Dumbproofing a lab power supply? (Input/Output protection)

Discussion in 'General Electronics Chat' started by TheLaw, Jul 18, 2012.

  1. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    I'm building a power supply based off this schematic: http://www.extremecircuits.net/2010/06/fully-adjustable-power-supply_17.html

    Overall a nice schematic, but I'm not sure how it would hold up if I hooked something up the wrong way or applied a voltage to the output by accident.

    What do you guys think would be some good provisions to make? What type of protections and where should I put them?

    Thanks.
     
    Last edited: Jul 19, 2012
  2. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    Do I just put big diodes on the output? Voltage drop issue?
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Do you require output short circuit protection too? (i.e. crowbar circuit)

    Normally the output of a benchtop power supply allows either - or + referenced grounds.
    Basically hook the output up one way and you've got positive voltage, hook it the other and you've got a negative voltage.
     
  4. #12

    Expert

    Nov 30, 2010
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    My first impression is: just put a 10 amp, 100 volt Shottky on the output and adjust the minimum out voltage to correct for the voltage difference. Here's one for 60 cents.
     
  5. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    Yes, I would like to be able to generate either positive or negative voltgages based on how you hook up the test leads. So in that case, do I just use one Schottky in series with the + output? Perhaps a PTCC also? Or a real fuse. I've seen people use TVS diodes, but I'm not sure the purpose of those.

    Thank you.
     
    Last edited: Jul 19, 2012
  6. #12

    Expert

    Nov 30, 2010
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    Simply reversing the way you connect the output leads reverses the voltage, so, what's the problem? You would like to be able to do that but you aren't able to do that?

    One diode will control current direction, no matter which way you connect the leads. A fuse is quicker than a PTCC.
     
  7. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    Sorry. I'm having some theory breakdown. There's no issue in me reversing the leads. I was just agreeing with what was said.

    Anyway, I think I will install a Schottky.

    Is there anyway to protect the LM317/pass transistor. Sometimes in regulator datasheets you see a diode reverse biased across pins 1 and 3 of the regulator?

    Or would I be covered with a diode at the output?

    Sorry! Thanks
     
  8. #12

    Expert

    Nov 30, 2010
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    The diode across the 317 protects it from the output capacitor discharging backwards through the chip if the input suddenly changes from a positive voltage to grounded. You can add that diode to this circuit and do no harm.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    To avoid the voltage drop you can place the diode across the output terminals (cathode to plus side). That way it will short any reverse voltage you might accidentally apply to the outputs. You might also fuse the output if you want to limit the current from this reverse voltage.
     
    #12 likes this.
  10. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    Right that makes sense. That doesn't necessarily have to be Schottky then..or?

    Fuse the power supplies output? Or fuse in series with that diode?

    Not sure if I got you on that second part.

    I appreciate all the help. Thank you.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    It could be a standard silicon diode.

    You want to fuse the power supply output after the diode. If you fuse the diode then you would loss the protection when the fuse blew.
     
  12. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    Thank you. I think I'll be good now. Thanks.
     
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    If you have current limiting (I have often used another LM317 as a current limiter before the voltage regulator) then you don't have to worry about reverse connecting a battery or something to the output.

    A reverse connected voltage source will act just like a short circuit and all it will do is activate the current limiting.

    Your only issue is connecting a voltage higher than the PSU voltage (not reversed). If you use the diodes as shown on the LM317 datasheet they will protect from that situation.

    I still think there should be a sticky or tutorial on the forum about how to make a simple LM317 adjustable power supply. :)
     
  14. TheLaw

    Thread Starter Member

    Sep 2, 2010
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    Well a regular old LM317 is pretty simple. But some theory could help silly people like me to understand how they work and not have to ask silly questions. ;)

    Thanks.
     
  15. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    There's nothing silly about questions. :) It seems the forum has lots of people with LM317 questions of late, things like the diodes, dropout voltage, how to wire it up etc etc.
     
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