# Simscape/Simulink persistent variable in algebraic loop

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Matthew Mishrikey on 11 Dec 2021
Answered: Alan Lian on 10 Apr 2023
I have a simple Simscape electrical model containg a circuit that trips if a current is exceeded. I want the trip to happen a certain time after the current excess, so I use Matlab function block and the Simulink Clock Block to track time.
The red highlighted portion is flagged as modifying a persistent variable in an algebraic loop.
But I am not sure how to acheive my goal without the MATLAB function block and the Simulink Clock.
The code in my function block is here:
% Monitors current. If over current, trip with a timed delay.
function switch_control = current_monitor(current, clockin)
persistent starttime;
persistent tripped;
if isempty(starttime)
starttime = 0;
end
if isempty(tripped)
tripped = false;
end
if (current > 10)
starttime = clockin;
if ((clockin - starttime) > 0.2) % fixed delay of 200ms
tripped = true;
end
end
if(tripped)
switch_control = 0;
else
switch_control = 1;
end
end
Any insights on what I am doing wrong?

Matthew Mishrikey on 13 Dec 2021
I think I managed to figure out a solution. As suggested elsewhere a delay block can "break" the algebraic loop.
I needed three persistent variables to make it work, not sure if there is a simpler way.
The timing was off so I changed the solver and also changed it to fixed step with time step siginificantly smaller than the delay I was looking for. Then it worked very well.

Alan Lian on 10 Apr 2023
I meet the same problem today, and I try a new way different from you. Put a switch after switch control, and use a step to turn on this signal channel in 1e-5 second. This solution works well in my condiction.

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