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stepping down voltage twice

Discussion in 'General Electronics Discussion' started by foTONICS, May 30, 2013.

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  1. foTONICS


    Sep 30, 2011
    So I wired a 7812 and a 7805 in series with the input to the 7805 connected to the output of the 7812. I was expecting 12V and 5V respectively but I got instead 11V and 5V. what happened to that extra volt?

    I supplied the 7812 with 15V (even brought it up to 20V same issue). I'm aware of the voltage drop across linear regulators but I thought that since there was a significant difference in voltage I would be safe.

    I quickly solved the problem by connecting the inputs to both regulators in parallel, I was just curious if anyone knew why this was?
  2. duke37


    Jan 9, 2011
    The power supply will drop its voltage under load, perhaps this goes too low.

    The regulators have a current limit, perhaps the current taken through the 7812 is over 1A.

    The regulators have a power limit, perhaps the 7812 got too hot and it decided to lie down !
  3. john monks

    john monks

    Mar 9, 2012
    I don't know if this applies but from my experience I noticed that sometimes the 7812 and the 7805 do not function well without an output capacitor.
    I would try a 5uF capacitor on the output of both devices and see what happens.
  4. foTONICS


    Sep 30, 2011
    I've attached a picture of the circuit I'm trying to breadboard. I've left out the diodes and I used a power supply to mimic the solar panel input

    The output of the 7812 is powering a 12vdc motor and the 7805 is the supply to a l293d quad open collector comparator IC. the east and west terminals on the left go to a pair of LDR's, the motor terminals go to the 12vdc motor, and the solar terminals are just the input from a photovoltaic panel. is something going on with using both the open collector comparator and the l293 that is a common mistake? I have a feeling it's the motor driver IC because the voltage dips whenever I add it to the circuit, any suggestions?

    btw, sorry for the late reply

    Attached Files:

  5. davenn

    davenn Moderator

    Sep 5, 2009
    yup well as stated by earler by John, you donnt have any caps between the 7812 and 7805
    I would be swapping the 1uF for a 10uF on the 7812 input and putting the 1uF right at the output of the 7812.
    And depending on the distance from the 7812 to the 7805, I would seriously consider putting a 10uF on its input and a 1uF on the output.
    Also put 0.1uF caps across (in parallel with) the two 1uF output caps on each regulator.
    what is the main voltage to the motor, via the L293, the 5V rail or the 12V rail ??
    check current drains on both rails

  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Let me guess. This is a circuit to rotate a solar panel to follow the sun, right? And the west and east LDRs are mounted on the solar panel frame, pointing slightly left and right of the panel?

    Yes, definitely add at least one decoupling capacitor on the output of the 7812.

    Also I would delete D2 unless there's a good clear reason for it. Otherwise it just wastes voltage.

    Also you can save a bit of space by replacing IC2C with a small MOSFET such as a BS170 and using an LM393 instead of an LM339.

    There's nothing fundamentally wrong with supplying a 7805 from the output of a 7812. If the 7812 output voltage is low, that's probably because it's being overloaded, as has already been suggested. How much current does your motor draw? The 7812 also has an overtemperature shutdown.

    Can you post some photos of your project? It sounds like it might be of interest to many people here.
  7. foTONICS


    Sep 30, 2011
    I'll post some photo's in a bit but for the time being I thought I should clarify something. I've attached a picture of the style of regulators I'm using. I'm not using any heat sinks, just straight up breadboard trying to get the basic's working.

    I'll get back to you on the motor, im not sure yet

    Attached Files:

  8. foTONICS


    Sep 30, 2011
    Can that be reason enough for my missing volt?

    Don't the comparators come in a quad package IC?

    I was touching them and not feeling anything close to hot
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    According to John Monks in post #3 on this thread, yes.
    I don't know personally, because I've never used them without decoupling capacitors!
    They're available in single (LM311), dual (LM393) and quad (LM339) versions.
    Well that's good!
  10. foTONICS


    Sep 30, 2011
    no load about 67mA (which is what I was testing it under)
  11. foTONICS


    Sep 30, 2011
    Hey all!

    First of all thanks for all the feedback, suggestions, and help. I've made the capacitor changes as suggested and after some circuit modifications have it working. Although I'm not 100% sure I think the problem was caused by the motor driver I was using. I added some pull down resistors that were beefy enough to not interfere with the pull up resistors at the output of the lm339n. I've attached a picture of my final, working, (phew) prototype. Next step will be drafting up a pcb design in eagle then giving it to my boss here at school so he can CNC a board. I'll keep posting here as I update.

    P.S. How can I get this thread pushed into the project area? Given KrisBlueNZ comments it probably suits that area better!

    Thanks again guys!

    Attached Files:

  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The projects area is probably more designed for showing people what you're doing than asking for help.

    You could start a new thread in there and post images and commentary about each stage of your design and construction. There's absolutely no reason why you can't post some of the stuff you've posted here to show a bit of history.
  13. poor mystic

    poor mystic

    Apr 8, 2011
    I'm thinking of the 7812 as a very high impedance voltage source, supplying another such voltage source, probably supplying a constant load R. So the 7812 sees the 7805 as a high impedance constant current sink.
    The path between the devices strikes me as being a nice potential home for a family of oscillations.

    Are your devices getting hot?
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Voltage regulators have a pretty low output impedance. Usually in the low ohms range or less.
    As long as both regulators are properly decoupled, I wouldn't expect problems.
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