MAX232 Overheat

Hello Federico,

This is very odd; please supply more information.

How many MAX232 units have overheated? Are they permanently damaged?
Does this happen when the RS232 cable is unplugged?  Do the 'system'  with MAX232 and device on other end of the cable share the same ground?  

The MAX232 should not overheat unless it is damaged by electrical overstress or electrostatic discharge.

Regards,
Ron Michallick

Hello Ron!

Some max232 are overheated.... it's very frequent . Of 10 system, one or two have this problems.

If I turn off the system at time (when overheated), the max232 aren't permanently damaged.
I take few minutes, turn on again, and max232 will work ok.

This happen when I turn on my system with the RS232 cable IS PLUGGED.

The system with Max232 and other device share the same ground.

I checked that the DB9 Chassis are not grounded.

OT: Today I replace the max232 with the TRS232ECN, but my system have some problem at start up. In my system have an RTC. When I change the Max232 for TRS232ECN, ocurred the RTC data loss. The RTC datasheet note the negative voltage glitches... but it's is another problem.

Federico,

Do you have a oscilloscope to measure to MAX232 pins.

Let me describe the expected signals.

C1+ is square wave 5V to 9V
C1- is square wave 0V to 5V

C2+ is square wave 0V to 9V
C2- is square wave -9V to 0V

V+ is +9V
V- is -9V

VCC is 5V without large ripple

DOUT pins +7V or -7V
RIN pins unknown

All numbers are approximate; look for differences between good and bad devices.
Is your application close to that in data sheet figure 4?  The +/-8.5V labels are for information only, they do not connect to external circuitry.

Regards,
Ron

Hello Ron

Ok, i will check it.

My application is the same at the figure 4, but I have ommited the Cbypass 1uF.

Do you know why my system don't work when I replace the max232 for TRS232ECN??

Best Regards

Federico

This is most interesting. Only today, I have been working on an application where I too experience the MAX232 overheating along with the 7085 that is providing power. Initially I observed it to be intermittent. I stripped out the capacitors one by one but still intermittent heating up when i power up the circuit. Further investigation found that if you place a short on the capacitor between pin 2 and ground for a fraction of a second and the thing takes off. I removed all the caps and it still does it. I then checked with a circuit i built several years ago and placed the short on ground and then quickly removed it. Sure enough the MAX232 takes off and heats up, drawing considerable current and making the 7805 way too hot to touch.

I suspect that my intermittent fault may be down to a bad cap but I am now concerned that I could be sitting on a small time bomb. I haven't experimented yet but perhaps a resistor in series with the supply might limit the current if it starts to do whatever its doing??

Dave - G4UFS

The plot thickens. After a little more digging and looking at other peoples circuits. I noted that all of them had the capacitor on pin 2 going to Vcc. This could be the problem with MY circuit and could be root cause of others.

The datasheet I have been working to was the first hit on google (see below) and it clearly shows the cap going to ground :-

http://focus.ti.com/lit/ds/symlink/max232.pdf

Yet a further search which brings up another datasheet shows the cap on pin 2 going to Vcc :-

http://www.chipswinner.com/ends/MAX232.pdf

I will check my circuit later but perhaps it could explain the observation of others??

David,

The C1 capacitor can be terminated to ground or VCC. The charge pump works equally well either way.

I will test the temporary short effect in the lab later this week.

Regards,
Ron Michallick

Ron,

This is most interesting. I have reproduced the fault on three chips now. Just to be clear, this is pin 2 to ground. Something else i noted, when the chip heats up, removing power and then repowering has to be done after it cools otherwise it heats again

Any one else have the problem?

Regards

Dave - G4UFS

Hi!

I experience the MAX232 overheating too.

My application is a little different that Figure 4. I have nonpolarized ceramic capacitors (1 uF) and the capacitor C3 (Vs+) is connected to VCC.

I turn off the system at time (when overheated). The max232 aren't damaged because if a turn on again, the circuit works!

Anyone else have the problem?

I too can confirm even with the cap connected to Vcc, I experience overheating.

That said, I have also managed to combat the problem by fitting a 20R resistor in series with the power feed. The resistor was initially intended to limit the current draw if the intermittent fault manifested itself but testing after installing has show the fault to disappear completely. Even when I induce the fault by momentarily shorting the capacitor, I observe no latchup.

Please let me know if this works for you.

Regards

Dave

G4UFS

I have the same problem with MAX202. It occurs randomly when power is applied to the circuit. Connecting the other end of the capacitor on pin 2 to VCC or GND does not make any difference.

Will anybody from TI suggest a real solution? I have many boards with this problem and the MAX202 part is exactly the same as the circuit in the datasheet.

Kmz Tan,

I have tried multiple times to create this effect in the lab unsuccessfully. Would you be willing to send in a couple units that are prone to overheating?

Regards,
Ron Michallick

Hi Ron,

Unfortunately, I cannot send any of these units since they are all on the boards assembled. We used 1 uF capacitors with MAX202 based on the statement "The MAX202 requires 0.1 uF capacitors, although capacitors up to 10 uF can be used without harm." on page 7 of the datasheet. When we replaced the capacitors (all of them) except the power supply decoupling one with 100 nF in a few boards, we observed that the problem does not occur. I think the problem occurs because the charge-pump circuit sometimes does not start with 1 uF capacitors.

Regards,

Kmz