My maxim is a twist on Murphy... if there's ANY chance of smoke, no
matter how minute, there WILL be smoke.
...Jim Thompson
On Oct 4, 1:22 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@My-Web-
Site.com> wrote: [snip]My maxim is a twist on Murphy... if there's ANY chance of smoke, no
matter how minute, there WILL be smoke.
...Jim Thompson
Sure. The question is then "What's the chance a transistor will have
20x its expected gain?"
In general use I'd believe a 4:1 spread, when selected for
optoisolators, I'd expect the spread to be a little tighter, and
surely not even 10:1. What say you, o silicon sculpting sage?
Cheers,
James Arthur
P.S. Inside info: actual CTR, measured in circuit, = 100%. ja
Then Microchip would keep you busy for a while.Rich said:I'm available to do proofreading - I'm the self-appointed chief of
the Apostrophe Police, after all. ;-)
And, of course, at times I branch out to the Spelling Police (I
used to win contests), Grammar Police, and "Does-it-make-any-sense"
police. ;-)
Cheers!
Rich
On Oct 4, 1:22 pm, Jim Thompson wrote: [snip]
My maxim is a twist on Murphy... if there's ANY chance of smoke, no
matter how minute, there WILL be smoke.
...Jim ThompsonSure. The question is then "What's the chance a transistor will have
20x its expected gain?"In general use I'd believe a 4:1 spread, when selected for
optoisolators, I'd expect the spread to be a little tighter, and
surely not even 10:1. What say you, o silicon sculpting sage?
You are correct that it's VERY unlikely to have a problem. But, in my
world, I NEVER take chances.
"Brutally elegant"?
Sounds a little creepy. ;-)
Cheers!
Rich
On Oct 4, 1:22 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@My-Web-
Site.com> wrote: [snip]My maxim is a twist on Murphy... if there's ANY chance of smoke, no
matter how minute, there WILL be smoke.
You are correct that it's VERY unlikely to have a problem. But, in my
world, I NEVER take chances.
On Oct 4, 12:47 pm, Vladimir Vassilevsky <[email protected]>
Let's do the math. With input floating, if John used 1.5k resistors,
the LED current would be from 4v - (2 led drops) passing through 2 x
1.5k ohm resistors. That amounts to (4v - 2.2v) / 3k ohms = 600uA.
At these currents the opto's CTR drops, so output current will be just
220uA (typical), for a safety margin of 13,600%. John's version is
_very_ safe.
Classic Larkin: manhandling electrons. He's a classy brute.
...
P.S. Inside info: actual CTR, measured in circuit, = 100%. ja
I actually sat at a bench and measured some opto's CTR, I think
one of the 4N series; anyway, I made a graph from about 0-20 mA,
and was astonished when my graph turned out to be a straight
line within a few percent. At a previous job, when they needed
a linear transfer function, they used either TWO optoisolators,
one for reference to linearize it, or they used a V-F converter,
opto, and F-V converter.
I had intended to use it in a feedback loop for a design we were
working on, but that project died on the vine - we couldn't afford
it. I almost bankrupted the guy. =:-O
There are optos where one LED lights two detectors (e.g. Infineon's
IL388). By closing a loop on one detector (on the isolated side) you
assure linearity. The 2nd, matched detector provides the signal on
the signal side.
Pretty good for phone couplers, but not much in the way of absolute
accuracy.
The led tempcos are rotten, and they degrade over time (integrated
current) too.
John
James said:Sometimes you need a low-power, moderately fast optoisolator.
Here's a simple-minded little rig I hooked up on my bench.
It's not much, but the circuits and measurements might hold
interest for some.
Here's the plain, standard circuit (view in fixed font):
FIGURE 1. +5V
========== -+-
|
R1 |
1.5k 1/2 |
+ >----/\/\/----. MOCD217 |
| OPTO |
.--. .- .---|---------/--.
| | | | --- |/ |
--' '--' | \ / ---> | |
4v p-p | -+- |>. | .--. .-
'---|----------|-' | | |
- >-------------' | --' '--'
+--------> 4.8v p-p
| 3.5KHz max
\ R2
/ 2.2k
\
|
===
GND
Above about 3.5KHz the output no longer swings rail-to-rail,
quickly becoming unusable at higher frequencies.
The opto's slow output transistor limits response time, but
that's easily improved:
+5V +5v
FIGURE 2. -+- -+-
========== | |
\ |
R22 / |
1k \ .'
R21 | |< Q21
1.5k 1/2 +--------| 2n3906 .--.
+ >----/\/\/----. MOCD217 | |\ | |
| OPTO | \ --' '--
.--. .---|---------/--. +------> 4.8v p-p
| | | --- |/ | | >100KHz
--' '-- | \ / ---> | | / tr ~300nS
4v p-p | -+- |>. | \ R23 tf ~1uS
| | | | / 4.7k
'---|----------|-' |
- >-------------' | |
=== ===
GND GND
Q21 prevents the opto's output transistor from saturating
and limits its voltage swing.
The same optoisolator, so equipped, now passes a 110KHz
squarewave, or 1.0uS pulses at 320KHz in my unit.
The speed limitation is still storage time in the opto's output
transistor, so anti-sat tricks on Q21 aren't helpful or needed.
Running the LED at low power ensures a very long lifetime.
Cheers,
James Arthur
View in a fixed-width font such as Courier.
.
.
.
. +5V +5v
. FIGURE 3. -+- -+-
. ========== | |
. \ |
. R22 / |
. 10k \ .'
. R21 | |< Q21
. 1.5k 1/2 +--------| 2n3906
. + >----/\/\/----. MOCD217 | |\
. | OPTO | \
. .--. .---|---------/--. +------>
. | | | --- |/ | |
. --' '-- | \ / ---> | | /
. 4v p-p | -+- |>. | \ R23
. | | | | / 3,9k
. '---|----------|-' |
. - >-------------' | |
. '-----------+
. |
. /
. \ R33
. / 1k0
. |
. ===
. GND
.
.
View in a fixed-width font such as Courier.
.
.
.
. +5V +5v
. FIGURE 3. -+- -+-
. ========== | |
. \ |
. R22 / |
. 10k \ .'
. R21 | |< Q21
. 1.5k 1/2 +--------| 2n3906
. + >----/\/\/----. MOCD217 | |\
. | OPTO | \
. .--. .---|---------/--. +------>
. | | | --- |/ | |
. --' '-- | \ / ---> | | /
. 4v p-p | -+- |>. | \ R23
. | | | | / 3,9k
. '---|----------|-' |
. - >-------------' | |
. '-----------+
. |
. /
. \ R33
. / 1k0
. |
. ===
. GND
.
.
Jim said:[snip]
View in a fixed-width font such as Courier.
.
.
.
. +5V +5v
. FIGURE 3. -+- -+-
. ========== | |
. \ |
. R22 / |
. 10k \ .'
. R21 | |< Q21
. 1.5k 1/2 +--------| 2n3906
. + >----/\/\/----. MOCD217 | |\
. | OPTO | \
. .--. .---|---------/--. +------>
. | | | --- |/ | |
. --' '-- | \ / ---> | | /
. 4v p-p | -+- |>. | \ R23
. | | | | / 3,9k
. '---|----------|-' |
. - >-------------' | |
. '-----------+
. |
. /
. \ R33
. / 1k0
. |
. ===
. GND
.
.
At first glance I thought that might work, but I think the negative
feedback will add Miller thru the opto NPN and keep it slow.
...Jim Thompson
John said:Won't that slow it down?
John
Jim said:[snip]View in a fixed-width font such as Courier.
.
.
.
. +5V +5v
. FIGURE 3. -+- -+-
. ========== | |
. \ |
. R22 / |
. 10k \ .'
. R21 | |< Q21
. 1.5k 1/2 +--------| 2n3906
. + >----/\/\/----. MOCD217 | |\
. | OPTO | \
. .--. .---|---------/--. +------>
. | | | --- |/ | |
. --' '-- | \ / ---> | | /
. 4v p-p | -+- |>. | \ R23
. | | | | / 3,9k
. '---|----------|-' |
. - >-------------' | |
. '-----------+
. |
. /
. \ R33
. / 1k0
. |
. ===
. GND
.
.At first glance I thought that might work, but I think the negative
feedback will add Miller thru the opto NPN and keep it slow....Jim Thompson
I don't have the MOCD217, but SPICE simulation on the MOC223 shows tr/tf
as about equal at 550ns. It easily handles 100KHz and he can back that
input drive off to 0.5mA or so, 2mA is way too much, and there is very
little danger of exceeding 100% CTR at that drive level. Yep, so NFB
does make the transitions slower.
1MHz according to the simulator:
Jim said:[snip]
View in a fixed-width font such as Courier..
.
.
. +5V +5v
. FIGURE 3. -+- -+-
. ========== | |
. \ |
. R22 / |
. 10k \ .'
. R21 | |< Q21
. 1.5k 1/2 +--------| 2n3906
. + >----/\/\/----. MOCD217 | |\
. | OPTO | \
. .--. .---|---------/--. +------>
. | | | --- |/ | |
. --' '-- | \ / ---> | | /
. 4v p-p | -+- |>. | \ R23
. | | | | / 3,9k
. '---|----------|-' |
. - >-------------' | |
. '-----------+
. |
. /
. \ R33
. / 1k0
. |
. ===
. GND
.
.At first glance I thought that might work, but I think the negative
feedback will add Miller thru the opto NPN and keep it slow....Jim Thompson
I don't have the MOCD217, but SPICE simulation on the MOC223 shows tr/tf
as about equal at 550ns. It easily handles 100KHz and he can back that
input drive off to 0.5mA or so, 2mA is way too much, and there is very
little danger of exceeding 100% CTR at that drive level. Yep, so NFB
does make the transitions slower.
Inspired by Fred's use of simulation, I plugged an MOC207 into LTspice
and found it surprisingly faithful in modelling my original Fig. 2
circuit.
So inspired, I explored some more.
Attractive as John's totem-pole idea was, I couldn't adjust things to
jerk the opto output transistors 'off.' Maybe I missed something, but
once they're saturated, they're stubborn.
Fred's idea of reducing LED current is a good one; I'd originally set
the LED current a bit high for CTR and aging allowance, but I was too
conservative. Lower LED current is better.
Here's a faster two-transistor version that minimizes the voltage
swing across the opto's output transistor, and thus manages to pass1MHz according to the simulator:
FIGURE 4.
========== +5v
-+-
R41 |
2.7k |
+ >----/\/\/----. MOC207 +---------.
| OPTO | |
.--. .---|---------/--. |
| | | --- |/ | |
--' '-- | \ / ---> | | \ R45
4v p-p | -+- |>. | / 4.7k
| | | | \
'---|----------|-' | --. .--
- >-------------' | | | |
| | '--'
R42 | | tf=90nS, tr=125nS
4.7k |<' Q41 +------------------->
+5>--/\/\--+------| 2n3906 |
| |\ /
| | |/ Q42
R43 \ +------| 2n2369
2.2k / | |>.
\ \ |
| R44 / |
| 560 \ |
=== | |
GND | |
=== ===
GND GND
It uses more parts, so we're straying from the initial charter of
simple, low-power, and medium-fast.
Common-base transistor Q41 is pretty close to a TIA, so we're
pushing the limits of easy improvement for this opto. In the interest
of speed I've left out any anti-smoke resistors. Besides the speed
advantage, this version uses half the LED current and the opto sees
less voltage, so it's even safer than its predecessors.
Best to all,
James Arthur
James said:FIGURE 4.
========== +5v
-+-
R41 |
2.7k |
+ >----/\/\/----. MOC207 +---------.
| OPTO | |
.--. .---|---------/--. |
| | | --- |/ | |
--' '-- | \ / ---> | | \ R45
4v p-p | -+- |>. | / 4.7k
| | | | \
'---|----------|-' | --. .--
- >-------------' | | | |
| | '--'
R42 | | tf=90nS, tr=125nS
4.7k |<' Q41 +------------------->
+5>--/\/\--+------| 2n3906 |
| |\ /
| | |/ Q42
R43 \ +------| 2n2369
2.2k / | |>.
\ \ |
| R44 / |
| 560 \ |
=== | |
GND | |
=== ===
GND GND
It uses more parts, so we're straying from the initial charter of
simple, low-power, and medium-fast.
