PWM with Lowpass Filter

I know that it is to get rid of ripple voltage and obtain a clean analog voltage. For example for brightness control of an LED, I don't think that I need it. But, when do I need to use it ?

Read this:- http://www.thebox.myzen.co.uk/Tutorial/PWM.html

You need to use a filter when you require a smooth voltage like in a linear motor speed control.

Grumpy_Mike: Read this:- http://www.thebox.myzen.co.uk/Tutorial/PWM.html

You need to use a filter when you require a smooth voltage like in a linear motor speed control.

Thanks, I'll read it

For brightness control of a LED, you generally must not filter the PWM, because you are using persistence of vision to obtain the relative brightness, but more critically because if you were to filter it, then your LED switch transistor would be operating in a linear mode and dissipating substantial power.

That said, because the LED is a constant voltage device, if you are already using the same transistor as a current driver, it actually makes no difference. And you cannot PWM switch or linearly regulate a switchmode constant-current driver unless it is designed to perform the dimming function itself.

Here’s a screen shot (literally) of the PWM output after going thru a low pass filter.
The PWM was adjusted from 0, 100, 200, etc to 1000 every 300mS

Paul__B: For brightness control of a LED, you generally must not filter the PWM, because you are using persistence of vision to obtain the relative brightness, but more critically because if you were to filter it, then your LED switch transistor would be operating in a linear mode and dissipating substantial power.

That said, because the LED is a constant voltage device, if you are already using the same transistor as a current driver, it actually makes no difference. And you cannot PWM switch or linearly regulate a switchmode constant-current driver unless it is designed to perform the dimming function itself.

Your point about not to filter PWM for transistor is totally reasonable. But I didn't understand your last remark.

Common LED drivers are DC-DC converters operating in constant current mode, but
many of them cannot be rapidly switched from off-on-off so are not PWM-able. This
is the sort of set up for high power LED lighting, not a single indicator LED though.

Your point about not to filter PWM for transistor is totally reasonable. But I didn't understand your last remark.

That said, because the LED is a constant voltage device, if you are already using the same transistor as a current driver, it actually makes no difference. And you cannot PWM switch or linearly regulate a switchmode constant-current driver unless it is designed to perform the dimming function itself.

If it's a constant current LED driver then it is not intended to be driven with a PWM signal. That should be easy to understand.

Depends on the driver, doesn’t it?
For example:
http://www.mouser.com/new/meanwell/meanwelllddled/
“Built-in PWM dimming function”
So it’s designed to accept a PWM output from say an Arduino for dimming.

YES, it does.