Author Topic: Constant current load not working properly  (Read 1510 times)

0 Members and 1 Guest are viewing this topic.

Offline jeet55Topic starter

  • Contributor
  • Posts: 18
Constant current load not working properly
« on: August 29, 2017, 07:49:35 pm »
I built a constant current load using a 741 ic and tip41.
But the voltage across the sense(1ohm) resisitor is staying constant(about 1v) regardless of the voltage adjusted by pot on non- inverting pin.circuit diagram is attached below.plz help me to figure out the problem.
 

Online Benta

  • Super Contributor
  • ***
  • Posts: 5839
  • Country: de
Re: Constant current load not working properly
« Reply #1 on: August 29, 2017, 08:06:10 pm »
Of course it doesn't. The 741 does not accept signals that close to VEE.
Either give it a negative supply voltage as well, or invest in a rail-rail opamp.
 
The following users thanked this post: jeet55

Offline danadak

  • Super Contributor
  • ***
  • Posts: 1875
  • Country: us
  • Reactor Operator SSN-583, Retired EE
Re: Constant current load not working properly
« Reply #2 on: August 29, 2017, 09:26:07 pm »
Generic bipolar 741 does not function off just a 5V supply either.


Regards, Dana.
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 
The following users thanked this post: jeet55

Offline not1xor1

  • Frequent Contributor
  • **
  • Posts: 716
  • Country: it
Re: Constant current load not working properly
« Reply #3 on: August 30, 2017, 04:26:24 am »
Generic bipolar 741 does not function off just a 5V supply either.

Regards, Dana.

Besides what has been already written, you (I mean the original poster) cannot connect any opamp to a power transistor pretending to get 1A out; opamp output current might not be enough for driving a TIP41 (depending on opamp feature and that TIP41 hFE).

You should also add a resistor to the base of the transistor (or mosfet or darlington) and probably a capacitor (or RC in series) between the opamp output and the inverting input to prevent oscillations.

And you might need another R+C to dump the parasitic inductance of the cables connecting the load.

You should better look for a proven schematic and read carefully what has been written regarding electronic loads in the https://www.eevblog.com/forum/projects/ section.
 
The following users thanked this post: jeet55

Offline jeet55Topic starter

  • Contributor
  • Posts: 18
Re: Constant current load not working properly
« Reply #4 on: August 30, 2017, 04:45:35 am »
So should i use  a mosfet? If yes then which one?
 

Online Zero999

  • Super Contributor
  • ***
  • Posts: 19345
  • Country: gb
  • 0999
Re: Constant current load not working properly
« Reply #5 on: August 30, 2017, 08:09:09 am »
You should replace the LM741 with the LM358. It will work with its inputs at 0V.

A MOSFET might work but will be more prone to oscillation, due to the high gate capacitance. You could add another transistor to form a Darlington pair, or use a Darlington transistor such as the TIP121 or TIP140.

Another thing you could try is connecting two op-amps in parallel to give more drive current to the transistor. The LM358 contains two op-amps anyway and all that's needed is a couple of extra resistors to help the op-amps share the load.
https://e2e.ti.com/blogs_/archives/b/thesignal/archive/2013/03/26/paralleling-op-amps-is-it-possible
http://www.intersil.com/content/dam/Intersil/documents/an11/an1111.pdf
 
The following users thanked this post: jeet55

Offline danadak

  • Super Contributor
  • ***
  • Posts: 1875
  • Country: us
  • Reactor Operator SSN-583, Retired EE
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 
The following users thanked this post: jeet55


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf