Making a power supply

Discussion in 'General Electronics Chat' started by Guinness, Jan 8, 2010.

  1. Guinness

    Thread Starter Member

    Dec 31, 2009
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    Hi,

    I have a little bit of experience with electronics, have been studying it for 6 months or so and have built some simpole circuits, 555 timers and transister switching circuits and stuff. But I want to make my own power supply.
    I have decided on a linear rather than a switch mode, as I like to take things one step at a time and try not to jump ahead of myself.

    I know how to make the basic transformer, bridge rectifier, smooothing cap part. I also know how to make a current limiter and over voltage protection circuit.

    What I want is a variable 0-30v 5A power supply, the problems I have are...

    1: I don't know how th make the voltage variable from 0-30v.

    2: Is it possible instead of using 1 big transformer, to use 2 smaller ones. Say one to supply the 30v at 1mA, and the second to supply say 1v up to 5A. Then combine the 2 at the output?

    3: I would also like to display the voltage and amps on a lcd display, I have looked into microcontrollers a little, and feel confident that I could use it. But need a source of information ( a link or something ) where I could get more info for this particular task.

    4: Anything else you could add would be nice as im always up for learning better ways of doing things.

    Thanks.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    On another website there is a power supply project that is also a kit but it doesn't work properly and many of its parts are overloaded.
    We fixed it. It has a 0V to 30V at 3mA to 3A output which can easily be made up to 5A.
    It has adjustable voltage regulation and adjustable current regulation so it is fairly complicated.
    Here is its schematic:
     
  3. Paulo540

    Member

    Nov 23, 2009
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    Is there a parts list, guru?
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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  5. Guinness

    Thread Starter Member

    Dec 31, 2009
    81
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    Thanks for the info, I will start gathering the bits to build it. Hopefully by the time I finish building it, I will have figured out what each part does.

    I didn't realise they had so much in them, will be a good learning curve for me, just really glad I didnt start with switch mode as I hear they are way more complicated.

    Audioguru: The main thing I was wondering, is you said it should be easy to modify it to 5A from 3A, could you tell me which parts I would need to change to do that please.

    Thanks to Jony130 for the web link aswell, I will stick with the more complicated one cheers. Will keep me busy for longer.
     
    Last edited: Jan 9, 2010
  6. Wendy

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    Mar 24, 2008
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  7. t06afre

    AAC Fanatic!

    May 11, 2009
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    Since you are a beginner why not take a look at lm723 data sheet. Here you will find many design ideas. The lm723 also have a current limiting option. With lm723 you will be able to go down to +2 volt. The lm723cn is a very cheap circuit
     
  8. Guinness

    Thread Starter Member

    Dec 31, 2009
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    t06afre - I had a look at the lm723 like you suggested, it is a nice component. But with only a current limit of 150mA, it falls way short of what I require.

    At the moment, I am using fixed voltage regulators to supply my varies voltages, like the 7805, 7812 and such. But would like a proper power supply, that will last a while.

    Also would love to have a proper project to get my head around, as up till now I have just been doing little projects, which have been usefull to realise that real life stuff is slightly different from what I read ( Have the Art of Electronics book ).

    As a re-think I realise I could use the lm723 for the variable voltage and just up the current with transistors or similar. But that would be to easy I feel.

    Thanks for the info though, I never seen the lm723 before and will sure use it in future projects.

    Bill - Cheers for the info, will look into them for in the future aswell.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    The power supply project at Electronics-Lab that was linked by Jony is the kit that does not work properly and many of its parts are overloaded. My schematic is the fixes for it.

    1) The transformer is 118VA for 3ADC output and is 198VA for 5ADC output.
    2) R7 is 0.47 ohms/10W for 3A and is 0.27 ohms/10W for 5A.
    3) Q4 and Q5 are used with their 0.33 ohm/1W resistors for 3A and a third 2N3055 transistor with a 0.33 ohm resistor is added for 5A.

    Here is the parts list:
     
  10. t06afre

    AAC Fanatic!

    May 11, 2009
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    Not a problem. You just use an external NPN power transistor to boost the current. Se figure 4 in the data sheet ;)
     
  11. Guinness

    Thread Starter Member

    Dec 31, 2009
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    Oh yea I see there are a lot of changes now, silly me.

    Thats great, cheers for all the info, Only a few small changes to increase the current aswell.

    I cant believe how easy it was to gather the information needed to build it, and how fast!

    Thank you again, you have just saved me so much time, now I can have a working power supply and study each part to see what it does.

    Im guessing as I am increasing the total power available, I should also consider putting a few fans inside the enclosure to keep everything cool?
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    The heatsinks for the output transistors need to be huge and fans will help cool them.
     
  13. Guinness

    Thread Starter Member

    Dec 31, 2009
    81
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    Thats ok then, I have a nice veriety of heatsinks and fans.

    I do seem to be struggling to source a transformer thats 28vac 200v/a.

    I have managed to find some 28vac, but they are in the mA range.

    Have found lots of 24v and 30v.

    Don't suppose anyone knows of anywhere I could find one, I know a few companies that can make to order, but they are expensive.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    A 30V transformer is fine.
     
  15. Guinness

    Thread Starter Member

    Dec 31, 2009
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    Thats ok then, wasn't sure if I needed to change component values with the increase of voltage.

    One last question then about the transformer :)

    They are both 200v/a but...

    I have a choice between either a:

    2 x 30v 3.33A which I would put in parallel to double the current
    or
    2 x 15V 6.67A which would connect in series to double the voltage.

    Just so there is no confusion, im not talking about putting 2 transformers together, its just the transformers have 2 seperate secondary windings.

    Is there any benefits or disadvantages to which way I do it?
    They are nearly the same price, same manufacturer and same specs in regards to regulation type (7%) and size.
     
  16. t06afre

    AAC Fanatic!

    May 11, 2009
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    Just a tip. Transformers are labeled with RMS value. So after the rectifier the filter cap will charge up to approx 1.4 times the labeled output value.
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    I'd go with this one:
    That way you could select whether to use just one of the secondaries if you were going to be producing low voltage output; no sense in heating up the room if you don't need to.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    A 28V transformer must be 200VA for a 5ADC output from the project.
    A 30V transformer should be 212VA but a 200VA one will not get too hot.
     
  19. Guinness

    Thread Starter Member

    Dec 31, 2009
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    Ok, that makes sence, would have a lot more scope for veriaty in future.

    Could build in low power 12v and 5v fixed outputs from the individual windings.

    Would make it easy to use multiple voltages at the same time.

    Cheers
     
  20. t06afre

    AAC Fanatic!

    May 11, 2009
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    A future I like with power supplies is to have a constant regulated 5 volt output also. A standard 7805 circuit will do fine. In your setup a separate transformer is needed. This because the 7805 can not have more than 35 volt as input voltage. The tip from SgtWooki to have a low and high voltage range was also very good.
     
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