Re: Speeding optoisolators
- From: D from BC <myrealaddress@xxxxxxxxx>
- Date: Thu, 04 Oct 2007 02:41:04 GMT
On Wed, 03 Oct 2007 19:13:35 -0700, James Arthur
<dagmargoodboat@xxxxxxxxx> wrote:
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
Yawnnnn... :O
Now..2.1Ghz RF coupling digital isolators are interesting..
Tprop < 10nS
150Mbps
2500VRMS isolation
http://www.silabs.com/tgwWebApp/public/web_content/products/Digital_Power/Isolators/en/Si844x.htm
D from BC
.
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