How to connect a 2-Wire sensor to Zelio Logic SR3B261FU.

About the SRxxxxxFU module :
The Input inpedence of the SRxxxxxFU is about 350kOhms

About the 2 wires sensor :
First at all, it is necessary to know residual current at open state of the sensor used.
Due to the residual current of the sensor at open state, the Zelio input will be always to ON.
In order to work around, it is necessary to add a resistor in parallel with the internal resistor of the Zelio Module.





How determine the additional resistor
In our example, we use a 2 wires sensor with a maximum residual current about 0,6 mA
On the Zelio input characteristic, the maximum voltage to guarantee a state 0 on the input is about
40V. In our example, we take Uo = 30V (< 40V)

1. Value of the resistor :
Uo = (Rext//Ri) x Ir
30V = (Rext//Ri) x 0,6mA
(Rext//Ri) = 30V/0,6mA = 50Kohms
Rt = (Rext x Ri) / (Rext + Ri) = (350 x Rext) / (350 + Rext) = 50kohm
Then Rext = 58Kohms We can use a 47Kohm resistor

2. Power of the resistor :
The power is about P = U²/R = 230²/47K = 1,12W.