# Definition of mA

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

1. ### GilesRetired tool & Die and Mechanic

Joined:
Aug 28, 2007
Occupation:
Retired
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. ### jimboPlumber

Joined:
Aug 31, 2004
Location:
San Diego, CA
1 amp is 1000 milliamp. 550 ma is 0.55 amps

4. ### CarlHNew Member

Joined:
Feb 4, 2007
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.

5. ### GilesRetired tool & Die and Mechanic

Joined:
Aug 28, 2007
Occupation:
Retired
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.

6. ### ThatguyHomeowner

Joined:
Aug 27, 2008
Occupation:
A bounty hunter like in "Raising Arizona"
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(0.5Â²))= 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