Electrical – Power calculation of circuits


Please read on….. this looks long but is entry level and easy

I have done many simple projects based on microcontrollers but have not considered power and required current(for circuit to work) calculations. But now as I was doing my another project i.e., "8051 microcontroller development board" on a perf board, I found that not all the functional blocks(16x2LCD, max232 for serial comn, ADC0848, relays, etc) can work at the same time with same power supply since the current required was not enough for all of them to work.

So to make them work I added additional supplies and they were working quite perfectly. But now I know I was missing some serious power and current calculations, I had to do before I solder them. These calculations are important when you are designing or making any commercial product and also to save world's energy.

So I decided to calculate the power in a simple 555 timer astable circuit. The astable 555 timer is as shown

enter image description here

Vcc=5V, R1=R2=1Kohm, C=0.1uF

To calculate the power consumed I could directly find it through the multimeter but I want to calculate the power theoretically from the manufacturer's datasheet info.

NE555 datasheet is here

What have I done?

We know that power is:

$$ P = IV = I^2 R = {V^2 \over R} $$


  1. power at trigger pin = max voltage at trigger * max current at trigger
    =2.2V(from the datasheet) x 2uA(from the datasheet) = 4.4μW

  2. Output power = Max voltage time the current flowing through the load.
    Let's say the output is connected to LED with a 1Kohm resistor
    Maximum output voltage when high =3.3V(from the datasheet) and current = 4.3mA(say when LED is connected to the output).
    So when the capacitor charges the output is high and when it discharges the output is low.
    low voltage maximum output = 0.4V, so I will calculate the power at output for one time period and then multiply for any time I run the circuit.

  3. power at reset pin : Here we assume that the reset pin is always connected to Vcc=5V so the maximum power = max voltage * max current
    = 1V x 0.4mA = 0.4 mW

  4. Power at control voltage = 0 since it is connected to a capacitor terminal whose other terminal is connected to ground.

  5. I don't know how to calculate the power going into the Vcc pin. I know the functional block diagram and I dont think it's as simple as the voltage divider since the power has to be given to the opamp's, SR flip-flop etc.

  6. Power dissipation in R1, R2 = (current through R1 or R2 squared) x (R1+R2)
    while charging and discharging. Since the current through these resistors are varying, so I thought to calculate the average current through charging and discharging and then find the power dissipated in one time period and than sum the power dissipation for one second.

  7. power at threshold pin would be negligble beacuse the max current flowing into threshold pin is 250nA(from the datasheet)

  8. How to calculate the power at discharge pin

I would now like to ask

  1. Are all my values taken from the datasheet right for calculations? Or did I take some wrong values?

  2. Are my calculations right?

  3. Assuming things are right with above question then how can I move further.. that is power calculations with microcontrollers etc.

Thanks in advance for spending your valuable time and bearing with me

Best Answer

The original issue was the over-taxing of the first power supply. Generally then you could use the max voltage/current supply figures from the 555 datasheet. Do the same for all the other expected devices. Many devices will have datasheets that you can use to approximate the actual power draw and that may be far below the maximum values. There may be a need to use separate power supplies for other than power delivery. i.e. to minimize noise in a high resolution ADC, or avoid using heat sinks ... etc.