Max 232 chip latching up

[AllTheGearNoIdea]

https://ae01.alicdn.com/kf/HTB1ddKYKpXXXXbAXXXXq6xXFXXXm/80070-Free-shipping-MAX3232-RS232-to-TTL-Serial-Port-Converter-Module-DB9-Connector-MAX232.jpg_640x640.jpg


http://e2e.ti.com/support/interface/rs232/f/392/t/123614


http://www.ebay.co.uk/itm/RS232-To-TTL-Converter-Module-Serial-Module-DB9-Connector-3-3V-5-5V-Arduino-Lc/292340889843?hash=item4410dff8f3:g:Zq0AAOSwx2dYCwXH


https://www.sparkfun.com/products/13712

I needed some dataloggers to record RS232  data and I’m writing this to an SD Card. I’m using a openlog to capture the data and write it to the SD Card.

I’m using an eBay purchased RS232 To TTL Convert as the logger will only talk at TTL levels. The converter is using a MAX232 and I’m getting a latch up condition occasionally when power is first applied. The problem only occurs when the serial cable is plugged in.

It does not always latch up but once it’s in the mood it latches up repeated. Once cooled down it will be fine for a while and will randomly latch up again when power is applied.  The max 232 is not permanently damaged but this is probably because I’m feeding it via a current limited supply.  I have built about 15 of these and they all have the same intermittent latch up issue. The original prototype I built works fine but the RS 232 converter came out the junk box and my 15 production converters units came from eBay.

My prototype looks identical to the production boards.  I bought these mainly because they are cheaper than I can even buy the parts for separately.

From what I have read it appears to be relatively common problem, but I have not really heard of a solution. I have increased decoupling around the max 232 and feeding VCC via a 20 ohm resistor.   Have used max 232 devices many times but never encountered the issue before.

At the moment I have the RS232 being fed by a laptop via a FTDI 9 way serial interface.  The laptop is not plugged in so everything is relatively isolated. I have plugged into various other sources to get some rs232 data and they all result in the same issue. Whenever I plug into any device the latch up periodically occurs.

I think there could be a link between powering up order the various pieces of equipment but not really hit on the one that guarantees latchup. It’s all very random until the latch occurs and then you Carnot stop the bugger until it’s cooled off.

Kind of wonder if I should buy some guaranteed genuine max 232 chips and swap them out as some of the markings do look dubiously fuzzy.  The problem does appear to be reported for genuine part also.


Apart from when the latchup occurs all works normally @ about 30ma for combined current for the rs232 level shifter and the open log. When the latchup occurs power supply is set to fold back at 100ma.

The circuit is fed via a 5 volt 7805 and I have a series 15 volt 5 watt zenner as my supply is coming from a unregulated supply 24-32v dc

I’m kind of thinking the problem occurs when plugging the unpowered converter into a live rs232 port.  When the converter is then switch on it sometimes latches up

Could do with some troubleshooting tips please.

Regards Chris

[texaspyro]

There are a LOT of fake/wonky MAX232A and 3232 chips on Ebay... maybe even close to all of them!  I just trashed 50 MAX232A's that would lock up, heat up, etc.  The charge pumps would not output proper voltages.   They would fail without any cable connected.  My boards have 0.47 uF caps on them.  I tried some MAX232 chips (spec'd for 1 uF caps) and they all worked fine.  I've never gotten a single 3232 chip to work.

[AllTheGearNoIdea]

I guess that’s the plan then I think the bare chips from RS cost more than the whole level shifter. If it turns out the chips are as fault it will be the second time I have been caught out with duff China parts in as many weeks

Bugger I guess it’s possible a easy fix if it works. I was I a hurry to sort these out as well.

Regards Chris

[duak]

Latch up in CMOS chips is usually caused by input voltages (and consequent currents) outside the chip's power supply.  I'd try some series resistance of a few Kohms on the inputs to limit current and see if it makes a difference.  I'd also try a diode after the resistor to circuit common to conduct negative currents away from the chip.  If memory serves, the RS-232 mark state is negative relative to circuit common.

I've used the MAX232 and never had this problem.  If these are indeed knock-off chips I could see that they could have a poor design that doesn't isolate the input resistors from the substrate properly as it adds a number of processing steps.

Cheers,

[texaspyro]

I've used the MAX232 and never had this problem.  If these are indeed knock-off chips I could see that they could have a poor design that doesn't isolate the input resistors from the substrate properly as it adds a number of processing steps.

I've used thousands of them without any problems. 

The circuit fails without any connection to the RS-232 connector.  All logic level inputs on the chip were either terminated with a pullup or connected to a logic level output from a receiver gate in the MAX-232 (RS-232 receiver section connected to transmitter).   

The chips that failed had a small circular indent in the package molding near the pin 8/9 end of the package.  The top of the good chips were smooth, with no indent.

BTW, MAX232A chips seem to work well when run off of 3.3V (at least to 115.2 kbaud).

[AllTheGearNoIdea]

Thanks for the advice. I don’t think this little boards will be that easy to modify. I think I will try a couple of branded max232 devices as it will be easier to swap out than doing other mods.  The ones I have  work fine unless powered cycled when connected to an active port. Thanks

[AllTheGearNoIdea]

Just an update> the IC in question is MAX3232-ESE  rather than a MAX232. Im not sure what the difference is between these devices so just going to print off the data sheets and have a look.

Thanks 

I have found some more information relating to this problem it apears to be an extremely common using these MAX232 type devices but I have never had a problem.   I can posted bellow a number of methods to help prevent the latch up from occuring but it does appear to be a scatter gun fix without really identifying the actual problem.   This post was from the Electronics StackExchange thread.

https://electronics.stackexchange.com/questions/122769/max3232-overheating-burnt-after-connecting-to-pc


I have used these 10 x 15mm converter modules from AliExpress for more than 3 years now, probably around one hundred of them. Just recently, on my last 20 or so circuits, I stumbled on this fire hazard issue. I used all the advice I could gather from different forums and I want to share my full recipe in the order I tried them because I reached a really stable behavior which doesn't consume much current and is also protected somewhat:

    First, I tried 10k pull-up resistors (on some forums they say that unused TTL pins should be grounded, but I measured some UART pins and for me it makes more sense to pull them up). This increased the stability a bit, but it did not solve it completely.

    Next, in addition, I limited the current that module consumes by putting that 20 ohm resistor in series with VCC. Here the behavior was the same as at the previous step, with the change that when the chip would latch up it would get warm, instead of very hot. So I left it like this, considering it a minor improvement.

    Next I added 100 ohm resistor on the RS232 pins. Personally I did not notice a change, but there is at least another person on some forum saying that he could see improvements. I tend to believe this, (I'm sorry I cannot mention him. I'm bad with ID's after reading all the posts in a 3 google results page) it will serve as a current limiting protection...

    Next, I added a 47uF electrolytic cap on the power pins in between the 20 ohm resistor and VCC. Again same story, somebody else stated that it fixed him. Of course not mine. I left it in circuit.

    Next I added a 20 ohm resistor in series on the TTL pins. No change, left it there. May serve as a protection, not to draw too much current from the micro-controller pin.

    I moved the power rail from 5V to 3.3V, keeping all the parts mention above. This totally solved the issue for me. At this point I had a procedure developed to make the thing latch. I was powering the converter first, and then connecting RS232 line then disconnecting it and also removing the micro-controller link.

Now I tried all the things I could and I cannot get it to heat up again.

[AllTheGearNoIdea]

Stupid me! Perhaps I should have used the forum search!

https://www.eevblog.com/forum/reviews/fake-max3232-any-additional-details/

[TomS_]

I guess that’s the plan then I think the bare chips from RS cost more than the whole level shifter. If it turns out the chips are as fault it will be the second time I have been caught out with duff China parts in as many weeks

Bugger I guess it’s possible a easy fix if it works. I was I a hurry to sort these out as well.

Regards Chris

Maxim branded chips seem incredibly expensive for what ever reason. I personally tend to use TI equivalent parts and haven't had any issues.

[AllTheGearNoIdea]

Yes chips are on order I did also consider a 202 this was recommended as being a totally different internal architecture and less prone to latching up. I am going to try and pull down the unused pins as it’s worth a try and easier that changing the ICs with my poor SMD technique but then again I need the practice.  I will report back when I have done a bit more investigation. Thanks for the replies and my best regards.

chris...

[AllTheGearNoIdea]

Ok for the record swapped out the fake MAX3232 with a genuine branded part and problem solved. You know I would have happily paid the extra for the genuine part to have been installed in the first place. It’s annoying to be put in a situation where you have no real choice but to buy crap and take your chances.

Chris

[JohnnA]

Hi,
I bought a batch of "MAX3232" chips on Alibaba and they were all latching up/running hot.
I tried with connecting open inputs to Vcc or GND and it did help somewhat but not sometimes they were still latching up.

Then I tried connecting the decoupling cap (1uF) from pin 2 to Vcc (5V) instead of GND. (Is indeed an option according to the 3232 datasheet.)
After that I have seen no more latchups.

I got the idea from looking at a working TTL to RS232 converter board that I have which seems to have a fake RS3232 device on it too.
There pin 2 was decoupled to Vcc.

That this change of decoupling could make a difference makes some sense since it gives the chargepump kind of a flying start at 5V rather than 0V.

[rstofer]

On a larger scale, why does anybody ever buy chips directly from China or by way of eBay?  You have to know they are either a) fake or b) rejects.  Rejects are usually destroyed but there's destroyed and then there's stolen.  It was such a problem at one company I worked for, we used a jack hammer with a flat plate anvil to completely mangle the package of 2N3055s before we sent them out to have the gold recovered.  Otherwise, they get sold to end users, fail (of course, they're rejects) and then we had to replace them on warranty.  Not only did we have a reject, we had to make it up with a known good device.

Considering all the time and effort to build a project, and again, the time and effort to debug the faults plus the inevitable expense of doing the job over properly, why are people still buying from eBay or China?  When I buy from DigiKey, I get traceable parts and I get them before my short term memory forgets why I ordered them.

[mdijkens]

I had exactly the same issues last month ((you can read about it here)
Went through the same internet topics and tests.

In my case the issue always appeared when I first connected the RS232 part and then the TTL part (incl power). I've tried all ports on the chip in every combination with pull-ups/downs etc.
If I connected the RS232 RX & TX after powering up the MAX3232 via TTL side, there was no issue.
By now I'm convinced I had a bad eBay batch from China also.
I've finally found a good working board in a RS232 to USB cable

I buy most parts via eBay and although probably most are not genuine, I've never had any problem
But these MAX3232's are now definitely on the 'reputable store' list ...