Before you begin
To help you troubleshoot inrush issues and what causes them, familiarize yourself with these terms:
Capacitance—The capability of a material to store electrical charge. Capacitors are the physical component that adds capacitance in electrical circuits.
Inrush current—Also called input surge current or switch-on surge—the maximal instantaneous input current that is drawn by an electric device when it is first turned on. Generally speaking, the higher the capacitance of a device, the higher the inrush current draw that an individual device will have.
Understand how an inrush current causes problems
The image shown illustrates a large current draw when a device is first powered on that decreases into a steady-state current. The peak and the inrush current magnitude depends on the number and type of capacitors that are in an electrical circuit.
The image shown below is an example of 2 different motion sensors that have different capacitance levels despite the data sheet reporting similar power requirements.
Motion Sensor | Power | Example |
Honeywell IS335 (2 Capacitors) | 9–15VDC @ 12mA |  |
Honeywell DT7235T (10+ Capacitors) | 7.5–16VDC @ 24mA |  |
The Honeywell DT7235T has a significantly higher capacitance and inrush current compared to the Honeywell IS335 due to the number and size of capacitors needed to support its DUAL TEC functionality.
How to identify capacitance issues
Generally speaking, capacitance issues present in 1 of 2 ways, as shown in the example of the BP41 Alarm Panel.
Immediately after wiring in a high capacitance device. The inrush current of the device is so high that it causes a brownout where the panels lose power due to an internal protective mechanism to protect the system from a short.
After a power cycle, reboot, or firmware upgrade on the BP41. The combined power requirements of system boot and inrush current of all devices at once exceed what can be supplied by the panel, again, causing a brownout where the panel loses power. This issue is not seen during the initial install because:
The panel is already in a fully booted state.
External sensors are added and wired in one-by-one, and each individual component is given enough time to charge its capacitors.
There is an "AUX Output Failure" warning on the device and "AUX power disabled" warning shown in the alarm site. The AUX is disabled to avoid the inrush issue from the 4-wire sensors and allow the system continue to boot and remain online.
What to expect
This can cause the panel to:
Fail to boot up completely until the sensors are removed from power
OR
Introduce delays in boot up until the capacitors on the devices are charged enough to bypass the internal protective mechanism.
The panel typically has its Panel Status LED (OFF) and the Power LED (ON).
How to solve problems with inrush current
We recommend wiring a 100Ω resistor inline to the power of each of the sensors to help slow down current draw from the sensors and avoid a brownout.
After wiring in the resistors, confirm system operation by power cycling the panel and to ensure that the panel boots up properly.
Need more help? Contact Verkada Support