Hi there,

I am building a simple DC PSU. 18v DC in 12v DC out. I have been using an LM317 and its been fine. Now I require the output to trun off / cut out if the output or indeed the input voltage drop below 12v. Is there a simple IC or transitor that I can use. My current draw is 200ma max.

Thanks in advance for your help!

 

Shouldn't be too hard to adapt a zener possibly with a transistor into the adjusting leg to set it to minimum should the condition occur, a complete cutoff could also be designed into the 18V coming in line but you'd need a small amount of delay so it would allow the regulator to start up at first.

 

Thanks for that. Would you able to suggest said zenner and transistor please?

Cheers, Ben

 

I believe any 12.1V Zener will serve the purpose. Since it's just a signal for a comparator, I guess no high power is needed.