PCE Instruments Crowcon Xgard Bright Manuel De L'utilisateur Et De L'opérateur page 23

Xgard Bright
4. Repeat this process for each detector, accumulating the voltage losses that will
occur between each detector.
5. The maximum detector voltage of 30V must not be exceeded.
Example Calculation using the above rules
How many Xgard Bright can be put on a multidrop cable if:
1. The controller has a guaranteed minimum output voltage of 18V.
2. Cable resistance is 12.1Ω/km.
3. There is 20m between each detector and 20m from the final detector to the controller.
4. The worst case current draw (Xgard Bright toxic) is 70mA.
Xgard Bright Xgard Bright
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So consider the voltage to the detector furthest (n=1) from the controller has to be 10V.
Each cable segment has a there and back resistance of 12.1 x 2 x 20/1000 = 0.484 Ohms.
So the cable volts drop to detector (n=2) is:
Vc = 0.070 x 0.484 = 0.03388V
V(n=2) = V(n=1) + Vc = 10.0338V
Now the voltage at detector (n=3) is
V(n=3) = V(n=2) + 2Vc (as there is twice the current supplied through this cable segment)
V(n=3) = 10.03388 + 0.06776 = 10.10164V
Xgard Bright
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Control
Panel
Xgard Bright
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