Will a skin conductance project, be recognized as "safe" project. I am planning to publish a skin conductance project. It will use a AC voltage in range 80- to 200Hz about 70mV RMS. A pc sound card as AC generator, and for data collection. The system will use synchronous demodulator technology used in "Lock-in amps" for signal recovery. But all this will be done in software which I provide. So the project is a hardware only project. I work with medical equipment so the design will have safety measures built in. But to control the use will be out my hands

 

Some years ago I made a device to administer shocks for the psych department. I used an audio transformer to step up the voltage. It also electrically isolated the circuitry form the subject and limited power to something on the order of 10 mw. That was easy, as it only had the aversive output.

For doing skin conductance, you might look for an isolating transformer to apply the AC signal, and use an isolation amp to read in the conductance. That should be close to foolproof.

 

I may use 3 audio transformers. As I need 1 output and 2 inputs. But it could also be so simple as strongly advice to use a battery powered laptop when measuring

 

Not bad advice, but you can never predict when an old Windows 95 computer with a SMPS will get pressed into service. That is why products that carry the Underwriter's Lab sticker to state inherent safety cost so much. It takes a lot to make something idiot proof.

Still, if the transformers carry a 5KV hipot rating and are power limited, it should be very safe.

 

So bottom line the project will not be removed as a potential hazardous project?

 

That is not completely up to me. For myself, it sounds as if you are taking proper precautions to prevent possible harm.

 

Will a skin conductance project, be recognized as "safe" project. I am planning to publish a skin conductance project. It will use a AC voltage in range 80- to 200Hz about 70mV RMS. A pc sound card as AC generator, and for data collection. The system will use synchronous demodulator technology used in "Lock-in amps" for signal recovery. But all this will be done in software which I provide. So the project is a hardware only project. I work with medical equipment so the design will have safety measures built in. But to control the use will be out my hands

It's safe is you use about 1 megohm series resistance in the leads.. Depending on the nature of the institution, there may be other requirements as well. Look up references on "medical electronics certification" and such.

These measurements can be effectively achieved with mere microamps, where the danger is non-existent...so why use any more?


Eric

 

Something that may interest you is electrostatics. You can deliver a substaintial voltage, yet by using the air gap, control current flow. The charge is carried onto the subject by a vehicle of compressed air, laden with a controlled amount of moisture particles. The moisture particles are attomized and electrostatically charged in a nozzle. A grounding, or collection plate recieves the subsequent flow.

There is a curious example of this found in the Mystic tanning system, an awkard take off of electrostatic herbicide deployment.

 

It's safe is you use about 1 megohm series resistance in the leads..
Eric

And how did you come up with this number? I agree with you then it comes to add series resistance in the leads. But 1 Mohm is much higher than it has to be. A series resistance in the leads with this magnitude may cause a lot of problems then it comes to signal recovery.
A skin conductance system will be classified as "TYPE BF APPLIED PART" If we look at the data for PATIENT AUXILIARY CURRENTS we can see which current that are allowed. For the NC condition we will be using 70mV AC. A 100uA current will then be same same as a skin conductance equal to 700uSiemens. Not very likely to happened. Then it comes to the SFC condition a typical fault may be that a opamp break down and the supply voltage for that component is coupled to the electrode. In this case the current should not be larger than 50uA. If we use say 5 volt as supply voltage then we must use at least 100Kohm as series resistance in the leads.