From: Doug Sutherland (wearable_at_earthlink.net)
Date: 2001-12-12 13:01:52
The sensors mag article mentions that manufacturers of
bioelectric amplifiers use isolation amplifiers that
provide as much as 10^12 ohms of isolation.
(http://www.sensorsmag.com/articles/0199/iso0199/main.shtml)
These Scimar OP232 isolators provide complete electrical
isolation with restistance of 10^12 ohms. They cost big
bucks though, more that you folks want to pay for the
entire EEG ...
http://www.scimar.co.uk/iso_prod.html
It seems that the opto-isolators with true optics-only
paths, breaking all leads, are quite expensive (>$100).
The Brainmaster build specs say ...
<snip>
There are several ways to isolate the device, including
simple optocouplers, or an infrared remote chip set. The
Circuit Cellar HAL has an optocoupled output stage, with
its own battery and regulator. It is reported in Byte,
June 1988, 273-285. If you want to optoisolate your
device for safety, use this design.
<snip>
The HAL manual is here:
http://www.cc-concepts.com/products/hal4/hal4.pdf
Looking at the HAL manual ... serial data output
drives the TIL111 opto-coupler by means of a BS-170
FET. A MAX232 U6) converts the opto-coupler's output
to RS232 levels compatible with the serial input of
the PC host computer. It is powered by a separate
9V battery so that when it is connected to the PC,
isolation between HAL and the PC will be maintained
(do not try to use the 6V C cells which power the
main HAL circuit to power U6!)
INTERESTING: The MAX232 (RS232 level converter) is
powered by a separate 9V battery from the main HAL
circuitry to maintain the isolation. Do we need this?
continuing with the brainmaster build specs ....
<snip>
You may purchase a suitable optoisolator from Patton
Electronics voice: (301) 975-1000 fax: (301) 869-9293.
e-mail: sales_at_patton.com and ask about the Model 590
RS-232 to RS-232 Optical Isolator. It comes with an
external wallmounted power adapter, or it can be
powered by 12VDC @45mA on pin 9 of the DTE.
<snip>
Here is that page on this opto-isolator:
http://www.patton.com/cgi-patton/catalog-bin/display.cgi?model=590
The brainmaster build specs also say this ...
<snip>
NOTE THAT this does NOT yet isolate the RS-232 from
the computer. If your computer is faulty, and puts
110VAC on the ground line, for example, this device
would NOT protect you. If you do not understand the
possible hazards, or are uncertain, please do not
attempt to construct this device.
<snip>
So I wonder then ... what level of opto-isolation is
necessary? None seems to be the wrong answer to me.
Does it need to be one of these expensive isolators?
I don't think so. Joerg already has 6N139s in the
design. Does the MAX232 need to be powered separately
per the HAL specs for proper isolation? Is the irDa
idea better or is it really overkill?
Here are the TIL111 and 6N139 data sheets ...
http://www.isocomoptocouplers.com/isocom/til111.htm
http://www.fairchildsemi.com/pf/6N/6N139.html
-- Doug
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