A little help needed with LM741 dual tracking power supply

Discussion in 'The Projects Forum' started by knoba, May 1, 2012.

  1. knoba

    Thread Starter New Member

    May 1, 2012
    3
    0
    By way of an intro, from a first timer, Hello! Great site.

    I'm fairly new to electronics, too (this will be my 3rd completed circuit), btw...

    I'm struggling to make myself a; bi-polar*, dc, auto-tracking^, bench-top power supply from various circuits around the internet. This is the one I want to make...

    [​IMG]

    I think I'm good up to the outputs from the voltage regulators. It's the op-amp that's killing me (the design calls for a LM301, is the LM741CN a suitable substitute?).

    I can't get the LM337 voltage regulator to alter its output. The LM317 adjusts fine from 0v - +17v, using the potentiometer. The LM337 side of things just stays at -17v.

    The only other components that are different to the stated design is:
    * my transformer is a centre tapped 72va 240v.
    * my bridge rectifier is a W01G.
    * my ripple capacitors (directly after rect.) are single 4700μF 35wv's.

    Any help would much appreciated!


    * bi-polar = 17v - 0v - +17v outputs. Is "bi-polar" the correct word for this?

    ^ auto-tracking = that both sides, of the minus and positive power supply outputs, can be simultaneously altered through a single control?
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,123
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    D2 is backwards, BTW.

    The op-amp is powered from filtered but not regulated (and therefore rippling) power. That seems like a problem.
     
  3. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    So many problems! No local capacitors on the 3 legged regulators. No output capacitors. You are trying to get the op amp to balance the voltages with an input equal to its negative supply (out of common mode range). The 741 does not have a place for that 100p frequency compensation capacitor. and I'm in a hurry to get to work!

    C U later.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
    1,605
    Most op amps have incredibly good power supply ripple rejection so unless your ripple gets close to where it is opperating then you should be OK with that.

    If not, add another diode and cap so it gets it's own filter cap. Then it's current draw will be so slight as to make it all good.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,011
    3,233
    The power supply rejection of the op amp is high enough (77dB minimum) that the power supply ripple will likely not cause significant ripple in the output.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,123
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    Got it! Op-amps are OK with ripple. Makes sense - that's their job.

    Plenty of other things wrong to work on.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Actually, it's a problem with the V+ on pin 7 being the same voltage as the noninverting input. If pin 7 were brought about 3v above ground, then the circuit would (mostly) start working - with the caveats already mentioned.

    If the voltage on the filter caps will never exceed ~±20v, then the 741's pin 7 (V+) could be connected to the positive filter cap. However, the absolute maximum supply voltage for the 741 is ±22v; and it is never a good idea to operate a component at a maximum specification.

    The output of the positive supply won't go below ~1.2v, and the output of the negative supply won't go above ~-1.2v, due to Vref (the voltage from the OUT to ADJ pins). If you really want the outputs to reach near 0v, there are ways to get there - but it's usually not worth the trouble.
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,011
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    As SW noted, the 741 or 301 op amps you show won't work properly. You need a rail-rail type op amp for it to work down to the minimum negative voltage of about 1.2V.
     
  9. knoba

    Thread Starter New Member

    May 1, 2012
    3
    0
    WOW... Thanks for all the great replies. :;)

    (...most of what has been said went straight over the top of my head)

    I only realised, after I posted, that I forgot to mention the pin-out for the LM337 is not the same configuration on my component. On mine; pin 2 is Vin, & pin 3 is Vout.

    My knowledge of electronics is pretty lame, so if I get to be too much hassle, no worries.

    wayneh - D2, nicely spotted.

    Reading the other posts, on this thread, seems to suggest whether the use of the 741 is advisable. It wasn't until your response about op-amps & ripple that it finally registered with me what was being discussed.

    #12

    No local capacitors on the 3 legged regulators.
    How would I know it needed them?
    Is that something that would be identified with an oscilloscope?
    Would the capacitors sit between the input-pin and voltage-adj, of each regulator?

    No output capacitors. - I'm not doing too well, am I.
    How would I know it needed those?
    Is that something that would be identified with an oscilloscope?
    Would these capacitors sit between the +17v output and 0v output, & the -17v output and 0v output?

    Appols, #12, the rest of what you said, apart from going to work, I didn't understand at all... OUCH!

    ErnieM - That's an oscilloscope thing, again, isn't it? I'm running Linux here. The only app for my operating system seems to be Xoscope. I have an old laptop here. It's my very next project, to kit it out, as a stand-alone oscilloscope and make-up an interface & some probes.

    Maybe I should bring that ol' o-scope project to the front of the queue... Would you say that should be a priority, for me?

    crutschow - The power supply rejection ratio - I see it , now, in the 741 datasheet. Min & Typ. Would this be measured, in my circuit, by an oscilloscope? How did you know what ripple frequencies are in my circuit (experience or supplied data)?

    You need a rail-rail type op amp for it to work down to the minimum negative voltage of about 1.2V - Like the LM7322 (but a through-hole package, a lower & wider voltage range)? How would I go about finding one? Is the right part something I should just know, or a datasheet research thing?

    SgtWookie - That is a fantastic reply. ....&, I bet I'm one of the few (among many who learn from it) that doesn't understand most of it. I don't mind the power supply getting down to 1.2v, or something. When I know more about electronics, I can maybe alter it, make it better, rebuild it like Steve Austin...

    Electronics has been a decade-long bug-bear for me to get to grips with. I'm trying to get "a feel" for it, this time, to see what it can do for me (in terms of practicalities). Most of the stuff I'm attempting is being done on a breadboard with through-hole components. I'm competent with software, but I don't seem to be able to take this subject-matter in, unless I have something to physically hold.

    I figure; If all else fails, at the very least, I can put in a dual-gang potentiometer in place of the IC. But... I'd like to learn more about electronics, if possible. I was reading about the 741, a little, and a few terms were mentioned about the way it could be used; "voltage follower" & "comparator". Are either of these terms in the ball-park of what functions I should be getting familiar with, in this IC power supply circuit model?

    I can; look up things, read & research terms, if anybody wants to throw me a few directional pointers... :)

    Thank you, all, for your time & efforts. Much appreciated.
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,123
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    Reading the datasheet for any and every component you use is a best practice. You couldn't have missed this if you'd read the datasheets.

    In addition to datasheets, it's always good to read general info about a component, for instance the wiki article on "MOSFETs" or "op-amps" or "555 timer" if you're using such a component for the first time. You really need to know what every piece is doing. It can make you feel pretty humble and ignorant sometimes, but that's how anyone learns it.
     
    #12 likes this.
  11. crutschow

    Expert

    Mar 14, 2008
    13,011
    3,233
    Reading some of the many on-line tutorials about electronics (including some of the "Stickies" on this forum) are a good way to improve your knowledge. And always read the data sheets on any devices you plan to you use. I know they can be a little obtuse but they generally tell you what you need to know about using the device. If you come upon unfamiliar terms, Google will generally come up with an explanation.

    And of course you can always ask questions on this or the "General Electronics Chat" forum. Someone is sure to have an answer.

    Have fun.
     
  12. knoba

    Thread Starter New Member

    May 1, 2012
    3
    0
    I got it... <dancing banana>

    A massive piece of the datasheets are now open to me...

    I swear... I looked over those datasheets a dozen times. Until your post I didn't realise the data could be related to my project. It was like some kind of blind-fold had been lifted.

    The datasheet for the LM337 said "typical application for a programmable regulator". I honestly didn't think it related to me. How does one know, what one doesn't know...

    Thank you, both, for your patience & understanding.
     
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