On 23.06.11 12:01, andy pugh wrote:
> Yes, it appears that I put the optos upside down too.
> You can see why I always buy 3 of every component...
During an 18 year stint as an electronics engineer at a German
multinational, I learnt a Murphy's Law in a word: "Vorführungseffekt",
which translates as "Demonstration effect". I.e. Your design works fine
in the lab, but as soon as management is invited to see the
demonstration, it goes belly up. It is curious how often the same thing
happens to an idea when you press "send", despite having cast two quick
glances over it first.
Please don't tell anyone, but even after 30 years of hardware design and
programming, it's still surprisingly easy to mix up pins while flipping
a board over for soldering, or otherwise let the magic smoke out.
(One way to take a "professionally" cool attitude to that is to buy
transistors a hundred at the time, for $5 or less, I find.)
> I think that it needs a 330R on the high side of each LED. The output
> side should be OK I think (with the spurious 15k resistors omitted)
>
> The drawback of this circuit is that wherever it is put it needs both
> the logic positive supply and the controlled positive supply, but I
> can't see any way round that if you want to actively discharge the
> output cap on one part of PWM cycle. Having a passive discharge
> resistor leads to non-linearity and pwm frequency dependency.
Oh, I had assumed the feed to the top of the opto H-bridge would be from
the top of the removed pot, so no additional supply would be needed, now
that the H-bridge resistances are low enough to obviate the need for a
buffer. That also automatically scales the output voltage for
compatibility.
> The "classic" one resistor / one cap converter doesn't have this
> problem, as 0v PWM discharges the cap and 5V charges it. With a single
> opto you lose that symmetry.
Yes, in that case symmetrical charge & discharge would make half speed
the lowest achievable, IIUC. And even if symmetry were sacrificed, a
10:1 charge:discharge resistor ratio would limit the range to 90% of
full scale.
Erik
--
Never worry about theory as long as the machinery does what it's
supposed to do.
- Robert A. Heinlein
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