Help me drive a LED (max current 20mA) with a PWM

[david90]

say u have an LED that has a max current of 20mA and u would like to drive it with a PWM to save energy. how do you determine the pulse width of the output that would give u 20mA so that u don't burn out the LED?

 

[IanP]

Re: PWM and LED

Just calculate the average current..
For example, if you have 50% duty ratio, the ON current will be 40mA.. but no more than maximum tolerable peak current.
The maximum forward current IF Max. at Ta = 40°C is 20 mA for red, orange and yellow, and 25 mA for green. Therefore, the maximum tolerable peak current under these conditions is as follows :

Red, orange and yellow 20 mA @ 3.0V = 60 mA

Green 25 mA @ 2.4V = 60 mA
Regards,
IanP

 

[Btrend]

Re: PWM and LED

say u have an LED that has a max current of 20mA and u would like to drive it with a PWM to save energy. how do you determine the pulse width of the output that would give u 20mA so that u don't burn out the LED?

if ur Imax=20mA, and u want to achieve Iout=Imax=20mA, though PWM method, then u will always get 100% duty cycle. and no PWM is achieved.
as I know, the PWM using in LED control is for dimming purpose, which is make
Iout(avg) =D* Imax,
so the luminance will be changed with the duty cycle.

 

[haymeron]

Re: PWM and LED

In the datasheet of the LED lamp.
Look for the Peak Forward Cuurrent.
According to this parameter, caculate the duty-cycle and peak current.
Attached PDF file

 

[VVV]

Re: PWM and LED

Limit the current with a resistor, as you would do normally. R=Umax/Imax. I would not use 20mA, though. Something lower, just to be safe.

Now, the light intensity is proportional with the AVERAGE current through the LED. So you just need to turn on the LED with a duty cycle that will give you a moderate average current through the LED. Iave=Ipk*DC

But, as you can see, this method does not save any energy, since the average power is still the same, for the same intesity. You draw current less often, but higher current.
If you need to save pwer, this has to be done with a proper switching regulator, using an inductor (which dissipates little power).

Therefore, if the goal is to save energy while using a simple circuit, I would recommend you limit the current to a low current, that makes the LED bright enough.
Then have the LED flash. That way, the average current actually drops and you save power.

For example, let's assume you have an LED that needs 5mA to be as bright as you want it to be. If you go with the first method, you need a duty-cycle of 25%, limiting the max current to 20mA. You get an average of 5mA, but drawn continuously. There is no power saving. You could just limit the current to 5mA with a resistor and not need the PWM, saving money, space

Now, limit the current normally, with a resistor, to 5mA, but turn on the LED for 0.25 sec once a second second. Then your average current is 5mA*0.25/1=1.25mA. Now you save power.

They key is to have the LED on long enough for it to produce enough brightness at the low curent. Then just make it flash, to draw attention or indicate something.