From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2001-12-13 21:41:34
Hi,
----- Original Message -----
From: Doug Sutherland <wearable_at_earthlink.net>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Wednesday, December 12, 2001 2:01 PM
Subject: Re: [buildcheapeeg] Necessity of ir isolation
...
> The brainmaster build specs also say this ...
> <snip>
> NOTE THAT this does NOT yet isolate the RS-232 from
> the computer.
I assume this note refers to the DIY-version of the
brainmaster that has no opto-isolation.
(At least I have not seen any opto-couplers in the
schematics of the bm-DIY-version )
> 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?
AFAIK that depends on the classification of the device.
If it shall be a medical device we have to isolate 5500 volts
(DC ? AC rms ?)
For a non-medical device less isolation should be sufficient.
>>>>>>>>>>>>>>>
www.bapcoinfo.com/IEC601safetytestingarticle.htm
...
Insulation breakdown
IEC601-1 Clause 20
This test is actually a series of tests designed to test the strength of insulation. Basically you apply a high voltage, as much as
5500 volts, to an insulation barrier to determine if it will "withstand" the voltage and not short across. This is commonly referred
to as the "hipot" test. Clause 20.1 and 20.2 outline the various situations where this test is to be used as well as the insulation
types that qualify. The measurement is basically made with a high voltage supply and two test points which are connected across the
appropriate insulation barrier. The test voltage varies based on the reference voltage U which is essentially the voltage normally
present the insulation barrier. See Table V in the IEC601-1 standard for details. Clause 20.4 outlines that the tests must be made
for 1 minute with the voltages outlined in Table V imediately after warming up to operating temperature, after heating elements warm
up to operating temperature, immediately after humidity preconditioning and after any sterilization procedure. Initially no more
than half of the test voltage is applied and then the voltage is gradually raised over a period of 10 seconds to full value. The
test is then run at full voltage for 1 minute and then the voltage is gradually reduced over 10 seconds. During the test, no
flashover or breakdown shall occur. Slight corona discharges are ignored, provided that they cease when the test voltage is
temporarily dropped to a lower value. See Clause 20 for complete details.
...
<<<<<<<<<<<<<<<
> None seems to be the wrong answer to me.
I agree.
> 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?
Some sort of separate power sorce is absolute mandatory.
Everything else would corrupt the isolation.
A separate (second) battery is possible but not really
state of the art.
A battery powered EEG and getting power for the opto-interface
from the com-port of the PC is possible but limited
to a few mA - definitely not enough to power the MAX232 chip.
Therefore I have chosen a DCDC-converter that provides the
second isolated power supply.
Regards,
Joerg
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