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Current Flow

Electron theory helps to explain electricity. Conventional theory (+ to -) and electron theory (- to +) both describe the flow of electrons, however conventional theory states that current flows from positive to negative, while electron theory states that current flows from negative to positive. The use of conventional theory in calculations is primarily for ease and consistency in the industry.

  • States that current flows from Positive to Negative.
  • This is called conventional current flow.

Note: If one volt is applied to a circuit, which has a resistance of one OHM, then a current of one AMP will flow in that circuit. This is an application of Ohm’s Law, which states that the voltage across a circuit is equal to the current flowing through the circuit multiplied by its resistance. In this scenario, the current (I) would be equal to 1 Amp, the voltage (V) would be equal to 1 Volt, and the resistance (R) would be equal to 1 Ohm. This simple example is often used to demonstrate the basic principles of electrical circuits and Ohm’s Law.

  • Mathematically we say: V= I x R
  • A change of any value will change the others

Ohms Law

Say that you’re wiring a circuit. You know the amount of current that the component can withstand without blowing up and how much voltage the power source applies. So you have to come up with an amount of resistance that keeps the current below the blowing-up level. In the early 1800s, George Ohm published an equation called Ohm’s Law that allows you to make this calculation. Ohm’s Law states that the voltage equals current multiplied by resistance, or in standard mathematical notation

  • V=I*R
  • OHMS law also calculates Power, and Current
  • P = Power, measured in Watts
  • I = Current, measured in AMPS
  • V = Voltage, measured in Volts

Ohm’s Law works that way; you can rearrange its elements so that if you know any two of the three values in the equation, you can calculate the third. So, here’s how you calculate current: current equals voltage divided by resistance.
You can also rearrange Ohm’s Law so that you can calculate resistance if you know voltage and current. So, resistance equals voltage divided by current.

So far, so Now, take a specific example using a circuit with a 12-volt battery and a light bulb (basically, a big flashlight). Before installing the battery, you measure the resistance of the circuit with a multimeter and find that it’s 9 ohms. Here’s the formula to calculate the current:
I = V/I = 12 volts/9ohms = 1.3 amps

What if you find that your light is too bright? A lower current reduces the brightness of the light, so just add a resistor to lower the current. Originally, we had 9 ohms; adding a 5-ohm resistor to the circuit makes the total resistance 14 ohms. In this case, the formula for current is I = V/I = 12 volts/14 Amps = 0.9 amps

To calculate current using the power or ohms triangles

  • I=V/R = 10V/10 =1Amps
  • I=P/V = 10W/10V=1Amp
Power TriangleOhms Triangle

Calculate the current needed for a 15 Watt Alarm Siren? It’s important to note that the sound wattage and the power wattage are different and not the same thing. The power wattage refers to the amount of electrical power the siren consumes, whereas the sound wattage refers to the loudness or volume of the sound produced by the siren. Don’t confuse these concepts.

For a 15W alarm siren, using the formula: I=P/V

I=P/V=15W/13.5V=1.111A

However, this theoretical calculation does not reflect real world power consumption. Many 15-Watt sirens actually draw less current often around 400 to 500mA. If you take a 15W siren and do the calculation you get an answer of 1.1A, which is more than what a typical panel can handle, but a typical 15W siren does not necessarily consume 1.1 Amp. Let’s take an example where the siren consumes about 400mA.

  • Current draw: 404mA
  • P=I*V=0.404A*13.5V=5.252W
  • Sound Wattage is not Power Wattage. This means the siren is labelled as a 15W, its actual electrical power consumption is closer to 5W, with the 15W rating referring more to its sound output rather than its electrical draw.

So, based of a panels Maximum Output:700mA Bell of the Alarm System, if we do this calculation

Panel Power Considerations

Most alarm panels have a maximum bell output of 700 mA (manufacturer dependant 500 to 700mA)

  • P=I*V=0.7A*13.5V=9.45W almost 10 Watt, which most alarm systems is rated for

If the unit or siren draws or consume 0.400mA then we use

  • P=I*V=0.404A*13.5V=5.252W of the 10 Watt that typical alarm systems have available. However if we exceed 10W

We can Damage the panel output*** It is important to make sure that the alarm siren you use is compatible with the alarm panel and does not exceed the panel’s capacity. Overloading the panel’s output can result in damage to the panel and potentially render the alarm system inoperable. It is always best to consult the manufacturer’s specifications and consult with an expert if necessary to ensure proper installation and operation of the alarm siren.

  • Rule of thumb – Any siren bigger than 10W requires a relay.
  • https://www.amazon.com.au/SD-15WULF-SD-15W-ULF-Siren/dp/B0041X4DKA
  • Operating Current: 380mA Surge Current: 404mA Peak Current: 392mA

Key Takeaways

  • Always check the actual current draw of a siren rather than assuming based on wattage.
  • Overloading the panel can cause damage, so ensure the siren is within the panel’s rated capacity.
  • Any siren over 10W should use a relay for safe operation.