[sdiy] PWM, Vactrols & Opto-Isolators

[Dustin Withers]

Tom,

Is a pulse frequency of 30Khz considered high?

Thanks for the help,
-dustin

On Wed, Jan 13, 2010 at 12:36 PM, Tom Wiltshire <tom at electricdruid.net> wrote:
>
> On 13 Jan 2010, at 16:32, Dustin Withers wrote:
>>
>> I'm curious about what the differences are between Opto-Isolators and
>> Vactrols. I think they are the same thing, right?
>
> Pretty much, but you might find some opto-isolators are designed
> specifically for digital signals, whereas vactrols are specifically designed
> for analog signals.
>
>> Also I'm interested in converting PWM to a voltage. I know this can be
>> done with a low pass rc circuit. I remembered that you could also
>> control the brightness of an LED with PWM. Couldn't a
>> Vactrol/Optoisolator be used to convert PWM to a voltage? What are the
>> pros/cons vs the RC circuit?
>
> Probably pretty much the same. With the simple RC circuit, you've got a time
> constant to consider, which obviously has to take your PWM frequency into
> account. The situation is similar with a vactrol, only the time constant is
> the reaction time of the LDR.
> Either way, the smoothing isn't that good (it's basically a single-pole
> filter) so for applications where a bit of leakage is a problem, you might
> be better looking at active filters with more stages. Still, if the required
> output is low frequency or DC, and the PWM frequency is high, a simple
> filter will do.
>
> HTH,
> Tom
>
>