optoisolated power supply

bitrex · 2012-06-16T05:27:38+00:00

A friend has asked me to look into the possibility of constructing a one-off Arduino-compatable 8 channel muscle/EMG sensor for experimentation purposes, and gave the documentation of the commercial (single channel) product available here:as a spec for the type of device he would like. The product's design seems to be released under a fairly permissive license, so one could theoretically whip up an 8 channel version and distribute it as one wished with no problem.However...The design as it stands has some problems, the most obvious of which is that it does not make any attempt to meet any requirements for medical equipment, or provide any form of isolation between the body and the power/ground lines of the host computer's USB port.A purpose-built piece of equipment would use an isolated supply for everything and run the USB lines to the host through an optoisolator, but because the device is supposed to be used with an Arduino, which uses the USB ground as reference ground for signals coming in to its A/D converters, this makes proper isolation of the body connections somewhat more difficult.To provide isolation, my thought was to use a pair of 4 channel optocouplers to couple the EMG signal from the input instrumentation amplifiers into the rest of the circuitry. Since the current requirements of the input amps are pretty low, their supplies could be isolated by using an "optoisolator power supply," maybe as simple as some photodiodes charging up capacitors. The instrumentation amp IC used here can run with as low as +/- 1.35 volt rails. Does anyone make 4 channel transistor optocouplers with the base terminals brought out, so they could also be used as photodiodes?Please bear in mind that I realize medical standards are stringent, and I probably do not have the means to test whether a device so constructed adheres to all such requirements. I simply wish to provided an added level of protection for someone who is aware of the risks involved with such...

Jun 16, 2012 62 Replies

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Paul Hovnanian P.E. 14 years ago

how about magnetic isolation? Simple, established techniques involving switch mode power supplies. Some attention needs to be given to the design and winding of the transformer to ensure proper isolation. But that's not rocket science.

-- Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com

------------------------------------------------------------------ Disclaimer: This publication is the sole property of monkey #736903 and his typewriter. It does not represent the opinions of any other primate, either alive or dead, or any descendants thereof.

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Paul Hovnanian P.E. 14 years ago

Yeah, but ... the OP is referring to a medical application. All bets are off as to skin resistance since we don't know where he'll be shoving those probes.

Once you get beneath the skin, the human body has quite low resistance.

Paul Hovnanian mailto:Paul@Hovnanian.com ------------------------------------------------------------------ Great minds discuss ideas, average minds discuss events, small minds discuss people.

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bitrex 14 years ago

Hi Paul, thanks for the reply. The project fell through; the friend has decided to go in another direction and use an off-the-shelf product. In a way I'm glad - building such a device the "proper" way with isolation and safeguards would have taken an awful lot of my time for a one-off project, and even if the risk involved is low I'd frankly much rather leave the design of quasi-medical equipment to others who are more qualified for the time being.

I agree with what you say, magnetic isolation is probably the most straightforward way to go in such a project.

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