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Modifying power Supply - worth it?

Discussion in 'Electronic Design' started by quietguy, Nov 6, 2005.

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  1. quietguy

    quietguy Guest

    Bit the bullet and bought myself one of those 0-30V 2.5 amp power
    supplies from DSE. Nice unit, but two problems I didn't anticipate.

    1. It has a power switch on the front panel - just on/off. Doesn't seem
    safe to me, as switching it on with an unknown voltage setting while
    connected to one of my TTL/PIC projects might result in tears if the adj
    knob was jarred etc while the unit was in the off state - and it seems
    silly that I have to either disconnect projects every time I switch the
    PS off, or fit a 7805 etc on every board.

    2. The voltage control is rather 'iffy' in that it is difficult to set
    the output at (say) exactly 5.0v - a twitch and its 4.5, another twitch
    and its 5.6 (get the drift) fiddle some more and get 4.9 or 5.2 etc
    etc. I realise some variations may not be a problem, but I don't like
    it. Life should be easy. V should be easy to adjust

    So, I am thinking of

    1. Fitting a load on/off switch - possibly by fitting a mains power
    switch to the rear of the case, and using the current front panel on/off
    sw as a load switch

    2. If I can, changing the voltage adj. pot to a multiturn pot, assuming
    I can find one that will fit in the original pots space - bit doubtful
    there, but there is not really enough room on the front panel to fit
    another (fine control) pot.

    Any comments? Ideas? Anyone else done these (or other) mods?

    David - who knows the mods will nullify the guarantee, but these PS's
    should be easy to fix, so...
     
  2. Yep, good idea. Good power supplies have load switches. Very handy.
    Another good idea, and again, this is what the good supplies have.
    Add a few fixed outputs too if you can, say +5V and +12V, you should be
    able to tap directly off the filter caps. But watch out because a lot
    of supplies will relay switch the transformer taps to avoid excessive
    power dissipation. In this case you could add your own
    rectifier/filter/regulator board directly on the transformer output.

    Just a matter of space really.
    Yep, go for it.

    Dave :)
     
  3. Fred Bloggs

    Fred Bloggs Guest

    The adjustment pot is a feedback voltage divider to an error amp input.
    Find the exact value resistance it takes to make 5.00V, get a 1%
    resistor of this value and add a switch to switch pot out and your 1%
    resistor in. Then just double check switch is in 5V position before you
    turn power on to your ongoing breadboard. But I still wouldn't trust it
    to stay within limits on overshoot. Another alternative is to restrict
    the range of adjustment by paralleling pot with resistance. Find exact
    value you need to limit to 6V, obtain 1% in this in this value
    etc...this might be better because you can then turn supply on at 0V
    output and fine adjust to 5V at your circuit.
     
  4. Dave

    Dave Guest

    A multiturn pot with integrated on/off switch:

    1) always starts off at the lowest voltage
    2) more precise control over output voltage

    Do they exist?

    Dave
     
  5. Dave

    Dave Guest

    Or another idea, replace the the pot with a multiturn and add a simple
    logic cct to detect if the output voltage is above a certain threshold
    at switch on, (say 2.5V). If it is, power to the output is disconnected
    via a mosfet/relay/etc.

    Dave
     
  6. I read in sci.electronics.design that quietguy
    Can you not find space on the panel even for a miniature toggle switch
    for load switching? They have surprisingly high current ratings at low
    voltages.

    That would save you disturbing the mains wiring.
    If the pot is wirewound, consider replacing it with a cermet pot. They
    have much smoother variations of resistance with slider position.
     
  7. It's sure easy to make, or buy, a regulated fixed 5.0V supply. If most of
    your work is 5.0V logic, make or buy one. You'll thank yourself in the long
    run.

    When I built my first bench supply in my youth, I looked at existing
    supplies and thought the feature set was kind of dumb, and built one that
    had a load switch and a fuse. I quickly discovered that having adjustable V
    instead of a load switch, and current limiting, was a much better approach -
    turn on the juice quickly to a circuit, and one of two things happens:
    either the circuit was good and the fuse blows while you're charging up the
    various capacitors in the circuit, or the circuit was bad and the fuse blows
    shortly after the circuit does, or the fuse was rated too high and only the
    circuit blows :)

    So these days I find that I'm quite happy with a supply that doesn't have a
    load switch, just a smoothly adjustable output voltage and a meter to
    display it.

    But if your output voltage isn't smoothly adjustable, that's worth fixing,
    by replacing the existing pot with a better-quality one (cermet, or even
    multi-turn).
     
  8. Phil Allison

    Phil Allison Guest

    "quietguy" <

    ** Have you checked to see if the voltage jumps up to full at switch on ?

    One of the DSE bench PSUs was notorious for that.



    .......... Phil
     
  9. quietguy

    quietguy Guest

    Thanks for that Phil - I haven;t seen that at switch on - my concern was
    that the adj might get bumped while the PS is off. However, I noticed that
    at switch off the V meter jumped to a high reading (20+V) - but with my DVM
    on the OP terminals I could not see that high reading - but I need to check
    with the CRO as my DVM doesn't have a sample? facility and may not have
    'seen' the transient.

    David
     
  10. quietguy

    quietguy Guest

    Thanks for all the ideas and info guys - I got inside the case for a short
    Will look more closely when I have time to completely disassemble the unit,
    but at the moment I may have to...

    Bridge the V adj pot to limit its range somewhat, if there is no room to fit
    a cermet pot (don't know about these, have to research) - it wouldn;t bother
    me if that limited the range to 0 to 15v or such.

    The load switch may have to be external, as the connections to all the OP
    terminals are PCB traces - but will see if there is a chance of breaking the
    common earth to them

    Adding fixed 5v is a good idea, but am not sure if I can do that and have
    the OP metered - building a separate fixed 5V supply for my PIC and TTL
    stuff might be the way to go

    Cheers and thanks again

    David
     
  11. Phil Allison

    Phil Allison Guest

    "quietguy" <

    ** The CRO is the go.

    Most DMMs with "sample" only store ( max or min) readings from the display -
    not input transients.

    The actual sampling rate is only 2 or 3 times per second - so events that
    last less than 300 mS are not captured accurately.




    ......... Phil
     
  12. budgie

    budgie Guest

    On Sun, 6 Nov 2005 17:19:20 -0800, "Walter Harley"

    (snip)
    and/or consider putting a highish value resistor from the wiper pot to the
    "cold" end, so an open wiper doesn't hit your load with max volts.
     
  13. *Always* -

    With the Chineese gouging the earth for raw materials to change into IOU
    notes, there is simply no point in buying components and building something
    for ten times the cost of the commercial unit; for my hobby purposes it's
    cheaper to buy something "almost-there" and then hack it to fit.
     
  14. quietguy

    quietguy Guest

    I can 'see' some transients on the CRO at sw on and sw off, but they are so
    brief that it is hard to see the amount of deflection etc - probably need a
    storage/digital scope for that.

    I am discussing some of this with the mfgs engineer - he is sending me a
    circuit diagram and seems interested in my comments - will update.

    David
     
  15. Phil Allison

    Phil Allison Guest

    "quietguy"


    ** Gee I wish you would **NOT** top post - it really stuffs up writing
    replies.


    ** Turn the horizontal ( time base) off.

    Centre the dot and make it nice and bright.

    Amazing what brief transients you can see and measure then.



    ............. Phil
     
  16. John - kd5yi

    John - kd5yi Guest

    message


    Hi, David -

    I'd bet you are seeing transients induced into the probe. Run a null test.
    Leave your scope ground lead connected where it is but connect the probe
    tip
    to the probe ground wire (that's right, short it out). Run the test. Do you
    still see a transient? If so, it may be the transformer inrush inducing
    noise into the probe. Could be radiated or induced via grounds, inductive,
    or capacitive coupling. It sometimes takes heroic efforts to make sure
    you're seeing a valid signal in the presence of noise.

    You can read about the shorted probe thing here:
    http://emcesd.com/tt070199.htm

    In fact, you'll learn a lot if you read *all* his papers.

    Good luck.

    John
     
  17. ehsjr

    ehsjr Guest

    Modify the case to give enough room to add whatever controls/jacks/
    meters you want. Just screw a project box to it, and run the
    necessary leads into the project box. A fixed 5 V output makes
    sense. For smoother and more precise control, add a selector switch
    to select an appropriate resistance string for a range, and place
    your existing pot in parallel with part of it to lower the resistance.
    Or put a multiturn pot in series with a suitable resistance string
    for the range to increase the resistance.

    A nice "expanded precision" pot is this circuit:

    -----+-----+
    | |
    | [R]
    [R] |
    | [POT]
    | |
    -----+-----+

    Ed
     

  18. Or a transient caused by the rising edge of the power supply ringing against
    the inductance of the probe leads. Do you have the probe adjusted so that,
    when a square wave is applied to it, it looks like a square wave on the
    scope? (That's partly what the 'calibration output' of the scope is for.)

    But either way, this is just more reason (IMHO) why switching the power
    supply on abruptly, whether with a load switch or the mains switch, is not
    how you want to power up your circuit. The bypass caps in your circuit are
    doing their best to look like a short circuit; the power supply is doing its
    best to look like a zero-impedance voltage source; so you're whacking it
    with a heck of an initial current spike, depending on how fast the current
    limiting in the supply (if it has any) can kick in. Much better to ramp the
    voltage up slowly with the voltage adjustment pot. If this adjustment is
    jerky or uneven, fix the problem, don't work around it. Wait till you know
    your circuit works normally, before you go testing its ability to withstand
    power supply transients!
     
  19. There is a lot of stuff that doesn't like being powered up slowly, it
    can cause latch-up and other problems. Something to be careful of.

    Dave :)
     
  20. quietguy

    quietguy Guest

    Did as you suggested, but couldn't see any transients, even on the CROs
    sensitive settings - thanks for the suggestion though - however, due to my and
    my gears limitations I don't discount what you say.

    David
     
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