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Circuit layout

  1. Sep 27, 2013 #1
    As an ME major I am less than comfortable with circuit design, so I have been banging my head against a circuit I need to layout on breadboard for a couple hours now. Below is a link to a diagram of the desired circuit, as well as a picture of the breadboard setup I am using.
    Diagram and breadboard
    Thanks in advance!
     
  2. jcsd
  3. Sep 27, 2013 #2

    berkeman

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    Welcome to the PF.

    What problems are you having? How much of the circuit have you wired up so far? The breadboard picture is too far away for me to tell how much is wired up...
     
  4. Sep 27, 2013 #3

    meBigGuy

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    The layout isn't critical. What sorts of currents are you handling? What is the input voltage? What are you doing exactly? You could lay it out so it looked nearly like the schematic, or rotate components to take less space. Shorter wires are generally better. Hard to say for sure without more info.

    When you go above 18V+scr trigger, you fire the scr and blow the fuse. Why have you chosen that action?
     
  5. Sep 28, 2013 #4
    Thanks for the replies, I'll try and get back to you all with some more details. I just got done with an exhausting day working on a home renovation so am turning in for the night, but wanted to let you know I'm still interested. Thanks again!
     
  6. Oct 1, 2013 #5
    I think there is some thing wrong with this schematic,

    As you can see when the thyristor is conducting, the + and - terminals are shorted!

    What is the purpose of the circuit that you are designing?
     
  7. Oct 2, 2013 #6

    meBigGuy

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    As I said
     
  8. Oct 2, 2013 #7

    Baluncore

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    The circuit is called an over-voltage crowbar. It is designed to protect powered circuits from excessively high voltages on the supply rail, should the power supply fail.
     
  9. Oct 2, 2013 #8

    meBigGuy

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    Obviously it will crowbar. Is that really what he intended? I was wondering why he chose that. Maybe he has good reasons. Just asking.
     
  10. Oct 3, 2013 #9
    Sorry its taken a while to get back to you all. With some help from others I have put together what I think is a serviceable, if ugly, layout. Here is a picture of it here: http://i.imgur.com/DDol18I.jpg

    As to the function of the circuit, it is meant to take an input of 12V that is constantly turning on and off and output a constant voltage, as well as protect the device on the output side from reverse polarity, etc.

    Can I get a check from you all on the way I've laid out the circuit?

    edit: Here is useful pic for my nomenclature: http://i.imgur.com/HUWIhU3.jpg
     
  11. Oct 3, 2013 #10

    Averagesupernova

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    It is to protect equipment from over voltage. There really is no other reason to do it. It is not at all uncommon to do this. A good 12 volt power supply will have this on the output in case of regulator failure. If the over current protection still works then the supply will go into current limit. If the over current protection has been damaged as well then usually the fuse on the primary side of the transformer will blow provided it has the correct fuse installed.
    -
    Think about what can happen if this is in a power supply being used to float a battery in parallel with equipment. Not uncommon to do this for battery backup in communications systems.
     
  12. Oct 3, 2013 #11
    Is there a reason he isn't just using a MOV to do the crowbarring?
     
  13. Oct 3, 2013 #12

    Averagesupernova

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    Typically aren't MOVs designed for higher voltages? They are pretty fast and clamp the voltage off but do not short circuit like an SCR will. Once the SCR is fired it will stay on until the current is removed. Very little power will be dissipated in the SCR. To allow an MOV or zener type device to just clamp off at the preset voltage and pass max current will get cause it to dissipate lots and lots of heat compared to the SCR.
     
  14. Oct 3, 2013 #13

    Baluncore

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    A MOV has a wide voltage range and cannot be accurately adjusted. An SCR with a zenner diode is much more predictable.
     
  15. Oct 3, 2013 #14
    Interesting, thanks for the info.
     
  16. Oct 7, 2013 #15
    Any opinions?
     
  17. Oct 7, 2013 #16
    The connection is correct as per single line diagram, however you have to verify the connection to thyristor terminals with the data sheet of the thyristor (what is the thyristor number that you are using?).

    Regarding the function of the circuit, i have doubt about its functionality as you described. If you can explain more how it will provide constant voltage at the output.
    As per your single line diagram, this connection with zener diode, resistor and thyristor will make over voltage protection, so when the voltage increase above certain level, the SCR will conduct thus shorting the +ve and -ve terminals and the fuse will burn and opens the circuit, thus clearing the fault.
     
  18. Oct 7, 2013 #17

    meBigGuy

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    When the input voltage goes away the 1000uF will supply current and droop accordingly until the input comes back. Have you calculated and are you OK with the ripple voltage that will naturally be created?

    I would lay it out such that the ground for R1 C2 went to SCR1 rather than the long way around like you have it. Or move SCR1 closer to R1-C2
     
  19. Oct 8, 2013 #18
    I was not the one who actually designed this circuit, but was simply trying to lay it out. I plan on testing it with the end device today, so fingers crossed, hopefully everything will work according to plan.
    I took your advice on the layout meBigGuy; It looks a bit better now to me. http://imgur.com/aMwCtdL Thanks!
     
  20. Oct 11, 2013 #19
    I got the circuit up and running on Wed, and it is giving exactly what we want: a constant 12V at the output. Thanks for all the help. We'll see later today whether the end device functions properly.
     
  21. Oct 11, 2013 #20

    meBigGuy

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    The next improvement in layout would be for the fuse and SCR to be next to the diode and capacitor with the zener and stuff on the far side. Make the input to output ground and power as short and fat as possible. But what you have will work.
     
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