power supply Isolation for microcontroller to eliminate Industrial Noise

[gne_577]

Hello ,
I'm Working on Industrial Project based on PIC18F series micro-controller. I'm using TQFP packaging pic18f97j60 controller. In this project i used some modules such as PWM for AC motor control . LCD , 4x4 Keypad , I2C for external EEPROM , L297-L298 and TIP122 for relays & Solenoids of 24v DC

I'm using two Omron +24VDC 4 A ( For microntroller & TIP122 ) and +12VDC 4A ( For Stepper Motors) Professional power Supply for whole panel and further i used two 7805 for voltage converter. I just make sure that there is fully isolation between Inputs / Outputs operations by using optocoupler EL817. Only LCD and 4x4 Keyboard is directly coupled to uC.

My question is that is it necessary to use different power supplies for load and Micro controller to eliminate noises and hang type problem. Because i find that there are blocks appeared on lcd some type and whole data is disappeared.

Is this due to single power supply..? or i have to give full isolation to ground and vcc both. If yes then please suggest me circuit or any idea.

Thanks in advance.

 

[KlausST]

Hi,

power supply isolation is mostely for safety reasons. It is to avoid electrical shock.

EMV problems are high frequency (or pulse currents) coupling problems.

So often one combines both.
An isolation splits the ground signals. So it prevents EMV signals to travel from display to microcontroller.
But EMV problems may still occur. Capacitive coupling or inductive coupling is the cause.

Often a good old transformer power supply has better HF isoltion from mains to microcontroller than modern switch mode supplies.

My hint for a good EMV design is to find out where the HF travels. Adding HF filters at each PCB I/O (signal, power supply...) may improve stability more than a true isolation.

Keep an eye what is important: if the controller updates a disply every second, then a accidentally wrong transmitted character is corrected within a second. But the measurement and control loop runs stable - and often that is that counts.

To even avoid a wrong character i often use differential signalling (without true isolation) to the display within a (big) case.
This often decreases installation effort and decreases signal filtering effort, because now i transmit the data serially instead of parallel.

******

If you need more detailed answers you shoud give us a draft or pictures of your application.
Mechanical, sizes, length, signal levels, signal count, shielding... and so on....

Klaus

 

[gne_577]

First of all thanks sir for your time and reply.

As you said in your reply. Yes the controller updates the LCD display every second or more than 5 seconds. I just little confuse that should i use extra supply for the micro-controller separately .? or any circuit which will totally isolate the controller from the main ciruit.?

whats your opinion .?

 

[KlausST]

Hi,

There is no general answer.
There might be benefits in using different, isolated power supplies.
I, personally, tend to use one power supply and use filters and differential signalling inside one case.

For external signals i often use filters, protection circuits and (optically) isolation combined.

But as said: no general rule.

Klaus

 

[gne_577]

Thank you sir for your opinion.

Can you suggest me filter and protection circuit which should be used in electronic circuits for the Industrial environment. Thats huge help.