I tried several different circuit configurations (ie. current mirror, howland's current source, etc) to achieve a constant current through a load independent of its resistance but no success. How is this possible? It doesn't make sense to me...

For instance, I tried this circuit configuration - http://www.falstad.com/circuit/e-howland.html - but the current load is affected (it changes) as the load resistance is changed which it shouldn't?! What's happening in here?

By the way, do you know any feasible circuit configuration which will provide me a constant current through a load independent of its resistance?

 

I was about to answer ur post but after seeing the link..
How do u do that man, it's really cool (Simulation thingy)
Pls tell

Rifaa

 

In order to achieve this without violating nature law of V = IR you need a circuit that constantly raises the voltage to keep the current magnitude at where you want it to be.

You can't violate physics. So in order to "force" some amount of current trough a circuit you need a circuit that senses the current magnitue and increases the voltage until you have achieved the current magnitude you want. Most likely that will destroy conventional circuits if they are designed to run on lower current then you are trying to force into it.

 

I was playing with the circuit...coooool!
refer to http://www.falstad.com/circuit/e-howland.html
As for ur problem, there will be no current via load when u by pass it buddy.
The current will chose the lowest resistance and since a close switch has no voltage drop, the load has no voltage across it, no V, no I in load
Get the point

Rifaa

 

A constant current source should do that - http://en.wikipedia.org/wiki/Current_source

Remember that I = E/R, so the load resistance has to be within limits. As R gets larger, E has to rise in order to force current. As R gets small, the ability of the source to supply current becomes limited. Both the voltage and current available to a constant current source are limited, and so therefore is the upper and lower value of the load.

 

The circuit configurations I've used are regarded as typical constant current sources so they compensate any loss/gain within some limits, right? Well, it didn't work for me... I was trying to get a constant 5 mA which I think is within the limits...

 

I have a schematic of a constant current sink circuit at
http://tinyurl.com/6cbn6h

Scroll down to the "Electronic Load" circuit. By adding cells
you can get to hundreds of amps. Be careful of the power
dissipation.

(* jcl *)

 

Google it. Simple circuit is an op amp, possibly driving a power transistor. All these circuits crap out if the required voltage exceeds the power supply voltage ( usually minus a few volts ) The general idea is that you put a fixed low reistance in series with the load on the low side and use the op amp to maintain a constant voltage on it. Works on the high side if you have a high side current sense, which is a bit more complicated. There is a 2 transistor solution often used in the optput of power amps to limit the current. Good if the current need not be adjusted or very well regulated. Look at battery charges for ni cad these are normally constant current sources.