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voltage keep dropping why?

Discussion in 'Electronic Design' started by [email protected], Aug 9, 2007.

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  1. Guest


    In my last post (few months ago I believe) I told you guys that I was
    building a voltage regulator, and well, I have, expect I built on a

    Anyway, one of the problem I have with this circuit is it drop few
    voltages. For example, I put my pot on a 12v, but few minutes later it
    droped to 11.49v. I find this very anonying..

    I am using a HP printer adaptor -- regulated adaptor with two output
    31v, and 15v.

    This is the circuit -- I have replaced R2 with a pot

    Is there anything I can do?

    Answering my own Q: --- let me know if u think I am wrong.

    I have removed two capacitors from the output, and voltage sticking at
    the same place 12.03V :) I really wanted to had two capcitors out the
    outputs, specially tantalum capacitor... Oh well.

    I am going to give myself a mental hug!

    In future I want to build a voltage regulator with current, and I am
    hoping I can get one of the digital pot, cause this really sucks! I
    hate tuning right and left. I want it to stick at one point and stay
    there as long as I wish it to stay there.

    See I really to build this sexy boy expect I would want to replace pot
    with a digital pot, and digital screen.

    PS: Also does anyone know what kind of zino diodes I need to drop the
    voltage to zero -- where do I add them?
  2. Gravio

    Gravio Guest

  3. Guest

    I am measuring with Multifunction Volt Meter, and I have put three 5
    volt bulbs, strangly voltage remains same at that time.

    The reason why I need this is b/c I am testing some series-parallel
    circuits, rheostats, and superposition theorem, and few other stuff.
    Mostly college work. I figure if I have a voltage regulator at home I
    can do my work twice, and verify the result -- lab is only one hour
    durning summer, and no extra time....
    It has a head sink, very good one I might add.
    I think I need to build a bit more advance regulator, something that
    can handle 30v load, and is very precise. This circuit is very much
    like unregulated adaptor -- expect this takes the voltage down. I am
    very unhappy with this.

    I am trying to do my next week lab, and 1/2 of times I am worried of
    the voltage going down.. No good.
  4. Ross Herbert

    Ross Herbert Guest

    You said you were using 3 x 5V bulbs as a load but you didn't give the
    wattage ratings of the bulbs or whether you had them in series or
    parallel or a combination of these modes. You also have not provided
    any details of the output capability of the regulated plugpack supply.

    You say you do have a substantial heatsink on the 317 but I would
    question the wisdom of using a TO220 device in a proto-board while it
    has a substantial (read heavy) heatsink attached. The weight of the
    h/sk can cause the device to lean over thus creating possible bad

    I, and many others, have constructed regulated and adjustable power
    supplies based on the 317, and I can vouch for the ability of these
    devices perform as intended. There is little chance that you can
    construct a decent power supply in time for your project completion
    date by going about it in the manner you appear to be. It takes a lot
    longer than that to build something from scratch unless you know what
    you are doing and you must have a good grasp of the technicalities of
    designing and building such a power supply. I really would suggest
    that you borrow a ready built unit for your immediate requirements and
    then design your own as you gain more experience.

    There are many suppliers of kits such as this unit complete with a pcb which can provide what you are
    looking for. This one is available from KitsRus in HK and it won't cost much.

    If you want a unit with higher long term current capability (bigger
    heatsink) then use K124
  5. neon


    Oct 21, 2006
    is this happening without load variations? is the input filtered DC or what? where did you put the caps close far away is the gnd solid? does anything gets hot? answer all this quastions then you probably find your problems.
  6. Guest

    Okay bit of an update. I just soldered it in a pcb board, and it seems
    the thing is staying at 10.05 -- i set it at that speed at 6 o'clock
    in the morning -- same circuit as above, I have change the pot to 50k,
    and no capacitors. It's very stable now. Althought it goes from 10.05
    - 10.06.

    Also, beside bulb test, I am testing series, and parallel circuits
    (which I have just finished it), and now I am starting on
    superposition theorem circuits. I thing I'll be okay now. Ever since I
    soldered them on a pcb, it's been very good to me. I should have done
    it months ago...

    I will do some research on this, cause I really need to build one, and
    soon. Cause next semester I will be taking AC circuits...

    PS: How come there are only 2 boys in my Electronic I thought this
    used to be men domain?
  7. (snip)

    This (the stability of soldered connections over plug
    connections) is a lesson almost as useful as the
    superposition theorem.
  8. Ross Herbert

    Ross Herbert Guest

    50K is too high imo - you don't need anything higher than 10K.

    What do you mean by "and no capacitors"?

    You must have capacitors connected as specified in order to maintain
    loop stability and dc filtering. I strongly suggest you include
    capacitors as used in the KitsRus designs because this circuit is
    typical of those used by knowledgable designers.
    Well you may have learned something from this exercise. Never trust
    temporary connections (eg. non-soldered) in power circuits. Even a
    tiny amount of resistance in a joint can drop a relatively large
    voltage when appreciable current flows through it.
  9. Guest

    :) I think so, this circuits has been very good to me, my professor
    just cancel the lab class! And we girls had to two more labs. Good
    things I brough my stuff in the car, and we are doing it in the
    library :) So far so good.

    I am also building 2nd voltage regulator.

    here is the diagram

    I don't have 1.2v LM313 Reference Diode, but I do have 3v or 3.3v
    lm133 (how do I check it?), and I am wodering if I can do something.
  10. A single 1.5 volt chemical cell might be good enough for the
    negative reference, though it will get discharged when you
    turn the AC off, so you will have to disconnect it.

    A simple, low precision 1.25 volt shunt regulator can be
    made by connecting two diodes in series, or by using an IR
    LED (like those that produce the light signal for remote
    controls) or by making a Vbe multiplier with any small

    The last is made by connecting two approximately equal
    resistors in series, one collector to base and one base to
    emitter, so that when the voltage across the transistor
    rises to two diode drops, the transistor turns on and the
    collector current keeps the voltage from rising much higher.
    You need one additional resistor to the negative bias
    supply to feed it. Adjusting the ratio of the two resistors
    trims the shunt regulated voltage. None of these is as
    temperature stable as the integrated shunt reference
    regulator, but at room temperature, are probably good enough
    for your use.

    I see that the above schematic does not show where the low
    current negative 10 volt bias supply comes from. Do you
    need help deriving that from your transformer and rectifier?
  11. Robert Baer

    Robert Baer Guest

    ?? "A simple, low precision 1.25 volt shunt regulator can be made by
    connecting two diodes in series..." ??
    By using an oven to control the temperature of the diodes within 0.1C
    and using a constant current?
    LEDs in that respect are just as bad.
  12. MooseFET

    MooseFET Guest

    No, an LED is a lot better. Its voltage is higher but the tempco is
    about the same.

    A diode followed by an emitter follower is better yet.

    An LM7805 is even better.
  13. Maybe I should have written that as LOW precision, as in
    somewhere between 1 and 2 volts.
    Since a negative ~1.25 volt source capable of sucking up no
    more than 10 mA is what is needed, an LM337L would also work
    pretty well.
  14. Mike

    Mike Guest

    Do yourself a favor and and change the 50K pot to a 5K linear taper pot.
    You'll find it much easier to adjust unless your 50k has a log taper.
    Also use the capacitors for sure and maybe some protection diodes.
    See Fig 1 of this datasheet for diode connections.



    "The scientist is possessed by the sense of universal
    causation...His religious feeling takes the form of
    rapturous amazement at the harmony of natural law,
    which reveals the intelligence of such superiority
    that, compared with it, systematic thinking and acting
    of human beings is an utterly insignificant reflection."
    Albert Einstein (theoretical physicist)
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