Total power of a pulsed source over time?

[dnyberg2]

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!

 

[Carrock]

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.

 

[dnyberg2]

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!

 

[dnyberg2]

I guess what I'm after is a power totalizer type calculation over hours...

 

[Tom.G]

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.

 

[dnyberg2]

Cool! Thanks so much!

 

[dnyberg2]

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

 

[Carrock]

I'm wondering if you want the LED power or (LED + dropping resistor) power.

I plugged some values into https://www.vishay.com/resistors/pulse-energy-calculator/

Pulse width 0.04 sec
Voltage 2V
Current 0.005A

and got Pulse Energy = 0.4mJ

Then cycle time = 2S gave 0.2mW
I'm guessing you had a decimal point problem.
I note Tom.G has just posted; his is a better approach for understanding than just putting in values at https://www.vishay.com/resistors/pulse-energy-calculator/

 

[dnyberg2]

Interesting...

 

[CWatters]

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

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.

 

[tech99]

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?