hi,
i've a doubt. Comparators require +ve and -ve power supplies to work. If a ckt has only positive power supply and if i connect the negative power supply pin of comparator to ground of ckt., will it work properly?

 

Look at the data sheet for the device.

 

A LM339 is a single supply comparitor. Again check the data sheets.

 

It depends if the comparator can work with a single supply and what signal you want to compare.

 

Ok.. LM339 does have a single power supply...But what abt LM319?? it has V+, V- as well as GND pins.. Can u explain?

 

http://www.national.com/mpf/LM/LM319.html

Operates from a single 5V supply

 

http://www.datasheetcatalog.org/datasheet/nationalsemiconductor/DS005705.PDF

in the datasheet there are 3 pins... V+, V- and GND

 

Look at the Input Voltage Range spec on the datasheet. If your input voltage is 1V or more above GND, you can connect the V- pin to GND. If your input range is lower than +1V, you will need a negative supply for V-.
The high limit on the input range is dependent on V+, also specified in the datasheet. You will need a pullup resistor from the output to some positive voltage less than +36V.

 

Here is an example:

http://sxlist.com/images/www/hobby_elec/e_speed2.htm