Definition of mA

Discussion in 'Electrical Forum discussion & Blog' started by Giles, Jan 26, 2009.

  1. Giles

    Giles Retired tool & Die and Mechanic

    Messages:
    112
    Location:
    N.W. Alabama--Florence--
    I have a PowerMax+ Security system that has a maximum output of 550mA at 12dc volts for a remote siren. I have a large 6v bell that requires .5 A.dc
    I hooked the bell up and tried it for about 5 seconds and it is really loud.
    I am conserned that damage may be done to either the bell or the security system, main consern is security system.
    I guess what I am asking is 550mA considerably less then .5 amp. I am also aware that I am using a 6volt bell with a 12volt source.
  2. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    1 amp is 1000 milliamp. 550 ma is 0.55 amps
  3. CarlH

    CarlH New Member

    Messages:
    175
    Location:
    Northern VA
    Hooking up that 6V bell to a 12V power supply will roughly double the current draw (Amps) of the bell. That would be about 1000 mA or 1 A for the bell running off of the 12V supply. I would not hook that bell up to that system or you may either blow any over current protection on the power supply or damage the power supply.
  4. Giles

    Giles Retired tool & Die and Mechanic

    Messages:
    112
    Location:
    N.W. Alabama--Florence--
    THANKS to both of you for the quick answers. I felt that this was the situation and that is why I only ran the bell for 4 or 5 seconds. Even that could have caused problems! I guess I could use a relay and separate power source but that is a thought for later. I have a 12v external siren but the bell is, of course, much louder.
  5. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Assume the 12v supply goes to 14v with no load. This gives an internal impedance of (14-12)/0.55 = 3.6 Ω.

    Your load is 6/0.5 = 12 Ω at 6(0.5) = 3W, rated power.

    Your bell saw 14(12)/(12+3.6)= 11v @ 14/(12+3.6) = 0.9A for a power of ~10W.
    You might have shortened the life of your bell.

    Use a series dropping resistor.
    You want a half amp, so 14/R = 0.5, so R = 28 Ω; the bell uses 12 Ω so you need another 28-12=16 Ω, with a power rating of 2x(16(0.5²))= 8w.
    Believe it or not, a 75w incand. household lamp will do pretty well for this resistor. It won't light and probably won't be warm. If the measured bell voltage is less than 6 then use a larger bulb.
    Last edited: Jan 27, 2009

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