Hello!
I have discovered how to convert Arduino's PWM to an actual analog signal using a low-pass filter. I also found some calculator for low-pass filters: (Sample)RC Low-pass Filter Design for PWM - Result -. I have some questions:
What is the cut-off frequency?
The peak-to-peak ripple voltage is the voltage difference between the peak and trough of the noise in the signal, correct?
Is it possible to amplify the current, but not the voltage of the signal using a transistor (I want to test the minimum voltage to light an LED, but the 10k resistor lowers the current too much [the LED is very dim, even at 3V])?
I may have more questions!
Thanks!
You need a cut-off frequency that is below the PWM frequency. The lower it is the less ripple you'll get, but the longer it will take to react to changes in the duty cycle.
Either method would be virtually worthless. Unless I an mistaken you asked the same question a week or so back about a pot and an led.
An LED requires a Maximum voltage supplied to it and a Maximum current flowing through it. both are recommended ratings and if you exceed one you will automatically exceed the other. An led can be thought of as a (for example ONLY) 1.8V (red led only) .04W resistor for a typical red led (1.8V X .02A = .036W. So in an LED if you exceed the voltage by for example a volt... you now have (1.8 + 1V(excess) X .02A = .054W almost twice the recommended power dissipation of .036W over time this condition will shorten the led's expected lifetime and may well cause a failure any way as led's are fragile internally.
Forgive my lengthy explanation but you already have the answer in front of you... the minimum voltage is the voltage applied to the LED to cause it to just begin to be visible or emit light. I have a blue (2.6V) led on a breadboard connected to 5V through a 10K ohm resistor and it is quite bright and only draws 250 uA from the 5V source, @ 2.5 mA it is Very bright @ 25 mA it is normal Super Bright and @ 250 mA it is dead... forever dead as I have exceeded the max recommended current by a factor of 10. Above the minimum applied voltage an led can be thought of as a Short Circuit, it Must be current limited. There is a second answer here too, all leds are slightly different and will require different amounts of current to be just barely visible even from the same manufacturer and the same part#. I hope this helps it is about as simple as II cam make it and cover the salient points.
Docedison:
Unless I an mistaken you asked the same question a week or so back about a pot and an led.
You are mistaken.
Docedison:
Above the minimum applied voltage an led can be thought of as a Short Circuit, it Must be current limited.
When did I say that I would not use a resistor of some kind?
I just wanted to see the minimum voltage needed for some LED to light up. Multiple LEDs will be tested.
Is the "forward voltage" the minimum amount of voltage to light an LED?
no it is the Junction Voltage much like a simple silicon diode will have a .6V drop across it, an LED will have a 'forward voltage' that can range from 1.8 to about 3.8V depending of material (color) and manufacturing process. it will also have a recommended current above which you will be exceeding the recommended power dissipation as well If you truly would like a real answer to your several questions over the past week or so then post a COMPLETE circuit or design requirements and someone will provide an answer for you. If I see it and feel that the answer given you is wrong or inadequate, rest assured I will add my $.02 to it. What you are asking has no real answer I feel, beyond what you were given so far.
Take your led, put it in series with a 220 ohm resistor and power it with 5v,
then take your multimeter and put it on the led's terminals
you will see the actual vf of the led, and take into consideration that it will change(lower) as the led warms up, which leads into why leds are normally considered "current" driven in that the actual voltage is less important than that the current stays at the proper level designed regardless of temperature or variations in leds
Is the "forward voltage" the minimum amount of voltage to light an LED?
Forward voltage is the voltage that will cause current to just start to flow, and not necessarily light the led. This value will change somewhat with temperature and slight variation values from led to led. LEDs are not voltage controlled devices, they are current controlled devices and must be driven by a voltage source well above their forward voltage value and the current controlled by external means, either a resistor or a constant current source.
