"Fred" wrote in message
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"Ian Iveson" wrote in
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Thanks Fred
Sounds like a very good current regulator to me,
capable of a 100:1 dynamic range. You, as the
designer, control how low the dynamic R goes in
your circuit by controlling how low the dynamic
voltage across the device goes during operation
- the lower the instantanious voltage, the lower
the instantanious R required to maintain the
design current.
The same would be true of a resistor. The greater the
voltage you have to play with, the greater you can make
the resistance for a given required current.
I hadn't read far enough into the datasheet to see under
what circumstances that dynamic R might vary, so thanks
for the info.
But is 100k OK? Fine if it were a resistor in most cases,
IMO, and for most ppl maybe fine even for a non-linear
device. But I would bet that some would argue that 100k
is not great enough to guarantee that the sound of
silicon can't seep in under the back door.
OTOH, I have seen many claims that simple discreet
silicon circuits achieve many Megohms, but with no
mention of frequency response, as if it didn't exist or
didn't matter.
If the ixys device achieves 100k at all frequencies of
interest, it's just about good enough, IMHO. It's
certainly convenient.
cheers, Ian
The IXYS device achieves many megohms at all frequencies
of interest, Ian, unless your only frequency of interest
is 0 Hz.
It's a current regulator, not a solid state resistor.
The AC resistance, or impedance, of a current regulator
is in the megohms because the current through it doesn't
change when the voltage across it changes - it looks like
an
open circuit in that respect. But it does conduct a
(fixed by
the designer) DC current, so depending on the voltage
across
it at any given moment, one can calculate a DC resistance
for
that moment by ohm's law. But it still looks like an open
circuit
to an AC voltage because, again, the changing voltage
fails to
cause a changing current as it would in a resistance.
DC resistance calculations have about as much bearing
on current regulator performance as they do on output
transformer performance, only less.
This isn't rocket science, Ian, and you don't need
calculus to
understand it. It's covered in any modern electronics
text.
All of which appears to me as a total nonsequitur in toto,
and pure nonsense in parts.
You won't find understanding in books, particularly if you
have difficulty reading. You need to think a bit too.
I took it that "dynamic R" meant the dynamic resistance of
the current source. Since it is given as 100k max, and that
is very different from your "many megohms", I felt it worth
discussion. I may be mistaken, but you have failed to spot
where my mistake is, smart-arse.
And you are wrong, but I can't now be arsed to tell you
where.
bye, Ian