Bidirectional Current Source

[Shehroz]

Hi
I am trying to build a bi-directional current source. The simulation I made using Proteus isn't working: no current flows when either polarity of input is applied. Can anyone point out what am I doing wrong?

 

[BobK]

Is R3 supposed to be the load? What it is connected to? You show a connection to pin 5 of the opamp, what is that? I would think it should be connected to ground.

Bob

 

[Shehroz]

Is R3 supposed to be the load? What it is connected to? You show a connection to pin 5 of the opamp, what is that? I would think it should be connected to ground.

Bob

yes R3 is the load through which current is supposed to change direction when input polarity changes.
Also, grounding pin 5 doesn't change anything.

 

[Shehroz]

Is R3 supposed to be the load? What it is connected to? You show a connection to pin 5 of the opamp, what is that? I would think it should be connected to ground.

Bob

oh, pin 5 and 1 of the op-amp are both offset nulls. I grounded both and it solved the problem. Thank you.

 

[BobK]

Great!

Bob

 

[Harald Kapp]

I grounded both and it solved the problem.

Did it? I see no feedback to close the control loop across the opamp. The opamp is operated in an open loop. As I see it, the 741 will immediately go into saturation. Either positive or negative, depending on the input signal.

What am I missing?

 

[TedA]

Shehroz,

I'm with Harald, I'm sure we are missing a few things here.

Any chance you could explain a bit more about how this circuit is supposed to work, and what it will be used for?

For instance, do you need an output current that changes amplitude with input voltage changes? Some sort of linear operation?

There is a pretty good 741 data sheet here:

http://www.datasheetarchive.com/dl/Datasheet-02/DSA0032521.pdf

On the last page it shows the intended connections for the offset null pins. I'm not sure what will happen when these pins are grounded on an actual device. For sure, the device's input characteristics will not be improved!

Ted

 

[BobK]

Harald,

It is supposed to do that (go into saturation). Look at how the transistor bases are connected. They sense the current from the power supply into or out of the op amp and multiply it, much like the pass transistor used on a 3-terminal regulator.

I am not saying that the design is good or will work well, but I have see audio amps that use this same method.

Bob

 

[Harald Kapp]

Look at how the transistor bases are connected. They sense the current from the power supply into or out of the op amp and multiply it, much like the pass transistor used on a 3-terminal regulator.

Bob, I know that design is being used to boost the output currrent of an opamp. Still for controlling the current some feedback is required. See e.g. here, page 2 (original page # 132)

 

[Shehroz]

The idea was to check if the current flows bidirectionally, which may not be a variable source yet.
It looked like the grounding worked but it didn't, and I forgot to update on the forum.