Dear all, I found the problem it is related with max232 ic infact triggered by contactor switching noise that ic get hot and only receive from the pc but unable send to the pc. When I disconnect and reconnect vcc for this ic, normal operation resumes. All the capacitors of max232 are as recommended on the data sheet. As a temporary solution I make the mcu to reset max232 ic using relay that way the whole project run reliably longer than before. But is there any better solution for the max232

 

Does the MAX232 get hot when the problem occurs?
If so read here: https://www.edaboard.com/showthread.php?352767-MAX232-MAX3232-heating-up

 

Dear all, I found the problem it is related with max232 ic infact triggered by contactor switching noise that ic get hot and only receive from the pc but unable send to the pc. When I disconnect and reconnect vcc for this ic, normal operation resumes. All the capacitors of max232 are as recommended on the data sheet. As a temporary solution I make the mcu to reset max232 ic using relay that way the whole project run reliably longer than before. But is there any better solution for the max232

I never had any problems with max 232 chips used a lot of them over the last 30 years but
switched to ISL83490IBZ better and cheaper.
This heating could occur due to:
Internal oscillation ( use a cap between vdd and vcc as close as possible to chip. 0.1 uF parallel to 1 uF parallel to 10 nF)
Use all caps parallel as mentioned they form a filter for different frequencies.
check all ground leads.
A ground loop could occur when using different (switched) supplies feeding one system.
This loop is caused by the caps to GND to reduce EMC however it will produce a few micro amps running at the switching frequency.
Good luck.

Picbuster

 

Dear all, I found the problem it is related with max232 ic infact triggered by contactor switching noise that ic get hot and only receive from the pc but unable send to the pc. When I disconnect and reconnect vcc for this ic, normal operation resumes. All the capacitors of max232 are as recommended on the data sheet. As a temporary solution I make the mcu to reset max232 ic using relay that way the whole project run reliably longer than before. But is there any better solution for the max232

Hi,

Wow that is both amazing and surprising. I didnt think the Max232 could be subject to such a problem. Maybe i should look at the data sheet again it's been a long time now since i worked with one.

 

It happened to me and R/C snubbers across the A.C. contactor coils cured it.
Max.

Dear MaxHeadRoom, I have done some work in the code that handle the noise but I also returned to the hardware. So I have searched the market and found capacitor only 2.2microf. So with what value of resistor should I use this. The contactor is AC3 7.5kw 380V-400V

 

The max232 chip is going into "SCR latch-up" mode - probably due to large transient currents flowing in the input protection diodes.

https://en.wikipedia.org/wiki/Latch-up

This is why a power-cycle is required to recover.

You might want to try adding some TVS diodes to both the TX and RX serial lines to prevent this.
Additionally, bypass the IC with several low-impedance caps, right on the IC pins - close as possible.

 

The max232 chip is going into "SCR latch-up" mode - probably due to large transient currents flowing in the input protection diodes.

https://en.wikipedia.org/wiki/Latch-up

This is why a power-cycle is required to recover.

You might want to try adding some TVS diodes to both the TX and RX serial lines to prevent this.
Additionally, bypass the IC with several low-impedance caps, right on the IC pins - close as possible.

I tried 33ohm resistor in series with the power supply and bypassing caps but the problem not removed completely. TVS diode is not available in my location. What I did as a solution is to reset the max232 ic periodically with the help of the microcontroller. But as a preventing method I am about to add snubbers but the only available cap is 2.2microf so what value of resistor would much for 2.2microf as a snubber across AC3 7.5KW 380V contactor coil?

 

Dear MaxHeadRoom, So I have searched the market and found capacitor only 2.2microf. So with what value of resistor should I use this. The contactor is AC3 7.5kw 380V-400V

Here is some help, you need an R/C snubber, there are a few manuf. out there also.
Max.

 

Here is some help, you need an R/C snubber, there are a few manuf. out there also.
Max.

Thanks

 

You dont want the snubber across the coil, it should go across the contacts that drive it, and the contactor itself.

 

I have always used it across the coil, worked for me.
Easy to test, remove contactor switched power and leave just coil circuit active.
Max.

 

@Tinsae Can you confirm the contactor coil voltage and AC/DC?
Max.

 

@Tinsae Can you confirm the contactor coil voltage and AC/DC?
Max.

The contactor coil voltage is 220v ac. And the contactor is for 7.5kw 380vac load(if it helps to guess the size of the coil hence inductance)

 

As well as the manuf such as Electrocube, many suppliers of contactors have ready made snubbers for their line of contactors.
A good starting point for a DIY version is 220ohm res in series with .5uf cap.
Max.

 

Dear all, as maxHeadRoom pointed, today I purchased two .22uf 275v caps and wired them series also 100ohm resister (

 

Dear all, as maxHeadRoom pointed, today I purchased two 0.22uf 275v caps and a 100ohm resistor,wired all in series and tested the project it runs 90 complete cycle without max232 latch and no mcu reset. I didnt run this much correct amount before therefore it mean the snubber across the coil gave me solution for me. Infact I have upgraded the code such that it disconnect max232 before a contactor switched off.

Thank you everyone!

 

Also another reason i always spec in DC contactor coils!
They seem to be less prone with a diode snubber across them.
Max.

 

Dear all, as maxHeadRoom pointed, today I purchased two .22uf 275v caps and wired them series also 100ohm resister (

Very glad this worked for you.

Note that your capacitor voltage rating is too low for the application.
The peak voltage of 220VAC is 311Volts, I would use a 600 V cap at the minimum here.

 

Very glad this worked for you.

Note that your capacitor voltage rating is too low for the application.
The peak voltage of 220VAC is 311Volts, I would use a 600 V cap at the minimum here.

Unless the rating is 275V AC which would be fine.

 

Very glad this worked for you.

Note that your capacitor voltage rating is too low for the application.
The peak voltage of 220VAC is 311Volts, I would use a 600 V cap at the minimum here.

I was also worried about it (but I dont know why it is made that way, the cap, may be it is inyended for 110vac system? ) but since I connected two caps in series the voltage drop per cap will be half dont you think so?