[SOLVED] looking for help with my circuit

[zeyad]

i created my circuit in order to have output current 2 mA and below to be connected on the body (forehead and side of the head), but the problem i get more than 4 mA which i dont' want.
what i need to change in the circuit in order to get 2 mA output and below

 

[adi0803199]

Remember Zeyad that any practical current source provides a current of a particular value for a particular load .All current sources are designed keeping in mind a particular load resistance.The human body is fairly a decent conductor of electricity(follow the particular link description

http://van.physics.illinois.edu/qa/listing.php?id=6793

to know the resistance value(i.e) load resistance.Follow the data sheet and see whether a load resistance value has been specified.I suspect that the load resistance of body is much lesser than the specified load,that is why you get more current flowing.I would suggest that you increase the resistance before giving it to load(new series resistor),of course this value has to be calculated.
Hope this helps.

 

[zeyad]

hi
thanks a lot
is there any possibility to have output current varies between 2-6 mA no matter of the load. i know from ohms law that they are related to each other. but with some voltage regulator you can have constant voltage no matter of the the load. do you think i can have same but with the current?

 

[PA4TIM]

but with some voltage regulator you can have constant voltage no matter of the the load

No, that is not tru, for instance a 7812 can deliver 1A. If you connect 1 Ohm to the 7812 the current should be 12A according Ohms law. But ther 7812 can not deliver 12A. A voltage regulator can deliver a stable voltage if the load is bigger as X Ohm.
And a current source can deliver the specified current is the load is smaller as X Ohm.

 

[zeyad]

is there any circuit that its output is giving constant current upto 6 mA via a load of 200k ohm?

 

[PA4TIM]

Yes, it can be made. To drive 6mA through 200k you only need 1200V ( Just Ohms law) But if you do not mind, I'm not gonna design something that dangerous for you ;-)

But what do you want, first you construct a circuit that should source not more as 2 mA but you get 4 mA and now you need 6mA ? First make up your mind or tell more about what you want. You need AC or DC for instance ? The impedance ( R+jX) of a body is not the same as the DC resistance.

The math: Devide the maximum voltage of you source without a load with the resistance of the load to get the current

Or divide that voltage with the current and then you know the load.

So if the unloaded voltage is 5V and the load is 200k there will be 5V / 200k = 0,025 mA . Or the other way around, If you measure 4mA and you use 5V the load must be 5V / 0,004A = 1250 Ohm (just random examples, you can fill in the values you need)

A body is not a constant impedance, it varies very much. Even one person can and will have differences in impedance between different parts of the body and different times. So a current source is not a good idea, better use a source with a fixed voltage that is limmited to a max current. Then make a very sensitive receiver. But first read a lot about safety. You do not want to electrocute or torture people (I presume)

 

[zeyad]

hi
thanks for your reply
i can't use 1200 v, the maximum voltage i can use is 18V.
therefore i have 18 V, 200K R, and i would like to have 6 mA current.
it is not normal ohm's law.
yes as you said i need current limiter to get maximum of 6 mA, but how can i have get that?
and don't worry mate it is not dangerous
thanks for your cautious

 

[PA4TIM]

Sorry, that is not gonna happen because it is" Ohms law", and not "Ohms recommendation" ;-)
So 18V and 200K wil allow a current of 0,09 mA. There is no way you can get 6mA through 200K with a 18V source.

 

[adi0803199]

Best way to design current sources having large ranges is Widlar Current Source but the designing part will be a bit tricky as low resistance may be required.But also consider cascaded current mirrors which uses more than two transistors but will also give the required current at that particular rating(around 200k).
Hope this helps.

 

[PA4TIM]

I once made a Widlar current source, a very clever circuit.
I have never used a cascade current mirror but how you get 6 mA through 200kOhm with only 18V ?