Douglas Krantz - Technical Writer - Describing How It Works

Does the Current for a Fire Alarm Strobe Pulsate?

By Douglas Krantz | Maintenance

Does the Current for a Fire Alarm Strobe Pulsate?

Does the Current for a Fire Alarm Strobe Pulsate?

Greetings Douglas,

Does the current of a strobe circuit pulsate as the strobe flashes, or is it a constant current.

Also, can a strobe be operated by a bench power supply?

Thank You, DS

Strobe Specifications

The current used by each model of strobe, including the variations during the once-each-second flashing, is different for each model of strobe. There isn't any standard of the industry specification.

However, just from how the power supplies, the building's wiring, and general-type fire alarm strobes operate, there's quite a lot that can be learned.

Power Supply Voltage

The power supply provides the voltage that's needed to operate the strobes. Connected to the same two wires from the power supply, both the strobes, and the fire horns get their power to operate.

Many Notification Appliance Circuits (NACs) power both horns and strobes at the same time, so the voltage provided by the power supply is pretty much constant; the power supply voltage does not vary during the during the strobe-flash cycle, or even during the horn's temporal or march-time cycle.

Power Supply Current

Used to power the strobes, the voltage from the power supply pushes the current in the NAC circuit. One of the ratings for the power supplies is the allowable current that can be used in the circuit.

The current on the NAC (the building's Notification Appliance Circuit) can't exceed the power supply's current rating. I've had to troubleshoot NAC circuits because the devices on the circuit exceeded the rated current on the power supply.

In essence, the devices in the NAC circuit, like the strobes or horns, have to be consistent in their current use, not pulsed.

Notification Appliance Circuit Building Wiring

With fire alarm NAC wiring, the drop in voltage is very sensitive to current. When designing a fire alarm's NAC circuit, the voltage loss for each NAC circuit is normally calculated by the installer or engineer.

They use Ohm's Law. The voltage loss is calculated from the resistance of the wire, times the current being used.

Swings in the current (pulsed current), between the flash time of a strobe and the time between flashes, produce swings in voltage loss; the swings in voltage loss would wreak havoc on the calculations. Just to make realistic calculations, the real life current, used by the strobe, has to remain relatively constant throughout the charge-and-then-flash cycle.

Slow Capacitor Charge - Fast Flash Discharge

To keep the current consistent, constantly, during the entire cycle time of the strobe, a capacitor is charging; it's storing up power (energy). That means the current being used from the fire alarm's NAC circuit is pretty close to constant.

On the other hand, the duration of the strobe's flash is less than 1/100 of a second. Rather than using a rush of current from the NAC circuit, the flash-tube inside the strobe gets it's rush of current from the capacitor.

Even the photographic strobes used by photographers have that capacitor. The energy to power the flash tube is stored over a period of time because the battery in the camera can't provide the current all at once to power the flash.

Current Used by a Strobe

I'm not sure about any particular strobe model, but in general, the current use by most fire alarm strobes cannot go up and down very much between the capacitor's charging time and flash time. The power supplies and the NAC wiring cannot take it.

Bench Power Supply

Using a bench power supply is powering the strobe in a default mode. When using a bench power supply, or even a battery, conventional strobes and addressable strobes operate differently.

Conventional Strobes - From what I've seen, when synchronization signals are missing, all conventional fire alarm strobes default to working at a once-per-second flash rate. They won't be properly synchronized, but at least they'll work.

In other words, as long as the voltage is correct for a conventional fire alarm strobe, a bench power supply will work. A conventional fire alarm strobe will work even on a battery.

Then again, if the synchronization signals are for a different system than what the conventional strobes are designed for, the strobes might not work at all.

Addressable Strobes - Each manufacturer is different; I do not know how addressable fire alarm strobes operate in a default mode. I suspect that they won't work unless they're turned on by the addressable fire alarm control panel. To find out for sure, you'll have to contact the strobe manufacturer's technical support team.

Douglas Krantz
Life Safety
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