I want to know the name of such a device, if anyone uses something similar.

author: Swake
author_rank: Frequent Contributor
timestamp: January 05, 2025, 11:32:01 am
content: I assume that your use case it the classic one that compares two analog signals and outputs a digital signal, or something very similar. In such a case you might want to inject some signal from an external source directly on the input pins to verify if the output acts as expected.If you encounter this frequently in the same context/equipment: thoroughly document the behavior of the circuit, make and write down many measurements and compare good vs bad case. You might want to use older HP style service manuals as example. btw, if it really happens frequently you should suspect an issue with the design. Comparators fail like any other component, but certainly not often.Unless the fault is obvious like a short between pins or a burnt hole in the case, the only good method I'm aware off to verify if an op-amp or comparator is still working is to measure it out of circuit.We might need more context to give you a better answer. What is the comparator used for? How many devices do you have to repare and what is the value of these? -> The investment in additional equipment might not be justified. In certain cases: just swap the component as it is quicker and cheaper.

author: Postal2
author_rank: Frequent Contributor
timestamp: January 05, 2025, 02:22:13 pm
content: Quote from: Swake on January 05, 2025, 11:32:01 amThis does not work in case of the defects described above. While input leakage can somehow be dealt with in this way, I was unable to deal with cross leakage.Quote from: Swake on January 05, 2025, 11:32:01 amZAPI (photo). There are already more than one a year with such a defect. Basically, such defects are associated with marginal people trying to fix them.

author: Poroit
author_rank: Frequent Contributor
timestamp: January 06, 2025, 05:18:30 am
content: G'day Postal2,I have only ever used a digital multimeter for testing these.I was fortunate and had a known working PCB so was able to measure and record all the Input and Output voltages and then compare them to other faulty PCB's.Purchased replacements from Element 14.

author: Postal2
author_rank: Frequent Contributor
timestamp: January 06, 2025, 05:23:05 pm
content: Quote from: Poroit on January 06, 2025, 05:18:30 amThat's what I'm saying, it's a lottery. That's right, you've found a leak at the input. But in rare cases (I had one like that), there are no leaks that cause voltage changes. And each comparator reacts separately to pull-ups and pull-downs. Replacing them all solves the problem. I came to the conclusion that this is due to cross-leaks, I may be wrong, but each element was checked carefully separately.That's right, a reliable store solves the problem and replacing several pieces is not difficult, although this is done blindly.The problem is that good original LM239s run out and those come that have a defect of 1 out of 10. Thus, when carrying out a mass replacement, I encounter a defect in new chips, and everything starts from the beginning.And if checking logic chips or operational amplifiers is not difficult, then checking LM239 is a non-trivial task. That's why I created the topic.

author: Poroit
author_rank: Frequent Contributor
timestamp: January 07, 2025, 06:24:54 am
content: I appreciate your dilemma.Could you use something like this to build a test jig?https://learn.adafruit.com/smt-prototyping-using-breakout-pcbs

author: Swake
author_rank: Frequent Contributor
timestamp: January 07, 2025, 06:41:47 am
content: Quote from: Postal2 on January 06, 2025, 05:23:05 pmFrom its datasheet:Quote

author: Postal2
author_rank: Frequent Contributor
timestamp: January 07, 2025, 05:17:02 pm
content: Quote from: Poroit on January 07, 2025, 06:24:54 amI have zif sockets of different widths for testing logic chips (which usually have a break). To test LM239, you will need 8 DACs, if you do it head-on. But I'm trying to figure out if there is an easier way. For example, if the inputs are shorted, then different copies have different output voltage (on the load). Knowing the subtleties, the testing scheme can be significantly simplified.Quote from: Swake on January 07, 2025, 06:41:47 amLM139,239,339 are the same chip, qualified by operating temperature. I ordered LM339, I did not like that when heated, the chip case can be easily bent.I suppose putting the LM239 into op amp mode and using a single ADC that can be switched between outputs.

author: Poroit
author_rank: Frequent Contributor
timestamp: January 08, 2025, 09:27:09 am
content: My application used 2 x LM239 to monitor the status of 4 power supplies via attenuators.+24V, -15V,+15V,+5V.If all supplies were healthy, the unit continued through it Self Testing routine.TL431 was used as a Reference.

author: 2N3055
author_rank: Super Contributor
timestamp: January 08, 2025, 10:19:48 am
content: Quote from: Postal2 on January 07, 2025, 05:17:02 pmWhat kind of heat and force did you apply to bend IC case?New LM339B from TI has same temp range as LM239.

author: Postal2
author_rank: Frequent Contributor
timestamp: January 08, 2025, 05:28:40 pm
content: Quote from: 2N3055 on January 08, 2025, 10:19:48 amI can bend it for you, measure the temperature with a thermal imager and take a photo. Will this advance you in any way or be useful?

author: 2N3055
author_rank: Super Contributor
timestamp: January 08, 2025, 05:49:05 pm
content: Quote from: Postal2 on January 08, 2025, 05:28:40 pmLanguage barrier.What I meant is to ask what the hell are you doing (what kind of abuse) to the chips to be able to bend them.I'm sure I can bend them if I use hydraulic press on chip that was heated to 400°C for a long time.But usually IC will unsolder and detach from the PCB before you will bend the case...

author: Postal2
author_rank: Frequent Contributor
timestamp: January 08, 2025, 05:56:57 pm
content: Quote from: 2N3055 on January 08, 2025, 05:49:05 pmNo. Due to the flexibility of the case, the legs extend when unsoldering, when you pull the case.Quote from: Poroit on January 08, 2025, 09:27:09 amYes, that's right. When applied to motors, comparators measure multiple thresholds of current consumption, since the MCU cannot handle the overload quickly.

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 08, 2025, 07:04:17 pm
content: I also believe that something is becoming lost in the translation. You cannot bend a DIP or SOIC package, it will crack long before it actually bends.Perhaps you meant something else?

author: Postal2
author_rank: Frequent Contributor
timestamp: January 08, 2025, 07:19:35 pm
content: Quote from: schmitt trigger on January 08, 2025, 07:04:17 pmNo, I'm trying to find these chips in boxes now, I'll find them and take a photo. I can wrap them like hot chocolate. LM339, should be TI, but I don't remember exactly.

author: 2N3055
author_rank: Super Contributor
timestamp: January 08, 2025, 08:22:27 pm
content: Quote from: Postal2 on January 08, 2025, 07:19:35 pmOK I think I understand now.Plastic of the case behaves like thermoplastic, getting soft..Yes if you have that, those are very fake chips.I assure you that original chips by any real factory is made from different material that will not behave like that.Not only TI , but any real factory even from third world countries.

author: Zero999
author_rank: Super Contributor
timestamp: January 08, 2025, 08:39:23 pm
content: My go to is a Schmitt trigger oscillator. Here's an example of one I simulated, to test this simplest model I could come up with for an open collector comparator. The LED should flash at about 1Hz. You could build 4 to test your LM239.

author: Postal2
author_rank: Frequent Contributor
timestamp: January 08, 2025, 09:48:37 pm
content: Quote from: Zero999 on January 08, 2025, 08:39:23 pmYes, this looks very much like what is needed. Thank you. I would like to determine the leakage of 1 MOhm between arbitrary points of 4 comparators, I will try.Quote from: 2N3055 on January 08, 2025, 08:22:27 pmI haven't found the chips themselves yet, but I did find a photo of me soldering the LM339. When I find the chips, the markings will be clearly visible.