Controlling an AC current

Hello all.

At the moment I am trying to produce and control an AC voltage. I'm pretty new at electronics, though, and I'm running into trouble.

It seems like generating an AC current should be straightforward enough, but for some reason I can't find DC->AC switching ICs. I can't see any physical reason this oughtn't exist... So that's my first problem. Can I expect to find an IC that will convert DC to AC?

The second thing I want to do is detect the resistance the AC is encountering. I'm interacting with skin so the resistance will change based on several factors. How can I do this?

Finally, I want to change the AC voltage so that the current is strictly controlled. I don't think this will be difficult if I can do the first two, but I'm not sure how it'll work.

If it's important, I expect the AC voltage to be between 5 and 15 volts and around 50 Hertz, and my target current is around 5 mA.

Thank you for your help.

Can I expect to find an IC that will convert DC to AC?

It is called an oscillator, there are lots of them about.

The second thing I want to do is detect the resistance the AC is encountering

You do this by measuring the current that the alternating voltage causes to flow.

I don't think this will be difficult if I can do the first two

As you say you are new to electronics.

and my target current is around 5 mA.

I hope you are not going to put 5mA through a person, that can prove fatal.

You really want to find out something about electricity before you go trying to kill people.

Fibrillation requires at least 70 mA through the heart. 5 mA is about the threshold of sensitivity and if it's kept in the skin won't cause any problems whatsoever. What I'm working on now is testing and it'll be done safely.

I will look into the other items. Thank you.

Fibrillation is around 100 to 150 microamps at the heart.

If you are trying to force 5mA through a body then you need certifying and locking up. You are an idiot.

AWOL:
Fibrillation is around 100 to 150 microamps at the heart.

Okay, not exactly what I meant. If the electricity skips the skin, microamps (as few as 10, I've read) will do the job. If it's entering through the skin and past the heart (for instance, arm to arm or arm to head) quite a bit more is required. If the source and sink electrodes are attached to one arm there will be no electrical current through the heart and fibrillation will not occur. If the source and sink electrodes are attached to the skin within several millimetres, conduction will be entirely within the skin and the only physiological effect will be sensation and sweating.

What are you exactly trying to do? kill people?

If it's important, I expect the AC voltage to be between 5 and 15 volts and around 50 Hertz, and my target current is around 5 mA.

Nobody is going to die with 15V. And, you're not going to get 5mA through the skin either. :wink:

I'd suggest that you get a multimeter to measure voltage, resistance, and current [u]inexpensive example[/u] before you get too far into this project.

The place to start is with [u]Ohms Law[/u] which defines the simple relationship between voltage, current, and resistance. You'll probably want to feed-in a known voltage and measure the corrent. When you know the voltage & current, you can calculate the resistance. You can measure/calculate the current by measuring the voltage across a known series resistor. (It seems like circular logic, but it works.) The series resistor needs to be small, so that it doesn't mess-up the overall current flow. This small series/measurement resistor means that you are measuring small voltages.

Measuring low currents is tricky, you and might need an amplifier in front of the digital-to-analog converter. (You can build an amp with an op-amp chip and a couple of resistors.)

...and around 50 Hertz,

That could present a problem... Your body acts like an antenna and it picks-up the power-line frequency (60Hz here in the U.S.) This is why you hear a buzz or hum when you touch the input on an audio amplifier. You can use a filter (op-amps again) to pass only the frequency of interest. But, if you are just trying to measure resistance (i.e. a lie detector), you should use DC (zero Hz) and filter-out the AC. DC will also make it easier to use the Arduino ADC.

Nobody is going to die with 15V.

Not true! Don't mess with this if you don't know what you're doing. It's simply not worth it!

Whoever is wanting to hook up electricity to people must go to med school and EE school (and get the proper degrees) plus get national medical oversight boards/agencies involved. And probably should have a good deal of liability insurance, too!

A moderator needs to lock this thread.

jwatte:

Nobody is going to die with 15V.

Not true! Don't mess with this if you don't know what you're doing. It's simply not worth it!

Whoever is wanting to hook up electricity to people must go to med school and EE school (and get the proper degrees) plus get national medical oversight boards/agencies involved. And probably should have a good deal of liability insurance, too!

A moderator needs to lock this thread.

Are you kidding? Yes, electricity can be dangerous. You should be careful. But what is being discussed here seems something that can be safely done, if care is taken.

Of course, if you are designing a defibrillator, or something like that, do stop. Otherwise, best of luck with your project. If you can, let us know what you have in mind, and we may be able to offer more suggestions.