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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.

SamGibson
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BobNll
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    "Should I read such a rating as 12v/80mah or 12V/.0080h" - neither. It will draw 80mA from a 12V supply indefinitely. – brhans Feb 18 '20 at 01:06
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    The slash "/" is throwing you off. It is not a divide. 80 mA @ 12V would be a better way to spec it. – Mattman944 Feb 18 '20 at 01:11
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    No, the slash isn't representing division, it is simply the way the rating was shown and I copied and pasted it. Okay, how about 12v 80ma? – BobNll Feb 18 '20 at 01:17
  • I was not speaking of a power supply but of batteries. – BobNll Feb 18 '20 at 01:23
  • By 'supply' I wasn't referring to any particular kind of power source. It's whatever you choose to use to supply power to your device - batteries, wall-wart, solar panel, thermopile, whatever. – brhans Feb 18 '20 at 02:06
  • well take your typical car battery that would supply your desired 12 V (typical 14.x Volt without load) - and the 80 mA will last a good while - since the car battery typically holds ~ 80 Ah .. so 1000 hours in that case – eagle275 Feb 18 '20 at 10:07

1 Answers1

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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.
jonathanjo
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  • Thanks, that is exactly what I thought but I was not sure. You are no doubt correct that the holding power is lower at lower voltages. That means it would be a matter of the force being exerted against the coil's pull that would determine when the device would release the item being held by the coil, given the voltage applied as the battery drains. – BobNll Feb 18 '20 at 01:22
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    @BobNll, it would be a good idea to measure the actual current just to make sure. Either that, or generously pad the size of the battery. Also, keep in mind that batteries go bad over time. If you just leave a battery there for 5 years, it may not work correctly at the end of that time. Somehow you need to test the battery or just automatically replace it periodically. – user57037 Feb 18 '20 at 02:24
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    @mkeith - I'm actually going to set up a test and see just how long various batteries last with device. I'll try to post my results here but don't hold our breath as I may not get around to the test for a few days (weeks?) – BobNll Feb 18 '20 at 03:34
  • @BobNll Don't just use a "12V battery" for a 12V load and hope for the best. If you need a particular voltage (and you do), use an actual voltage regulator. – Sneftel Feb 18 '20 at 10:14
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    If that's the spec for the power supply the device needs, it doesn't need to mean it draws the full 80 mA all the time, constantly, just that it may need that much available at some point of its operation (in general). – ilkkachu Feb 18 '20 at 10:42
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    @BobNll Be aware that most (rechargeable) batteries (especially lead) don’t like deep discharge. – Michael Feb 18 '20 at 12:04
  • @BobNll: You want a special "deep cycle" battery for that. Car batteries usually aren't, but you can find them for marine and camping use. They'll take 400 discharges to 50%. I.e. a 80Ah battery will deliver 400*40Ah = 50.000 hours at 80mA. (about 6 years, ignoring that you need to recharge it 400 times). – MSalters Feb 18 '20 at 17:17