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Thread: Power supply help

  1. 5th November 2009 06:31 PM #1
    chris414
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    Default Power supply help

    Hi everyone. I'm trying to build myself a lab power supply - up til now I have been using a computer power supply with LM338 voltage regulator, and it has been fine for my purposes. What I'd like now though is:
    - greater voltage range, 3V to say 35 or 40 volts
    - output current 2A or more (preferably more)
    - positive / negative outputs, with a "ground" between the two (would like to be able to change the ratio between teh ground the positive/negative outputs)

    I've been looking at a bunch of IC's that do this kind of thing, but am getting quite confused by it all My current plan is to keep the ATX power supply (it produces 24V max, with 7A maximum current - forget where i read that, correct me if i'm wrong) and use a DC converter to either step the voltage up or down depending on what is required (90% of the time i'll be using a voltage under 12V).

    Is something like the LM5118 on the right track? Page 6 of the LM137 datasheet has a sample circuit of an adjustable lab voltage regulator which i like the look of. I was thinking maybe combining the two, ie. using the LM5118 to produce a large voltage, and using the LM137/LM337 combination to create the positive / negative / ground outputs.

    Any thoughts? Sorry, this is all unchartered territory for me
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  2. 5th November 2009 06:40 PM #2
    bountyhunter
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    Trying to build an 80W power supply that must both boost and buck (go above and below the input voltage) makes your job infinitely more difficult.

    What I would do is buy a decent 36V/3A power transformer and do the basic bridge rectifier/huge cap thing to make the unregulated voltage of about 50V ballpark. Then use a simple switcher 3A buck converter to step it down.

    having said that, you can probably buy a small 80W lab supply pretty cheap.
    Last edited by bountyhunter; 5th November 2009 at 06:41 PM.
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  3. 5th November 2009 11:58 PM #3
    k7elp60
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    I have built a lot of power supplys, most of them variable in voltage. I have found that using 3 terminal regulators such as the LM338 is not suitable for a large range in variable voltages because the internal current limit and in out differential sensors will shut it down no matter how big the heat sink is. I have use LM723's with external pass transistors with great sucess. I have used them to drive external Mosfets with equal sucess.
    The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
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  4. 6th November 2009 12:01 AM #4
    bountyhunter
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    Here's a really nice 1-36V/3A supply with digital readout that lists new for $139.

    Model 1550 - Switching DC Power Supply with USB Charger Output 1-36V, 0-3A - B&K Precision

    When you see how much time and grief it is to build one, you will see what a bargain that is. It also has full protection built in.
    Last edited by bountyhunter; 6th November 2009 at 12:03 AM.
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  5. 6th November 2009 12:06 AM #5
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    Quote Originally Posted by k7elp60 View Post
    using 3 terminal regulators such as the LM338 is not suitable for a large range in variable voltages because the internal current limit and in out differential sensors will shut it down no matter how big the heat sink is.
    Correct. They all have built in SOA (safe operating area) protection on the power transistor so the load current limits lower and lower as the voltage drop across the power transistor increases. For example: an LM317 is good for close to 2A (typical) with about 5V across it, but only about 1/2A if there is 25V across it.
    Last edited by bountyhunter; 6th November 2009 at 12:07 AM.
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  6. 6th November 2009 12:37 AM #6
    k7elp60
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    Here are some pictures and the schematic of a dual power supply that has two ranges 6-14V @ 8A and 12-28V @ 4A. I could of went to near 0V, but I decided it wasn't necessary.
    I used 4 transformers in parallel to supply the LV for the rectifier. At one time I had a large quantity of the transformers.
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    Last edited by k7elp60; 6th November 2009 at 12:42 AM. Reason: addition
    The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
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