Constant current load not working properly - Page 1
Forum section: electronics / beginners

author: jeet55
author_rank: Contributor
timestamp: August 29, 2017, 07:49:35 pm
content: 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.

author: Benta
author_rank: Super Contributor
timestamp: August 29, 2017, 08:06:10 pm
content: 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.

author: danadak
author_rank: Super Contributor
timestamp: August 29, 2017, 09:26:07 pm
content: Generic bipolar 741 does not function off just a 5V supply either.Regards, Dana.

author: not1xor1
author_rank: Frequent Contributor
timestamp: August 30, 2017, 04:26:24 am
content: Quote from: danadak on August 29, 2017, 09:26:07 pmBesides 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 thehttps://www.eevblog.com/forum/projects/section.

author: jeet55
author_rank: Contributor
timestamp: August 30, 2017, 04:45:35 am
content: So should i use a mosfet? If yes then which one?

author: Zero999
author_rank: Super Contributor
timestamp: August 30, 2017, 08:09:09 am
content: 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 aDarlington 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-possiblehttp://www.intersil.com/content/dam/Intersil/documents/an11/an1111.pdf

author: danadak
author_rank: Super Contributor
timestamp: August 30, 2017, 09:46:06 am
content: https://www.allaboutcircuits.com/technical-articles/how-to-buffer-an-op-amp-output-for-higher-current-part-3/https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjkxsyR1v7VAhVF5SYKHUjJArkQFggmMAA&url=http%3A%2F%2Fwww.ti.com%2Flit%2Fan%2Fsboa046%2Fsboa046.pdf&usg=AFQjCNG1P7RJee3xOsaFhuuS5cz_1jQzZQhttp://www.ti.com/apps/docs/litabsmultiplefilelist.tsp?literatureNumber=sboa046&docCategoryId=1&appId=308Regards, Dana.