I've got a MAX3232 Serial-to-TTL RS232 board which I am currently using in factory assembly lines. I don't have its picture, but here's a similar product:
This is the circuit I've managed to figure out of the board(I don't have the original schematic):
I'm using the RS-232 board to send streaming data from an electronic Weight scale to PC, where the data is grabbed by my software for display.
The problem right now, is that sometimes the board stops sending data. I used HyperTerminal to check if the weight scale is sending data or not. When it doesn't, I've tried disconnecting and reconnection the serial port connection, unplugging the serial cable and plugging again, turning off the weight scale and even the PC on/off again and such, sometimes with no results. Sometimes the weight scale (WS) stops sending data. When I left if turned off for a while, and turns it on again, the WS sends data again. I suspect the MAX3232 IC got hot, because I can see the glue I put on the board looking like it's melted off the IC. When it cools down it is operational again.
I've used an oscilloscope to check on burnt ICs – these ones clearly don't have output on pin 14 (refer to schematic), with voltage level nowhere near +- 7V.
I've searched on the internet, and also found other people with issues regarding their IC (well this is actually MAX232) heating up when connecting to PC:
So far I've done some countermeasures:
-
Replace the burnt/old IC with new MAX3232 ICs from element14. Since the boards were very cheap and made in China the IC were probably of low quality.
-
Connected the unused CMOS input (pin 10) to GND with 2.2K Ohm resistor.
-
Added 20 Ohm resistor in series to pin 16 (VCC).
I'm afraid that these measures wont be enough. Maybe I need to isolate my connection. Does anyone have any idea what's going on, and know what to do?
Best Answer
Just solved this overheating problem on a batch of max232 chips using a couple of resistors. See the input pins 10 and 11 http://www.maximintegrated.com/en/products/interface/transceivers/MAX232.html and click the picture to enlarge. There are supposed to be 400k resistors internally but measuring the resistance to pin 16, I got 40 megohms on this batch. So these pins will be floating and picking up RF and oscillating at a high frequency which in turn will be drawing a lot of current and making the chip hot. I used a couple of 390k resistors as pullups, but anything from 10k to 400k would probably do. Might be worth adding 5K pulldowns on pins 13 and 8 too.