12VDC 5A Power supply issues

Discussion in 'General Electronics Chat' started by newtopsupply, Jul 6, 2015.

  1. newtopsupply

    Thread Starter New Member

    Jul 1, 2015
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    Good afternoon guys and gals

    I have been trying to create a 5A power supply for a specific application.
    Am nearly there and have had much help from AAC people to get here.

    I have a system currently (attached) that runs a relay and electronic dc motor speed controller up to 5A
    It runs from a 60VA 240V to 12VAC electronic transformer.
    Voltage at the output of the current system however is less than 10 and fluctuates 1 to 1.5V.
    This is too low to run my motor controller which has a low voltage CO.
    I'm also worried about damage from the fluctuations to my motor speed controller.
    Initially I was getting 16V without the ZD and Transistor circuit (too much too much).

    AnalogKid has suggested the use of a couple of 1N4004's in series to correct and an 18 ohm resistor (thanks for all your help AK). See dwg attached.
    (if you're there AK a last bit of a look would be brilliant of you)

    Q's - Have I too much Capacitance?
    - Have I got the orientation of the 1N4004's right?
    - Should I go to a 15V ZD?
    - Will it stop the fluctuation

    Regards to all
    New
     
  2. #12

    Expert

    Nov 30, 2010
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    Those 2 helper diodes on the zener are backwards.
    Your first capacitor is running right on the ragged edge of, "not enough voltage rating". If your powerline voltage is too high you are in danger of having it puke. Measure the voltage to be sure it isn't above 16 volts.
     
  3. dl324

    Distinguished Member

    Mar 30, 2015
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    The voltage fluctuation you're seeing is the lack of voltage regulation. Is there a reason why you don't use a 12V regulator? You have a non-linear voltage drop after your zener; more current means more voltage drop that the zener won't compensate for. A voltage regulator will give you much better regulation than what you have now.

    The voltage rating of your filter cap is too low. Transformer secondary voltage is RMS; 12VAC will give you 1.414*12=16.97V.

    The 1N4004 diodes are backwards.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    1. The zener is oriented correctly, but the diodes in series with it are backwards.
    2. Start with only one series diode.
    3. As noted before, this will not be a tightly regulated output. An LM338 would be better.
    4. Decrease the resistor to 10 ohms. This gives an available base current of approx. .25A, so the 3055 needs a gain of only 20 to make 5 A out.
    5. Power in the resistor is 0.625 W. Power in the zener is 3 W.

    BUT -
    The first large filter cap is too small. At 5 A there will be 8 V of ripple, and you have less than 3 V or headroom. You would need 42,000 uF of capacitance to keep the ripple within your design limits. This is a common problem when the transformer secondary voltage rating is so close to the output voltage. If you really need 12 V at 5 A and this is your only transformer, I'm not sure how you will get there without a much more complicated low dropout regulator design.

    ak
     
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  5. newtopsupply

    Thread Starter New Member

    Jul 1, 2015
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    Thanks guys will fix that.
    Wasn't sure of their function in this scenario.
    I didn't know what a 1N4004 does exactly see.
    If it's a reg diode or not.

    OK will fix the cap.
    I don't really want to test it's upper limits. Have read it may be messy.

    At 42000 I think you may be right.
    A redesign may be in order.
    Will let you know how I get on.
    I could have probably bought something cheaper by this stage, but I'm bull headed about this now.
    Thanks for all your help again AK, really good of you to have the patience.
    Would a 15V Zener help? What would it do in this situation? Just trying to understand how they work.
    Or is that just barking up a tree?

    Thanks AK and thanks guys
     
  6. #12

    Expert

    Nov 30, 2010
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    Everybody ignored the 15 volt zener idea because it's just wrong for this job.
    The 1N4007 diodes are what I call, "spacers". They use up about 0.6 to 0.7 volts each and that difference jacks up the base of your transistor enough to compensate for its base to emitter voltage loss. One should be enough for most cases, but a 2N3055 can use up as high as 1.9 volts in its base circuit under some extreme cases.
     
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  7. newtopsupply

    Thread Starter New Member

    Jul 1, 2015
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    LOL.
    Gotta ask though. What would it do?
    It's just so I can understand better.
     
  8. #12

    Expert

    Nov 30, 2010
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    If you put a 15 volt zener in the place of the 12 volt zener, it would make the output voltage go up by 3 more volts.
     
  9. newtopsupply

    Thread Starter New Member

    Jul 1, 2015
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    But my output now is 9 V and I need 12.
    Would it do anything odd with the transistor or current/voltage pattern?
     
  10. newtopsupply

    Thread Starter New Member

    Jul 1, 2015
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    Also the 1N4004's are supposed to help this too aren't they?
     
  11. #12

    Expert

    Nov 30, 2010
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    Your output voltage is low because your first filter capacitor is too small. You have to read the instructions and do what they say before they will make the circuit work better.
     
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  12. #12

    Expert

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    No. We just suggested them because we own stock in the diode company.
    OF COURSE THEY ARE SUPPOSED TO HELP!
    Do you think we just type this stuff to play a joke on you?
     
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  13. newtopsupply

    Thread Starter New Member

    Jul 1, 2015
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    Ah, I think I see.
    If I were to fix the filter resister to a 25V and do the other things and then add a a 15V ZD my voltage could go past 14, yeah?
     
  14. newtopsupply

    Thread Starter New Member

    Jul 1, 2015
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    No of course not.
    Did not mean to offend.
    I have limited knowledge in this area and am trying to expand my knowledge of the goddess of electrons is all.
     
  15. dl324

    Distinguished Member

    Mar 30, 2015
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    First you have to address the problem with the filter cap; 42mF is a large capacitor.

    Then you need to address your current requirement. Your transformer is rated at 60VA. It will not supply 5A; it'll be closer to 3.5A. Since you need a different transformer, why not get one with a higher secondary voltage so you can reduce the size of the filter cap?

    You say you want better regulation. How much better? After you reduce ripple, you're still going to have a non-linear voltage drop that the zener won't compensate for.

    Trying to use a 15V zener to compensate for issues with the current component values is a hack. Better to put a diode or two in series with the zener to offset the BE drop of the pass transistor.
     
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  16. #12

    Expert

    Nov 30, 2010
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    Being a beginner is not an excuse to not read what you have been told. Between about 3 of us, you were told everything you need to make this work. Not reading the instructions, not doing what they say, and asking the same question over and over means you are wasting our time. This is a 2 way street. If you don't pay attention to us, we won't pay attention to you.
     
  17. newtopsupply

    Thread Starter New Member

    Jul 1, 2015
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    Thanks for that.
    AK tells me a LM338 circuit would do the job better and by the feel of it it's what I am going to have to do.
    But what you guys have all said to me increases my understanding of what's going on.
    Thanks for taking the time.

    I'm going to persevere until I get this right if it kills me.
    And if I hook up the wrong thing it just might!
    Just kidding

    I need to work through all this info I've gotten and replan
    Let you guys know how I go.

    Big thanks again AK and to you DL/#12.
    Regards
    New
     
  18. newtopsupply

    Thread Starter New Member

    Jul 1, 2015
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    That's fair enough man.
    My apologies
     
  19. dl324

    Distinguished Member

    Mar 30, 2015
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    The LM338 drop out voltage at 5A is around 3V. That means it's input voltage can't drop below 15V, so you still need a large filter cap or a higher secondary voltage. Using a bridge rectifier and filter cap requires the secondary current rating to be 1.4X the load current.
     
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  20. #12

    Expert

    Nov 30, 2010
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    newtopsupply likes this.
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