Power Calculation for Constant Current Supply

[krich]

I'm still learning electronics, mostly self taught, and there's one thing that's really nagging me at the moment. I am trying to reason out how to calculate the Power (W) consumed by an LED driven by a Constant Current supply. For example:

An LED with a Vf of 2.2V is driven by a 12V supply with constant current of 100mA (basic LM317 constant current arrangement, if it matters). How many watts is consumed and by which parts? Does the constant current aspect of this really matter over a simple resistor arrangement? How much is consumed by the LED itself and how much is dumped as heat in the constant current supply.

I have my opinions, however, I have the feeling that I'm missing something fundamental here, and I want to be sure I've got this right.

If it matters, this isn't homework. I've been out of school for decades. It's just good old fashioned curiosity.

Thanks.

 

[ronsimpson]

The LED is simple. Wattage is V x I. 2.2 x 100ma = 220mW

The LM317 is also simple. 9.8V x 100mA = 980mW

Power form the 12 volt supply is 12 x 100mW = 1.2 watts. .22W + .98W = 1.20w

 

[krich]

Okay, I can see that it really is no different than calculating power with a resistor rather than the LM317 source.

For the simple LM317 constant current circuit, I assume the power is largely dissipated in the resistor used to set the current, correct?

 

[crutschow]

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For the simple LM317 constant current circuit, I assume the power is largely dissipated in the resistor used to set the current, correct?

No. If you look at how the circuit operates, you will see that the constant current is determined by the LM317 maintaining 1.2V across the resistor (notice the formula for calculating the current). The rest of the voltage is dropped across the LM317. I'll let you figure out the relative power dissipation levels of the two.