I have an application with multiple components and multiple voltages in use and current calculations are giving me a headache.
A simplified version of my application would include:
- 1x device drawing 300mA @5V
- 1x device drawing 100mA @6V
- 1x device drawing 500mA @12V
- 1x 13.7V SLA battery being trickle charged
In order to trickle charge the battery I need a 15V PSU with current limiting resistor fed from the 15V PSU.
In order to supply 12V, I am using a voltage regulator MC7812 fed from the 15v PSU.
6V device gets power from a step down converter 62180 fed from the 15V PSU.
The 5V device feeds off the 62180 6V supply and uses current limiting resistors to reduce down to 5V.
But what wattage PSU do I need?
Using P=IV suggests P=0.3 x 5 = 1.5w but as my voltage supply is ultimately 15V and not 5V, would I need to multiply this by 3 (to get current draw at 15V) before adding to the other calculations or can I just calculate the wattage for each device (@ each devices voltage level) and then add them up?
using method 1 (excluding the SLA battery): (0.3*5) * 3.0 = 4.5w + (0.1*6) * 2.5 = 1.5w + (0.5*12)* 1.25 = 7.5w =13.5w However using method 2 (excluding the SLA battery): 0.3*5 = 1.5w + 0.1*6 = 0.6w + 0.5*12 = 6.0w = 8.1w