Troubleshooting Alarm Power Supply Problems.
Alarm Power Supply Problems – We’re having trouble with false alarms on a system running many PIRs and other sensors.
We’ve traced the problem back to the common power supply – short of using multiple power supplies, what can we do to solve the problem?
A: Step 1, first check the loaded voltage of your supply, making sure it’s in specifications – if this is the case, i.e. power supply is OK, check the sensors and note operating voltage range and, if recorded, current.
Armed with this information, you can calculate acceptable voltage drop for the sensors, e.g. if a 13.8V power supply is managing a sensor rated 10-16V, then maximum allowable voltage drop is 3.8V. Note this figure because you’re going to need it later.
Troubleshooting Alarm Power Supply Problems
Take into account current consumption and using Ohm’s Law (V = IR) it’s possible to calculate a maximum acceptable resistance that can be used in line. Using 20mA (I) as an example and taking our 3.8 (V) drop, rearrange the formula to read R = V/I. When the numbers are plugged in we’ll get the answer – it’s 190ohms.
Using the worst-case sensor in the system, that is the sensor with the lowest acceptable voltage drop and the highest current consumption, calculate the resistance value for your system.
The next step is to select a preferred value resistor (in the range of 10-100 Ohms). What you’re looking for is a resistance of half the figure measured as maximum possible resistance.
In series with the positive voltage line, install a resistor on each sensor, then install an electrolytic capacitor in the range of 10-100yF (watch your polarity) with the positive lead attached after the resistor and the negative lead attached to ground.
This provides isolation and filtering to all devices on a common supply. Consider that the higher the resistor and capacitor values, the more isolation you’ll have and bear in mind the maximum allowable voltage drop behind that resistor, as well as the fuse rating of the supply. Higher resistor and capacitor values will increase in-rush current at turn-on.
You can find an interesting attempt to reduce noise in a circuit here or read more SEN news here.
“Troubleshooting Alarm Power Supply Problems.”
