480VAC three phase

Thread Starter


Joined Sep 8, 2011
Hi I use this circuit to detect phase loose, so if one of the phase lost , the phase loss signal will be high, the problem in this circuit are.
1-if the input voltage less at 50% it give phase loss , i did try to increase the resistors (R99,R100 and R102) this make work but it cause another problem
is when i have phase loss and if input voltage more than 380Vac , the circuit does not detect phase loss becuase all optocoupler are on.
So how can i make this circuit work right like if i loss one phase , the phase loss signal be high and is not turn off at high input voltage and also how can avoid getting phase loss when the voltage at less than 50%


Joined Mar 30, 2018
I don’t think you will be able to modify the circuit easily to do what you want.

But if each front end bridge rectifier (feeding the opto coupler) is wired from each phase to neutral (rather than being across phases), you will have much better control of the voltage at which it detects/signals the loss of a phase and will not be influenced by increased phase to phase voltages.