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Homebrew power supply

Discussion in 'Electronic Basics' started by Byron A Jeff, Nov 1, 2004.

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  1. Byron A Jeff

    Byron A Jeff Guest

    Good for circuits that do audio. Also circuits that use opamps may or may
    not need a dual polarity supply.
    Usually less than 1 Amp. This requirement may change if you're building
    something with electric motors whough.
    Nope. The component is called a transformer. BTW you'll probably want to
    put a fuse or circuit breaker on your primary of the transformer in case of
    a short.
    As good as any. Did you want variable voltage? This it's the ticket. Just
    take care with the max voltage and power dissappation on the part. You'll
    probably have to heat sink it to draw off the excess heat.

    BAJ
     
  2. David Green

    David Green Guest

    Hey,

    Forgive me for the simplicity of this question but I'm rather new to
    electronics. I want to build a homebrew benchtop power supply and would
    like a bit of feedback before I try:-

    1. What potential uses does a dual polarity supply have - are they harder
    to assemble?

    2. What sort of current am I likely to require for simple electronics
    projects?

    3. Is isolating a power supply from the mains supply difficult?

    4. What regulator IC should I consider for a beginner project? I was
    considering LM137, only on grounds of cost and simplicity - is this a good
    choice?

    Thanks kindly for your time,
    Dave
     
  3. If you have an alternating voltage, set around a gnd point, the negative
    value will let you represent those voltages below the 0 point.
    Especially useful if you're using opamp, then I'd go as far as to say a
    requirement.
    Logic gates (CMOS gates that is) I usually run on 12v, since this is the
    ultimately most used voltage in electronics. A logic gate requires
    extremely small amounts of power, like 35mA maximum. Thus, you'd get 30
    of them with a 1A PSU. If you are looking at relays, motors and so on,
    you suddenly need bigger currents, as those pull more. Also, amplifiers
    and such needs more, but in my expirience, 1A gets you very far.
    No. A transformer (not an autotransformer or any other kinda xfmr with
    shared prim/sec windings) will isolate mains from outgoing voltages.
    LM317 is a good choice for having it regulatable. I would also recomend
    the following:
    7805 to provide +5V independent of the adjustable output
    7812 to provide +12V independent of the adjustable output
    7912 to provide negative 12, especially if you work with operational
    amplifiers.
    The +5/+12v I've found very useful, but that can be just me, since I'm
    working much with pc electronics wich require those voltages. Otherwise,
    +5V is pretty useless unnless you use TTL logic. CMOS will take anything
    from 3-18V, so you'd probably end up running cmos logic from the
    adjustable anyway;)

    Good luck:)
     
  4. David Green

    David Green Guest

    Would it be necessary to provide current limiting? I've seen some possible
    examples on circuit schematics of manufacturers data sheets but I'm not
    particularly sure if I need it or not?

    I don't plan to use motors at this point and would like to keep the
    project small and simple (low a current as necessary) until I get more
    adept.
    250v @ 2amp ok? Should I go for quick blow or surge resistant fuses? My
    mains supply is a pretty stable 230v (UK) and fluctuates little.
    Would a fan be a necessity or just passive heatsinks? Variable voltage is
    a definite requirement.

    As an asides can I actually increase ripple in the DC side of my circuit
    by adding capacitors across the load of my power supply? If I can't I plan
    to add a few selected values, mainly for experimentation to see what
    results I achieve.

    Appreciate the response,
    Dave
     
  5. Byron A Jeff

    Byron A Jeff Guest

    -> In article <>,
    ->>Hey,
    -
    ->>Forgive me for the simplicity of this question but I'm rather new to
    ->>electronics. I want to build a homebrew benchtop power supply and would
    ->>like a bit of feedback before I try:-
    -
    -<snip>
    -
    ->>2. What sort of current am I likely to require for simple electronics
    ->>projects?
    ->
    -> Usually less than 1 Amp. This requirement may change if you're building
    -> something with electric motors whough.
    ->
    -Would it be necessary to provide current limiting? I've seen some possible
    -examples on circuit schematics of manufacturers data sheets but I'm not
    -particularly sure if I need it or not?

    Nope. Parts like the LM317 are temp limited. If the current gets too high,
    the part heats up, then shuts down.

    The fuse/circuit breaker I referred to below is also a current limiting
    device. The current gets too high, then the fuse blows.

    -
    -I don't plan to use motors at this point and would like to keep the
    -project small and simple (low a current as necessary) until I get more
    -adept.

    Then 1A should be fine for starters.

    ->>
    ->>3. Is isolating a power supply from the mains supply difficult?
    ->
    -> Nope. The component is called a transformer. BTW you'll probably want to
    -> put a fuse or circuit breaker on your primary of the transformer in case
    -> of a short.
    ->
    -250v @ 2amp ok? Should I go for quick blow or surge resistant fuses? My
    -mains supply is a pretty stable 230v (UK) and fluctuates little.

    Much smaller current. [email protected] is almost 500W of power. It's very unlikely that
    you'll dissapate more than 25W of power. So you should be looking for something
    along the lines of a [email protected] It can be slow blow as you already have the
    LM317's thermal limiting as another backup.

    -
    - >>4. What regulator IC should I consider for a beginner project? I was
    ->>considering LM137, only on grounds of cost and simplicity - is this a
    ->>good choice?
    ->
    -> As good as any. Did you want variable voltage? This it's the ticket.
    -> Just take care with the max voltage and power dissappation on the part.
    -> You'll probably have to heat sink it to draw off the excess heat.
    ->
    -Would a fan be a necessity or just passive heatsinks? Variable voltage is
    -a definite requirement.

    Depends on the power dissapation. Design for the worst, then double it. If
    you need a fan at double the worst case scenario, then put one in.

    Of course with CPU fans both cheap a plentiful, it may be worth it simply
    to throw a small one on the part.

    BAJ
     
  6. David Green wrote:
    [...]
    Usually, the transformer is marked with a fuse symbol, and a number wich
    gives the maximum fuse that is to be used on the primary side. Use this
    size ±10%, and you'll be safe.

    [...]
    That was to get multiple voltages, and yes, input in paralell, but
    output as an single point, as it would be a entirely different voltage
    potential amongst those.

    Also, if you want an negative/positive voltage, you'll need an
    centertapped xfmr, or more effectively a xfmr with two secondary windings.

    Good luck with your little project:)
     
  7. David Green

    David Green Guest

    How about the mains side? 250v but what amperage? Sorry for the repetitive
    questions about current but building something running on mains power
    makes me want to be very sure I've got everything setup correctly.
    I'll put a cheap fan in the back of the case and use a finned heatsink -
    i have some that should fit the package type of the voltage reg.

    Vidar mentioned using multiple voltage regs? I presume I would wire these
    in parallel and they would draw current
    appropriately when used simultatenously up until the maximum limit, set by
    the fuse?

    Thanks,
    Dave
     
  8. Yes, this seems perfectly fine :) Also, you will have to rectify. Pass
    the +ve and -ve trough a rectifier bridge, and take out + and -, and let
    GND be all alone, do not run that trough any diodes or so...

    [...]
    I guess you should be able to that. PSU is damn expensive. Something
    like 300$ for a simple regulatable psu with voltmeter and short
    protection and such is quite usual...
     
  9. David Green

    David Green Guest

    I am looking at a transformer with dual primary and secondary windings -
    as suggested on the datasheet I was going to jumper the primaries
    together and then junction the secondaries to create a centre tap, then
    use the centre tap as the ground reference:-

    Something like this:

    -------------------- GROUND
    -- +240v |
    | |
    | ------------- |
    | | | |
    ----------------------------
    | 0 120 0 120 |
    | |
    | |
    | |
    | 0 20 0 20 |
    | |
    ----------------------------
    | | | |
    | -------------- |
    | | |
    | | |
    | GROUND |
    +ve (common) -ve
    rail rail

    Then run the variable output and +5 and +12v outputs of the +ve rail, and
    the -12v negative output of the -ve rail, providing an output of common
    groud for when I want a +ve supply only.
    Is it a good plan? Thanks for the input, hopefully I can build something
    far cheaper than an off-the shelf supply and learn a lot whilst building
    it.

    Regards,
    Dave
     
  10. Jack// ani

    Jack// ani Guest

    Would a fan be a necessity or just passive heatsinks? Variable voltage is

    Putting a fan it useless, a big heat will make it.
     
  11. Active8

    Active8 Guest

    On Mon, 01 Nov 2004 21:52:51 GMT, Vidar Løkken wrote:
    He means this:

    ____
    | |
    +----+---|7805|--+------ V+
    | | |____| |
    | --- | ---
    fuse -. ,-------+ | --- | ---
    )|( | +-+----+ | | |
    )|( | A A | | |
    |-' '-+ +------+ | | | |
    |-. ,-+--+ +------(-+ +-----+-----+----+
    )|( | | A A | | | |
    )|( | | +-+----+ | | | |
    -' '----|--+ | --- | --- ===
    | | --- _|__ --- GND
    | | | | | |
    | +----+---|7905|--+------ V-
    === |____|
    GND
    created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de

    Google for "Ian Purdie power supply" (or suplies). He covers that
    well enough - selecting caps for a specified load current and ripple
    and stuff like that.
     
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