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protection circuit is diodes the best way?

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  1. 1st May 2012 06:30 PM #1
    porthill porthill is offline

    protection circuit is diodes the best way?

    All,
    I am getting back into electronic design after being out of if for a while. I have designed a voltage regulator circuit and it works well as designed, and is a power supply. about 24VDC and could pull as much as 5A. I need to be about to protect it from someone connecting the terminals in backwards. I have an LED that will come up when the system is working properly. Are diodes the best way?
  2. 1st May 2012 06:40 PM #2
    KMoffett KMoffett is offline
    Please post a schematic.

    Ken
    0
    "To invent, you need a good imagination and a pile of junk."
    Thomas A. Edison (1847 - 1931)
  3. 1st May 2012 06:58 PM Thread Starter #3
    porthill porthill is offline
    KMoffett,
    here is the schematic that is connected to the bus. There is some other circuits down stream from this on but I figured if I could protect this one then the rest will also be protected.
    Thanks for the help...
    Vreg.JPG
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  4. 1st May 2012 07:16 PM #4
    KMoffett KMoffett is offline
    A single diode in series with the (+) input would prevent damage due to reversal of input polarity. A low Vf Schottky diode would be the most efficient. There are active polarity-reversing protection circuits with almost no voltage drop, but for your boost circuit it would be a waste.

    Ken
    0
    "To invent, you need a good imagination and a pile of junk."
    Thomas A. Edison (1847 - 1931)
  5. 1st May 2012 09:32 PM #5
    MrAl MrAl is online now
    Quote Originally Posted by porthill View Post
    KMoffett,
    here is the schematic that is connected to the bus. There is some other circuits down stream from this on but I figured if I could protect this one then the rest will also be protected.
    Thanks for the help...
    Vreg.JPG

    Hi,

    You say you want to protect it, but you dont say whether or not you want to protect the input, output, or both. You show a diode on the output so i am guessing the output but you should specify anyway.
    0
    One test is worth a thousand expert opinions, but one expert specification is worth a thousand tests.
    If i miss something you posted or something you think is important, feel free to PM me.
  6. 1st May 2012 10:12 PM Thread Starter #6
    porthill porthill is offline
    My goal is to protect the circuit from a user error. The most likely one will be connecting the positive terminal of the battery to gnd of my circuit and the gnd terminal of the battery to the vin of my circuit. I am engineering controls into the installation instructions but they are not always followed as they should be and don't want to be replacing the board all the time.
    Thanks!
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    Last edited by porthill; 1st May 2012 at 10:37 PM.
  7. 1st May 2012 10:17 PM Thread Starter #7
    porthill porthill is offline
    I tested a shottky diode in this configuration and it did not protect my circuit when I reverced my input terminals.
    Capture1.JPG
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    Last edited by porthill; 1st May 2012 at 10:22 PM.
  8. 1st May 2012 11:08 PM #8
    RCinFLA RCinFLA is offline
    Just about all IC's will take a negative supply of 0.7 vdc. You can use a large amperage rated diode but you need an input fuse that will blow when the diode conducts due to reverse polarity. The diode amperage rating needs to be higher then the fuse amperage.

    Most DC-AC inverters use this approach to protect against reverse polarity connection.
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  9. 2nd May 2012 12:40 AM #9
    KMoffett KMoffett is offline
    Quote Originally Posted by porthill View Post
    I tested a shottky diode in this configuration and it did not protect my circuit when I reverced my input terminals.
    Capture1.JPG
    So what happened when it did "not protect my circuit "?

    I had suggested that you place the diode in series with the (+) input, not in series with the ground. With the diode in the ground input, the output ground will be ~0.3V more positive than the input ground. This will be a problem if any of the load circuits needs to have a common ground with the 24V input.

    Ken
    0
    "To invent, you need a good imagination and a pile of junk."
    Thomas A. Edison (1847 - 1931)
  10. 2nd May 2012 01:57 AM Thread Starter #10
    porthill porthill is offline
    Ken,
    what happened was that it fried the the vreg chip.
    I know that you said to put it series with the + input. I have not had a chance to test that. I am wondering how that will work to protect it? Do you think it needs a fuse? If so what would be the configuration. I really appreciate this I am trying to learn it has been almost 10 years sence I have done circuit design.
    Thanks!
    0
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