Electronic – If a device is rated at 12 V/80 mA what does that mean as far as the length of time that it will draw 80 mA

amperagecurrent-rating

I have an electromagnetic device designed to hold a fire door open until either the circuit is cut or the building's electrical supply goes out. At which point the door is released and will close.

It is rated as 12 V/80 mA. What does that mean in terms of the length of time that the device will draw the 80 mA?

Should I read such a rating as 12 V/80 mAh or 12 V/.0080 h?

I'm doing some rough calculations as to how long a given battery combination would keep this device energized.

Best Answer

It will draw the 80mA current for the period it is on, ie, has power applied.

If you are powering it from a battery, that's when the time comes into play.

For example, an 8 Ah battery would power the device for 8 Ah / 0.080 A = 100 hours.

BUT

  • A battery's storage is often quoted under various conditions which might not be continuous discharge (and are often given as multiple values at different currents).
  • You might well also find that the holding voltage is lower than 12V, so that as your nominal 12V battery's voltage drops below 12V the device still holds the door open, it will depend on the strength of the spring or other door closer.
  • Battery charge capacity varies considerably with age, charging history and battery technology
  • If the device is a simple electromagnet, its current will vary with voltage (basically linearly)
  • Be aware that batteries and devices in general might give nominal, maximum, minimum, typical, average or "specification" values on the label without saying which. And caveat utilitor, nobody really expects to see the manufacturer's Highway Mileage or even Urban Mileage on their car. For a fire door, make sure your system is fail-to-safe (and tested!) and be clear about which responsibility is whose.
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