TL741 Op Amp - Using the Null Set Function? - Page 1
Forum section: electronics / beginners

author: t1d
author_rank: Super Contributor
timestamp: September 19, 2021, 04:01:46 pm
content: Here is the draft of my schematic:The circuit proposes to use a TL741 op amp. This op amp has a null set function.Here is what I read about null set:Here is what I think that means:If you are using the 741 as a buffer to fix the GND/0/Null rail to absolute zero (meaning the non-inverting and the inverting inputs are tied to GND/0/Null, set a trimmer between pins 1 and 5, with the wiper tied to the Negative rail.My circuit does not use the op amp as this type of a buffer. Meaning, my inputs are not tied to the same (GND/0/Null) rail... They are referenced to different rails.However, I do want the ability to set the GND/0/Null output to exactly absolute zero (as close as r-e-a-s-o-n-a-b-l-y possible.) The circuit includes a separate trim pot (RV3) to accomplish that.So. for my education, as well as for use in this circuit... Is there a way to use the op amp's null set function with two different references? If so, how is that set up? If not, how best to terminate the null set pins?Asides1) Should I add D1 to the circuit, to account for any back inductance voltage? I used poor terms, here; my apologies.2) The Power BJT Transistor I propose to use is the MJ4502G. See the data sheet, below. It is described as for Audio use, as opposed to General Purpose use. It appears to have all the specs that I need. Is there any reason to not use it, in this application?Thank you for your help!

author: iMo
author_rank: Super Contributor
timestamp: September 19, 2021, 04:11:43 pm
content: And what is your question? To use it or not to use it?With 741 the typical offset voltage is 1mV and the voltage offset adjustment range is +/-15 millivolts.

author: t1d
author_rank: Super Contributor
timestamp: September 19, 2021, 04:34:25 pm
content: Thanks, imo, for your super-fast reply!Quote from: imo on September 19, 2021, 04:11:43 pm"So. for my education, as well as for use in this circuit... Is there a way to use the op amp's null set function with two different references? If so, how is that set up? If not, how best to terminate the null set pins?"Quote from: imo on September 19, 2021, 04:11:43 pmWell, the performance for this power supply is certainly not going to fall in that (tight) range. So, how best to terminate the pins?I sure appreciate your help!

author: t1d
author_rank: Super Contributor
timestamp: September 19, 2021, 04:36:09 pm
content: Oh, yeah... As the other questions?"Asides1) Should I add D1 to the circuit, to account for any back inductance voltage? I used poor terms, here; my apologies.2) The Power BJT Transistor I propose to use is the MJ4502G. See the data sheet, below. It is described as for Audio use, as opposed to General Purpose use. It appears to have all the specs that I need. Is there any reason to not use it, in this application?"

author: iMo
author_rank: Super Contributor
timestamp: September 19, 2021, 04:58:52 pm
content: As the input offset of the 741 is concerned - do not care about the 2 offset adjustment pins unless you are going to chase single millivolt digits somewhere in your circuit..To your PSU - you have just opened a big can of worms..

author: t1d
author_rank: Super Contributor
timestamp: September 19, 2021, 06:08:31 pm
content: Thanks, again, imo.Quote from: imo on September 19, 2021, 04:58:52 pmNo, I am not interested in that level of precision... Well, not yet, anyway...Quote from: imo on September 19, 2021, 04:58:52 pmIf you are joking, because PSU projects can grow into a large amount of considerations and compromises, I get your meaning. If you are speaking of specific issues with the circuit, itself, please let me know, so I can move on to a better design. This design is just for learning. I do not think it is necessary a great design. I imagine it is rather old technology.Another aside... The Power BJT is rather expensive. I have lots of MOSFETs on hand. Might I use a MOSFET, instead? If so, can it be dropped directly in place of the BJT? If it would need additional circuitry, what would that be?Thanks for your continued support and sticking with me.

author: iMo
author_rank: Super Contributor
timestamp: September 19, 2021, 06:21:16 pm
content: There are experts on PSUs here who may advice you.Worms - I've been just joking as the PSU related threads are quite frequent here and always pretty longPS: you should change the Subject of this thread perhaps - as you now talking about your PSU, not about the zeroing the 741's offset.. Thus people can see you want help with the PSU design..

author: David Hess
author_rank: Super Contributor
timestamp: September 19, 2021, 08:17:35 pm
content: Quote from: t1d on September 19, 2021, 04:01:46 pmThe offset null pins adjust the input offset voltage no matter how the operational amplifier is configured and will do exactly what you want. If you do not use them, then leave them unconnected.However I do not think your overall circuit is going to function properly as a variable voltage power supply with adjustable current limit.QuoteAdding D1 is not a bad idea.QuoteThe MJ4502G, and audio power transistors in general, work fine for power supply pass elements.

author: t1d
author_rank: Super Contributor
timestamp: September 20, 2021, 11:50:21 am
content: Thanks, guys!@ imo: You are correct. I will start a separate post and link it, here.@ David: You have been a great help to me, in the past. Please go to the new thread and help me, there, with the design issues that you see.EDIT: Here is the new link:https://www.eevblog.com/forum/beginners/another-noob-psu-design-needing-review-please/msg3699610/#msg3699610

author: Terry Bites
author_rank: Super Contributor
timestamp: September 20, 2021, 04:25:13 pm
content: A decent trimpot will probably cost more than a precsion opamp!

author: t1d
author_rank: Super Contributor
timestamp: September 21, 2021, 03:37:49 am
content: Quote from: Terry Bites on September 20, 2021, 04:25:13 pmThat is a really excellent point, Terry. Please join us at the other thread:https://www.eevblog.com/forum/beginners/another-noob-psu-design-needing-review-please/msg3699610/#msg3699610