seemingly impossible class project

[thadinator]

For for digital electronics class I need to design a circuit similar to a shocking dog collar. The requirement is that with a 6 volt source I need to deliver a max of a 3 mAmp current no matter what the resistance (aka, nomatter how tight the collar is). The point is to learn how to deal with circuits tha don't follow ohms law, and I don't know where to start.

Iknow you can step up voltage with a transformer and that shocking circuits often use transistors, but that about it. IF you guys can either help or provide a few sites explaining this and the math behind what I need to do, I'd really appreciate it.

 

[jackorocko]

The point is to learn how to deal with circuits tha don't follow ohms law

Even though I can't really tell you how to do it, what I do know is Ohms law is important in all electronic circuits. Knowing this, one would have to assume that since you need to deliver a constant current and the resistance is ever changing, then you need to build a circuit that can vary it's voltage based upon the resistance to deliver the 3mA

Of course there is always a limit to what is achievable

 

[duke37]

Is a shocking dog collar legal in your country?
You could look up details of electric fence energisers. These work by generating a very high voltage and feeding through a high resistance.

Please tell me of a circuit that does not obey Ohms law.

 

[BobK]

Please tell me of a circuit that does not obey Ohms law.

Put a voltage across any device that is not a resistor and it does not obey Ohm's law, i.e. the current will not be constant and proportional to the applied votage. If it does obey Ohm's lay, it is, by definition, a resistor.

Bob

 

[duke37]

BobK
The current will not be proportional to the voltage if the resistance changes, it does not violate Ohms law.

 

[thadinator]

I realize no one can tell me exactly what to do, but like i said, I'm not sure where to start.
I can obviously step up voltage, but I will have a 6 volt source.

By shocking dog collar I mean something like an invisible fence where if a dog attempts to cross one's property line a shock is delivered.
There are lots of sample circuits online that show how to deliver a constant current with an unregulated voltage source or with a regulated resistance, but I have a battery source and keeping with the dog collar example, resistance can vary from 2-4 million ohms depending on how firm the contact is. I just don't know where to go.

 

[(*steve*)]

It seems weird to me.

Consider the perfect current source. Now consider what happens when it is connected to a high value resistance. What is the power dissipated in the resistor?

Whilst you can certainly get a lot of power out of any perfect current or voltage source, in real life nothing is perfect.

This seems very strange for a *digital* electronics class. (Did you rock up at the wrong lecture?)

 

[Sid723]

Three milliamps sounds like it would cause a nasty burn on the skin.

You should really leave the design of this device to the professionals. Check out this link:
http://www.hollysden.com/say-no-to-shock-collars.htm

 

[timothy48342]

"You should leave the design of this to the professionals." LOL
It is a school project!

I want to point out that the instructions were to "deliver a max of a 3 mAmp."

Constant current is not nessessary. The current can vary as long as it does not excede 3 mA.

You don't know the exact resistance between the color and the dogs skin. Where you given a range of possible resistance that you have to allow for? You mentioned 2-4 MOhm. Was that range from the teacher or do you have to guess at that?

What is the absolutely lowest possible resistance that might happen? (You don't want to the designers getting sued if someone sprays the dog with saltwater.)

Once you've decided on the lowest possible resistance that might happen, design a circuit using your 6V that stays at or under 3mA.

-tim

 

[Sid723]

"It is a school project!
............
You don't want to the designers getting sued if someone sprays the dog with saltwater.

-tim

So, are you saying that they will not try it out on a dog?

Or do you think they will have a DUMMY dog just for the class to try out their work?

 

[(*steve*)]

hahahaha, OK, max of 3mA.

With a 6V source, what value of series resistance would limit the current to 3mA (max)?

That's a simple ohms law question.

edit: OK, there are other ways. You could look up how to create a constant current source. realistically from 3V it will only deliver 3mA into fairly low impedance loads, but it's a slightly better option than a resistor.

Perhaps you could take a look at the sticky on driving LEDs because I'm sure there's something in there about constant current sources.

Take a look at this, especially the section on driving high power LEDs because that includes some circuits you may be able to use.

 

[jackorocko]

Three milliamps sounds like it would cause a nasty burn on the skin.

You should really leave the design of this device to the professionals. Check out this link:
http://www.hollysden.com/say-no-to-shock-collars.htm

Seriously, he has a 6V battery with 3mA output, that wouldn't even leave a tingle on the end of a human nerve. Relax a little!

 

[Sid723]

@jackorocko - The "tingle" does not come from the 6 volt battery, it comes from the thousands of volts the circuit is supposed to produce.

If all you want to do is zap dogs, then here is a starting point:

http://www.radiolocman.com/shem/schematics.html?di=71518

It uses 12 volts, so some circuit changes will need to be made to make it work for your use.


Good luck all.