I'm building an adjustable power supply

Discussion in 'Power Electronics' started by Mohamad Tarabah, Jul 4, 2016.

  1. Mohamad Tarabah

    Thread Starter Member

    Jul 4, 2016
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    Hi,
    I was looking into internet for tutorials about how to make adjustable power supplies at home DIY. Unfortunately all of the videos did not help me. So I got an idea, what about connecting a dimmer switch to a rewired MOT, and then adding a full bridge rectifier ? Will I get an adjustable DC on the output if I adjust the dimmer ? ( PS: The rewired MOT is about 20 rounds so it should give ~20v max on the output ) And do you have any idea about an adjustable current limiter that's easy to do
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Yes you will get an adjustable DC output, but it will tend to be noisy, and have poor voltage regulation with a change in load.
    You'd likely be better off using a linear regulator to control the voltage.
    Is that 20Vac?
    What's the maximum current output that you want and the adjustment range of the current-limit?
     
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  3. dannyf

    Well-Known Member

    Sep 13, 2015
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    Not sure about MOT but LTR should work here.
     
  4. Mohamad Tarabah

    Thread Starter Member

    Jul 4, 2016
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    Yes it's 20Vac, but I don't need it so I'll directly converted to DC
    For the current limiting, I need something regulated up to 5amp to not burn things up, but if I need more I'll add a switch to isolate the limiter and get the pure MOT output, so I can get up to 100 amps. So I'll just need a regulated limiter of ~5A
    I'm not using an LM317 because of the current limit, it won't give me enough power, even though it's nicely smooth
     
  5. crutschow

    Expert

    Mar 14, 2008
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    You can boost the output current of an LM317 with an added power transistor to get 5A, or use a different regulator, such as an LM338, that's capable of 5A.
    But note that 5A can generate lot of heat in the regulator (over 100W for a short-circuit) so you will need a large (and likely fan-cooled heat sink.
     
  6. Mohamad Tarabah

    Thread Starter Member

    Jul 4, 2016
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    Nice idea, can you plz show me a circuit
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Of which device?
     
  8. Mohamad Tarabah

    Thread Starter Member

    Jul 4, 2016
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    For the current limiter
     
  9. crutschow

    Expert

    Mar 14, 2008
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    What is the range of current limit that you want?
     
  10. Mohamad Tarabah

    Thread Starter Member

    Jul 4, 2016
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    5 amps
     
  11. crutschow

    Expert

    Mar 14, 2008
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    So you want a fixed limit of 5A?
    What voltage regulator are you planning on using?
     
  12. Mohamad Tarabah

    Thread Starter Member

    Jul 4, 2016
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    Yes I want a limit of 5A, what regulator you think can fit well (it should be popular so I can find it in nearby electronics stores)
     
  13. crutschow

    Expert

    Mar 14, 2008
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    Since you didn't post where you live I can't know what's available in a "nearby electronics store".
     
  14. Mohamad Tarabah

    Thread Starter Member

    Jul 4, 2016
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    Never mind, just tell me and I'll look for it if it's not available I'll tell you back, thanks for your help
     
  15. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Sorry to say this, Mohamad, but if you need to ask these basic questions (and you have the right to do so!) I wonder if that maximum limit of 100 Amperes is realistic from your side.

    What do you intend to use the power supply for? How much power that "dimmer switch" can control?

    BTW, you could burnt certain components with maybe just 1A.
     
  16. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello,

    For a simple 5 amp supply use your transformer with a bridge rectifier, then a filter cap. Regulate that with a high current linear regulator like the LM317 but handles more current directly.
    For example, the LT1083 chip which handles current up to 7.5 amps.

    That's the simplest you can get.
     
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  17. Mohamad Tarabah

    Thread Starter Member

    Jul 4, 2016
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    It is realistic, i'm using an MOT, it can melt metal on 2 volts, watch youtube
    I'm using a dimmer switch that can handle ~10 amps, to not burn when I connect the transformer
     
  18. Mohamad Tarabah

    Thread Starter Member

    Jul 4, 2016
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    I'll just stick to the LM833, because 7.5 amps may burn my stuff in case I made mistakes. So I'll just stay on 5 amps
     
  19. crutschow

    Expert

    Mar 14, 2008
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    You mean the LM338?
    If so, I suggest you mount it on a CPU type fan cooled heat sink to minimize the chance of overheating.
    You may need to add a small (LM317) regulator for the fan power.
     
  20. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    If you are worried about the current then you should install a current limiter. The LM338 can go over 5 amps too for example. A current limit circuit would be a transistor and a couple resistors in addition to the LM or LT device.
     
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