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basic power supply

Discussion in 'Electronic Design' started by [email protected], Mar 1, 2009.

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  1. Guest

    I want to built a basic power supply with 6 7[89]?? regulators. I have
    a line transformer capable of supplying 31VDC (15.5 + center-tap,
    scrap transformer). However, I do not know where I should connected
    the required capacitors (that is, I know they are required (and why),
    but I do not how to do it). Any help?
  2. Jon Kirwan

    Jon Kirwan Guest

    You probably mean VAC. Transformers on their own don't work well with
    DC. Where did you find the term 31VDC written?

    By the way, I love the idea of learning some electronics by trying a
    hand at building a power supply. So this is a great project to work
    on. You can start by ignoring some details that would probably be
    more confusing that helpful and later get better at it by bringing in
    those details later on when you are ready for them.
    Okay. So you have a transformer in hand and what to know what to add
    to it. Do you want a power supply that allows you to vary the
    voltage? Or do you just want a fixed output? Do you know what the
    rating of the transformer's secondary (output) is, regarding amps? How
    many different voltage outputs, regulated, are you looking for? Just
    one? Or more?

    If the transformer doesn't already have a bridge rectifier or other
    rectifiers on it, you will need to get two or four diodes. Since your
    secondary is center-tapped, you can go either way. Usually, after the
    diodes "rectify" the AC into deeply rippled DC, you can add
    appropriately sized capacitors with the right polarity hookup _across_
    (in parallel with, as often written) the + and - sides of the output
    from the diodes to help filter the voltage ripple to a somewhat
    steadier level before it reaches the regulator circuit. Usually,
    these are electrolytic capacitors that have a particular polarity you
    must be careful to follow.

    What other resources do you have? Voltmeter, at least? What other
    parts laying about?

  3. Guest

    Ah.. yes it is VAC, not VDC. sorry. And yes, I do have some bridge
    rectifiers (left over from a project using relays, where the relays
    only allowed switching in 1 direction, but I needed to be able to use
    2). 15.5VAC is written on the transformer, but it's center-tapped and
    the markings seem to indicate that is between two adjacent taps (and
    not the end taps). And this isn't really something that I'm doing for
    one of my first projects - I'm building an 8-bit TTL-based calculator
    right now. I just posted it in basics because it's a basic power
    supply - nothing fancy. So gimme the details, if you want.
    I want variable voltage from the regulators, which I understand can be
    done by either putting a pot on the output (relatively obvious) to
    lower it, or floating the regulator above ground with a resistor
    between ground and what the reg "thinks" is ground. I know that
    electrolytic caps have a polarity (though I believe I have blown at
    least one by hooking up a not-so-well-marked car radio up backwards),
    so that's not an issue.

    I found two pairs lying around which are 30uF and 1uf. Will that be
    good enough?
    I have plenty of resources - true-RMS multimeter with transistor
    checker and (!) capacitance measurement, 100MHz oscilloscope (it cost
    about 1/4 of the online price at a place called Stewart Smith's (I
    live in Syracuse, New York)), home-built parallel port logic analyzer
    (might even be powered off of this supply!), and of course the
    mandatory connection to the net.

    I have an old HP 524C counter which I have gotten numerous golden-
    years-of-electronics-no-clue-what-semiconductors-are-age components,
    including the above caps. And there are two electronics stores in
    town, so no problem getting parts.
  4. Jon Kirwan

    Jon Kirwan Guest

    Hehe. I had worried a little.
    Okay. I'm still not clear on the amps available. Do you have a way
    of finding out? Or, alternately, can you say what you want to be able
    to support?
    It might help to measure from the ends, as well. In other words, work
    out the details. I gather you have a meter, so use it. Doesn't take
    that much to hook it up to a snipped off AC cord (I have those laying
    around from stuff I throw away.) Alligator clip it, if you have to.
    Well, I'll do what I can. I learn as I go, too, being a hobbyist. The
    "big boys" will jump in when they feel I screw up badly, I'm sure.
    Yes, but really, really terrible, too. In the sense of -- "it just
    ain't done" except in VERY low current situations (your usual
    'divider' situation.) Certainly, not for a power supply.
    Yeah. I've hooked them up to AC power outlets back as a kid just to
    watch them blow up. Bad gases, I heard later. But the smell isn't
    the kind of thing you stick your nose into, anyway, so I mostly
    avoided getting too much into my system back then. And I didn't blow
    up more than a small number, even then. Gets boring.
    For some of the IC regulators, perhaps. What bothers me is that you
    may need something larger than 30uF to keep the ripple down leading to
    your regulator if the current is much of anything. The dV is easily
    seen from the I=C dV/dt equation, as dV = (I/C) dt. Your dt is pretty
    much determined by your AC Hz and the choice you make for the diode
    topology. With 60Hz and a bridge, you are talking about 120Hz bumps
    coming out. That's 8.33ms per. And the capacitor may need to supply
    current for a substantial part of that time. Assume for a moment that
    it is 72% of the time or about 6ms. Then you have a ripple dV of some
    200*I. If you can accept a 2V ripple, that means your load current
    needs to be 10mA or less. Basically, a heavy load suggests bigger
    caps to keep the ripple down to something manageable. And if you
    accept a lot of input ripple, just to keep the caps smaller, then your
    power supply regulator needs to be better at ignoring the ripple. If
    it is a linear one besides, you still need to make sure that the
    lowest point in the ripple is still large enough that it provides the
    output voltage required plus whatever overhead is required by the
    regulation system.

    So it's important to know what you plan for the regulator (switcher vs
    linear), what you hope to get out as a maximum load current, and over
    what range of output voltages, plus some idea of what you are willing
    to accept as ripple on your regulated output, too. Among other things
    I'm sure I will be told about.
    Well, that sounds great. You have some decent equipment and hopefully
    have reasonable understanding about applying them (I learn myself over
    time to use what I have better.)

    Are you interested in working through the design thinking? Or just
    want a circuit and don't care to understand why?

    And in the meantime, can you measure that transformer and let us know
    what range of output voltages are okay with you (both the low end as
    well as the high end -- does it really need to be able to go down to
    0.2V or to 0.00V, for example?) Also, what compliance you want for
    the current? (Which gets back to maybe telling us just how heavy that
    transformer is or if it has a rating on it, somewhere, or part

  5. Ecnerwal

    Ecnerwal Guest

    So figure out your connections for a proper center tap, and hook up your
    rectifiers. Go buy (or scrounge) some much larger electrolytic caps -
    you want (generically, without getting into the details of selecting
    them) 1000+ uF sort of size, not 30 uF sort of size - ie, those are not
    even in the right ballpark, as power supply filter caps go, IMHO.

    Here's a handy site (couple of page links - more are there, poke around)
    found per quick websearch (there may be better, but it seems OK for the
    basic idea):

    A real book might also serve well - having had some exposure to terrible
    books, I'm fond of The_Art_of_Electronics as being a very good one. If
    you have a college bookstore nearby, look for a used copy.
    Eww. Yuck-poo and merely yuck. There are much better ways to make
    variable voltage supplies. Old tech, but works well and still available
    (variable [+] regulator datasheet, variable [-] regulator datasheet).
  6. Guest

    Amps available: 0.75 amps is marked on the secondary.
    Ok, I lied. That's the FIRST secondary (the 15.5V one).
    The SECOND secondary gives 30 volts (I think - I'm at school and don't
    have it with me) but only 0.25A (I'm sure about that), so not enough
    amps for what I want.
    Ok. So max voltage out is 48V + or -, from the 7824 and 7924.
    It's a linear (I think - that's what I'm told) regulator, but I would
    think that a 2V ripple would be fine (I'm probably wrong, of course).
    I mostly just want to build it, but I could easily learn some design

    And for the low and high ends, I would probably like each regulator to
    be able to go down to half the rated voltage (though that is obviously
    not feasible, except for low current, as you said) and up to double
    the rated voltage.

    They're a TO-220 package, so I should be able to get a nice amp out of
    them... of course, the transformer doesn't handle that, but it's tiny,
    and I can always upgrade later.
  7. Guest

    I know someone who recommends this method. And he's very smart. And I
    know someone else who sees no problem, so I will simply ignore you by
    the democratic process. Remember, this is America.
  8. Guest

    Oh, also I already have my regulators. Maybe later, if I find one I
  9. Guest

    This is why democracy only works in theory. Your friends may be smart
    in their fields, but this is just stupid. Those are the two worst ways
    of doing it, horrific.
  10. Guest

    Oh, and
  11. I came up with this design in LTspice of a regulated power supply with
    current limiting that uses simple bipolar transistors. It would probably be
    a good learning project. The PWL voltage source should be replaced with a
    pot across a zener for an adjustable output. I used it only for simulation
    purposes. There are probably many ways this could be improved, but you
    could learn much more from this than a supply with a fixed integrated
    voltage regulator.



    Version 4
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  12. Guest

    And I suppose you're going to say, with your stupid e-mail address and
    "knowledge" of Wikipedia, that you're smarter than... let's see,
    what's appr - Ah. You think you're smarter than GWB? Interesting. I'll
    keep that in mind.
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