PIC16F877A-Dotmatrix LCD Display random character display

[sankar.m8]

Hi Friends
I have a problem regarding the 16x2 Dot matrix LCD display with PIC16F877A. I will just explain regarding the project. I need to control two relay (12V) for switching ON & OFF of a 230V supply. ULN ICs are used as a driver for relay interface. And a 2x16 LCD is interfaced in 4 bit mode.

Without providing 230V to the COM point of relay, the LCD interface is working fine. But when i am turning ON the 230V power supply, some random characters are displayed in the LCD panel. But the switching ON & OFF are working fine. I am switching the relay ON & OFF within a time of 250msec.

Is it an interference problem due to the AC supply switching? since the whole project is in a single PCB (15x10cm).

I couldn't figure it out the issue. Help me.

Thanks in advance

 

[wp100]

Hi,

What is the mains load your relay is switching, suspect its an inductive load like a fan ?

Try fitting a mains contact suppressor across the relay contacts ( note - ensure they are proper mains voltage components )

Have you got any switches going to the Pic with long connecting leads, if so shorten them and use a screened cable to minimise interference.

Any unsused pins on the Pic, set them to outputs or inputs with a pull up resistor.

Ensure you ULN chip is good, i.e. the internal flyback diodes are good.

 

[sankar.m8]

I am driving a stepper motor through the relay.
What do you mean by mains contact suppressor?
I don't have any long cables to induce the interference.
Only two pins are unused and i tried by terminating them. but still not working.
ULN chip also working fine.

 

[wp100]

I am driving a stepper motor through the relay.
What do you mean by mains contact suppressor?
I don't have any long cables to induce the interference.
Only two pins are unused and i tried by terminating them. but still not working.
ULN chip also working fine.

Hi,

In your first post you said the relay was switching 230v ( I assume AC) to a power supply.

Now you are saying a relay is switching a stepping motor, what voltage and current does the motor use ?

Any relay switching a current will generate interference when the contacts open and close.

Its often possible for that interference to affect the working of a Pic, so fitting a relay contact suppressor is the first thing to do.

The voltage and current being applied to the relay contact determine what suppression device to use.
There are a range of devices to achieve this, such a R/C or Varistors.
Most electronic shops will stock standard suppressors and should advise you which are best to use.

You say there are no long wires etc, but if you have used most of the 40 pins on a 877 chip then you must have a lot of devices wired to it ..

 

[sankar.m8]

I am controlling AC (230V) synchronous stepper motor. I have tried with placing my relay in a separate board. Also with moving the stepper motor source too far. But the problem still exist. By analyzing, it is sure that the the AC supply switching is the root cause. because, when i am switching the relay at a slower rate, the problem is disappeared. So i have decided to completely isolate my controls to the relay through an optocoupler. What you guys says?

 

[sammathew]

PIC16F877A is used to display message on the character LCD module. PIC16F877A is 8-bit micro controller based on reduced instruction set computer (RISC) architecture. It has 8kx14-bits flash program memory, 368 bytes of RAM.

Yes, it is an interference problem due to AC supply switching..