circuit using LM339

[cute109]

i want to use LM339 as a voltage comparator for switching a relay at 25V but i am having a problem in the circuit.can anyone help me for the circuit?

 

[Militoy]

Are you trying to switch a relay with a 25 (24) volt coil – or are you trying to switch 25V using the comparator to drive a relay? Either way, I’ve found that that for me, the simplest way to interface a relay with a comparator is to run the comparator in TTL mode – at 5VDC – and interface the relay using an ON Semi MDC3105LT1G relay driver. If all you have is a 25V source, you may have to add a simple 3-terminal regulator to supply the comparator and the relay / driver with 5V. The tiny 3-terminal relay driver really saves space and component count though – and I’ve found them very useful in my robotics applications.

 

[cute109]

i have limited resources and i cant get relay driver ic.i want to build it more simply in least cost as possible.

 

[NickS]

then you drive a transistor with the comparator. I will try to throw a schematic together for you but please explain better exactly what the 25V is for and what other voltage rails you have available if any.

 

[cute109]

i want to use this circuit for cutting out the supply line to the inverter sensing the battery voltage because i have the solar power for charging the battery in days.below 25V of the battery i want to charge it using inverter and if greater than 25V i want to cut out ac line in inverter input.

 

[cute109]

previously i use transistor to drive the relay but once relay switched at 25V DC it didnot come back at the voltage lower than 25 V so i am trying to use comparator.but i dont have enough knowledge about comparator.

 

[NickS]

Right, so you need the comparator to change states but the comparator cannot supply enough current to drive the relay coil(probably). So you buffer the comparator with the higher current transistor.