1. My circuit is powered by +5V. It then runs a MCU with 3.3V (LDO used) & then lcd powered by +5V.
2. Now if any case user coonects higher voltage than +5V then it will damage the lcd.
3. Is there any way to protect it.
4. One way is to place a zener of 5.1V. But usually zener conducts a lower voltage than rated & second its series resistor will limit the current.

 

use a Crowbar circuit with a zener and thyristor

http://www.hobbyprojects.com/thyristor_triac_and_diac/images/crowbar.gif

 

If using a crowbar circuit, don't forget the series fuse.

 

3. Is there any way to protect it

OVP is a well 'explored' area --- A basic crowbar should suffice

4. One way is to place a zener of 5.1V. But usually zener conducts a lower voltage than rated & second its series resistor will limit the current.

My recomendation is implementation of a 1N6373 (or NTE 4900 -- http://www.ntepartsdirect.com/CGI-B...ebrtn=search&ml=LANSA:XHTML&part=CEP&lang=ENG )

EDIT: Please be advised that the above linked page is maddeningly slow to load! -- Patience though, it is a working site...


Of course it goes without saying that you'll need to provide appropriate over-current protection lest the OVP device 'falls upon the proverbial sword' in vain...

Best regards
HP

 

@Dodgydave and @MikeML

Sorry about that! It seems I spent so much time waiting on 'NTEpartsdirect" (so as to copy the link) that your responses were 'in' by the time I clicked the post button

With abashed contrition
HP