Total power of a pulsed source over time?

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Discussion Overview

The discussion revolves around calculating the total power consumption of a pulsed LED over a specified duration, focusing on the average power during the pulse and the total energy used over a 10-hour period. Participants explore various aspects of the LED's operation, including current, voltage, duty cycle, and the role of the dropping resistor.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant states that the LED pulses every two seconds, using 5 mA for 40 ms, and questions if the calculated average power of 188 mW is correct.
  • Another participant suggests that the average power during the pulse might be around 9W but emphasizes the need for the voltage drop across the LED for a more accurate answer.
  • A participant clarifies that the LED operates at approximately 2 volts and that the voltage tapers off after the pulse, indicating a focus on the pulse for calculations.
  • One participant proposes a symbolic calculation for power drawn by the LED, suggesting to substitute the LED voltage into the formula for clarity.
  • Another participant uses the pulse energy calculator to derive a pulse energy of 0.4 mJ and an average power of 0.2 mW, hinting at a possible decimal point error in earlier calculations.
  • A later reply agrees with a previous calculation and reiterates the average power formula while questioning the circuit configuration that might affect current draw.
  • One participant emphasizes the distinction between energy and power, noting that energy is measured in Joules and is time-dependent, while power is energy per second.

Areas of Agreement / Disagreement

Participants express differing views on the calculations and interpretations of power and energy, with no consensus reached on the correct average power or total energy consumed by the LED over the specified time period.

Contextual Notes

Some calculations depend on assumptions about the supply voltage and the configuration of the circuit. There are unresolved details regarding the voltage drop across the dropping resistor and the behavior of the LED when off.

dnyberg2
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Lets say I have an LED that is being pulsed once every two seconds.
The LED uses 5 mA of current each time its fired and the pulse lasts for 40mS.
I know the current into the LED because I measured the voltage drop across the dropping resistor of 420 ohms with a good O-Scope. So I know all these factoids and I found a calculator that seems to be able to tell me the power of the pulse at https://www.vishay.com/resistors/pulse-energy-calculator/. It came up with about 188mW due to the 2 second dwell between pulses...

Does that sound about right? 188mW average power? Averaged over what is my problem?

What I really need to know is how much total power is used by the LED in say 10 hours??

Any help would be greatly appreciated!
 
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Seems like ~9W during the pulse - very high for one LED. You need to give the voltage drop across the LED, and whether the voltage during each pulse varies for a useful answer. Also I presume you want the energy used during the ten hours.
 
Ummmm... YES! The LED is a standard surface mount red LED at about 2 volts. The voltage applied looks like a logic level 2 volts for 40mS then it tapers off real slow down to some voltage before the cycle happens again. As I am not interested in anything but the pulse that is capable of turning the LED on, everything else is useless so just the pulse in the calculations...

And I need to learn to fish so don't just supply the answer as these conditions will vary over the experiment.

Thanks!
 
I guess what I'm after is a power totalizer type calculation over hours...
 
Since the source voltage was not given, the below is done symbolically. I'm assuming the energy supplied by the power source is needed. If just the LED dissipation is desired, substitute the LED voltage (2V) for <SupplyVoltage> in the calcs below.
  • Calculate the power drawn by the LED when on; <SupplyVoltage> x 0.005A = 0.0ZZW.
  • Calculate the Duty Cycle = 0.04 sec/2sec = 0.02
  • Multiply the power by the duty cycle
    0.0ZZW x 0.02 =ZZZW average power over each 2Sec. cycle.
For a 10 hour power consumption multiply the power-per-cycle (ZZZW) by the number of cycles in the time period.
ZZZW x ([3600SecPerHr] / [2SecPerCycle]) x 10Hrs = ?.?Watts from the power source.
 
  • Like
Likes   Reactions: CWatters
Cool! Thanks so much!
 
Interesting the LED seems to clamp the supply to 2 volts so I guess that's the end supply...
 
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Likes   Reactions: Tom.G
Interesting...
 
  • #10
I agree with Toms calculation..

For the LED alone..
Average power = Current * voltage * mark/space
= 5*10-3 * 2 * 40*10-3/2
= 0.2mW

dnyberg2 said:
Interesting the LED seems to clamp the supply to 2 volts so I guess that's the end supply...

There are several different ways to drive an LED. Do you have a circuit diagram for your set up? Some configurations draw more current when the LED is OFF than when it's ON.
 
  • #11
dnyberg2 said:
Lets say I have an LED that is being pulsed once every two seconds.
The LED uses 5 mA of current each time its fired and the pulse lasts for 40mS.
I know the current into the LED because I measured the voltage drop across the dropping resistor of 420 ohms with a good O-Scope. So I know all these factoids and I found a calculator that seems to be able to tell me the power of the pulse at https://www.vishay.com/resistors/pulse-energy-calculator/. It came up with about 188mW due to the 2 second dwell between pulses...

Does that sound about right? 188mW average power? Averaged over what is my problem?

What I really need to know is how much total power is used by the LED in say 10 hours??

Any help would be greatly appreciated!
In 10 hours the LED would consume a certain amount of energy, measured in Joules, not power.
The power consumption of the LED is not time dependent. Power is energy per second.
I am not sure which you require?
 

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