From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2001-12-12 16:59:43
Dear michalchik:
Several years ago I worked with a medical doctor helping him with his EKG related invention. I order to get the device into the hospital for testing,we had to meet the stringent medical isolation specifications. These specs seemed to be overly conservative, but after visiting the hospital and being shown what the problems might be, I have a different view. Initially Iwas only looking at the problem from my perspective, that being that my equipment works perfectly and will have no failures. What I never consideredwas that some other equipment in the room might have failed and have 120 or 240 volts AC on the chassis. So if you touch some high voltage source while hooked up through an un-isolated device you may subject yourself to lethal currents through the head in this case.
The spec says that you must not have more than 10 micro amps flowing through your equipment leads regardless of the source. In order to meet these specs, special medical grade transformers are necessary. In case of battery operation, the charger, if on board must also meet this requirement. The simplest way to meet these requirements is to battery operate the front end and optically isolate the data lines. As Doug pointed out, the investment is only a few dollars in Joerg's design. Don't even think of skimping on this part!
The argument might be that if you used a laptop running on batteries you would not need this. Murphy's law states that someone will plug the laptop in for charging and forget.
Juergen P. (Jim) Meissner
Check out my Website at www.MeissnerResearch.com
Read about the benefits of the Brain State Synchronizer sounds for improving your life and health.
----- Original Message -----
From: michalchik_at_aol.com
To: buildcheapeeg_at_yahoogroups.com
Sent: Tuesday, December 11, 2001 4:59 PM
Subject: [buildcheapeeg] Necessity of ir isolation
Hi All,
I still find myself disagreeing with the majority that feels that opticalisolation is necessary for safety. I posted the following message to sci.electronics.design to get a second opinion on this issue. I will post the replies I get.
Hi all,
I am in a list-serve on do it yourself EEG building. The devices we make need
to be hooked directly to a persons skull via conductive electrodes. The
consensus in the group seem to be that to avoid the possibility of
electrocution the actual recorder needs to be separated from the computerand
powered by a battery. Date would be transferred between the two with an IR.
link.
Personally I think this is daft and overkill. EEG's have been around for going
on a century and I have never heard of an electrocution death. Since the
recorder is passive and the signals we are picking up are less than 1 volt, I
would think it would be easy to build a cut off circuit that would ground
itself out or short out is the potential between electrodes or the electrode
and ground ever exceeded 1 volt.
I had in mind either using zener diodes in series with opposed biases or
setting up some threshold switch using an op-amp and a reference voltage from a
dry cell to cut off power if it exceeds the safety threshold. Or maybe both.
Also this device is going to be hooked into either the microphone, serialor
parallel port of a computer so the computer power supply is going to alsokeep
the voltages very low.
2 questions:
1) Do the safety circuits I described sound sufficient to you guys?
2) Do you know where I can look up the accepted standards for safety circuits
in medical equipment hooked up directly people.
I have surfed UL's website and have not found any useful information yet.
Thanks,
Michael Michalchik
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:33 BST