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Re: LM7805 Overheats VERY quickly, unloaded.. HELP!

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  • manifold_1
    A 7805 will not blow if the output is shorted; it will go into thermal shutdown. Something else is wrong. It sounds like the polarity is wrong or two of the
    Message 1 of 6 , Mar 2, 2002
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      A 7805 will not blow if the output is shorted; it will go into
      thermal shutdown. Something else is wrong. It sounds like the
      polarity is wrong or two of the pins are backwards.

      Double check your wiring and then take it off the heatsink. I am
      guessing you have a current path that is not obvious since it sounds
      like you have wired 7805's successfully in the past. Maybe the
      heatsink is connected to a potential that it shouldn't be connected
      to.

      Problems like this are the worst for me because I feel like I have
      lost my mind. It usually turns out to be something simple but not
      obvious. Tell us what the problem was when you find it. Good luck!

      --- In Electronics_101@y..., "Shawn W. McClintock" <kd6oji@a...>
      wrote:
      > Hey all,
      > I got a strange problem, I am unsure of what it is. If anyone
      can help I
      > would appreciate it.
      >
      > What is happening is, I am using a TO-220 packaged 7805 to deliver
      a stable
      > +5v for some TTL projects I am working on, I have wired hundreds of
      these
      > and never seen this, but the 7805 overheats within 5 seconds of
      plugging the
      > 9VDC @ 1200ma adaptor (wall wart) into the power jack, without a
      load on the
      > output of the regulator other than the LED indicator. I have double
      checked
      > my circuit, everything is wired properly. I thought maybe a bad
      7805, so I
      > replaced it.. three times.. still all of them overheat and cut off.
      so I
      > thought maybe the 9V wasnt enough (although it should be) so I
      plugged it
      > into a 12V battery (small gel cell) and the one in circuit at that
      time had
      > a very impressive failure (kablooey!!) all of the regulators are on
      a
      > heatsink and nothing shows shorted.. I made the circuit on perf-
      board the
      > first time, and even etched copper for another spectacular failure!
      >
      > Crude ASCII schematic follows:
      >
      > Pwr Jack (tip +) 560 Ohm Red
      > --O-------.-----[7805]--------.-----.-[R1]-.-[LED1]-------
      > ^ _ In | Out _ | |
      > | C1 ^ 3.3uF | 0.1uF ^ C2 -------X +5v Out |
      > |---------.-------------------.---------------------------
      > | GND
      > ------------------------X GND Out
      >
      >
      > the wal wart is very stable, and powers other circuits fine, some
      using
      > 7805's, with no problems.. The battery is fully charged, no
      problems keeping
      > an automotive bulb lit. I just dont get it!
      >
      > Shawn
      >
      > ---
      > Outgoing mail is certified Virus Free.
      > Checked by AVG anti-virus system (http://www.grisoft.com).
      > Version: 6.0.320 / Virus Database: 179 - Release Date: 1/30/2002
    • Hans
      All of this advice is good. 780X can oscillate and heat up if the input and output are not decoupling as per spec. sheet. Earlier 780x regulators requires
      Message 2 of 6 , Mar 2, 2002
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        All of this advice is good.
        780X can oscillate and heat up if the input and output are not decoupling as per spec. sheet.
        Earlier 780x regulators requires larger caps. than modern versions.
        Make sure you have at least a 1uF on input and output close (within 0.5" is OK ) of the pins.
        Make sure the common is ground using short thick connection.
        I remember buying a pre-packaged well known replacement brand 780x regulator, the kind where they print the connection on the package.... It was WRONG.
        Example:- 78L05 and 7805 have different connections. Looking at the "front" with legs pointing down,
        Left to Right
        T0220 7805  In Gnd Out
        T092  78L05  Out Gnd In
        hansw
        http://hans-w.com My web site for Projects, Code and More !

        manifold_1 wrote:

         A 7805 will not blow if the output is shorted; it will go into
        thermal shutdown.  Something else is wrong.  It sounds like the
        polarity is wrong or two of the pins are backwards.

        Double check your wiring and then take it off the heatsink.  I am
        guessing you have a current path that is not obvious since it sounds
        like you have wired 7805's successfully in the past.  Maybe the
        heatsink is connected to a potential that it shouldn't be connected
        to.

        Problems like this are the worst for me because I feel like I have
        lost my mind.  It usually turns out to be something simple but not
        obvious.  Tell us what the problem was when you find it.  Good luck!

        --- In Electronics_101@y..., "Shawn W. McClintock" <kd6oji@a...>
        wrote:
        > Hey all,
        >       I got a strange problem, I am unsure of what it is. If anyone
        can help I
        > would appreciate it.
        >
        > What is happening is, I am using a TO-220 packaged 7805 to deliver
        a stable
        > +5v for some TTL projects I am working on, I have wired hundreds of
        these
        > and never seen this, but the 7805 overheats within 5 seconds of
        plugging the
        > 9VDC @ 1200ma adaptor (wall wart) into the power jack, without a
        load on the
        > output of the regulator other than the LED indicator. I have double
        checked
        > my circuit, everything is wired properly. I thought maybe a bad
        7805, so I
        > replaced it.. three times.. still all of them overheat and cut off.
        so I
        > thought maybe the 9V wasnt enough (although it should be) so I
        plugged it
        > into a 12V battery (small gel cell) and the one in circuit at that
        time had
        > a very impressive failure (kablooey!!) all of the regulators are on
        a
        > heatsink and nothing shows shorted.. I made the circuit on perf-
        board the
        > first time, and even etched copper for another spectacular failure!
        >
        > Crude ASCII schematic follows:
        >
        > Pwr Jack (tip +)                     560 Ohm   Red
        > --O-------.-----[7805]--------.-----.-[R1]-.-[LED1]-------
        > ^         _   In   |  Out     _     |                     |
        > |      C1 ^ 3.3uF  |    0.1uF ^ C2  -------X +5v Out      |
        > |---------.-------------------.---------------------------
        >                    | GND
        >                    ------------------------X GND Out
        >
        >
        > the wal wart is very stable, and powers other circuits fine, some
        using
        > 7805's, with no problems.. The battery is fully charged, no
        problems keeping
        > an automotive bulb lit. I just dont get it!
        >
        > Shawn
        >
        > ---
        > Outgoing mail is certified Virus Free.
        > Checked by AVG anti-virus system (http://www.grisoft.com).
        > Version: 6.0.320 / Virus Database: 179 - Release Date: 1/30/2002
         


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      • Shawn W. McClintock
        Yep, that was it. Wal wart had enough AC to really goof the regulator.. I think the reason the one blew on the battery is because it was weak. Anyhow, adding a
        Message 3 of 6 , Mar 3, 2002
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          Yep, that was it. Wal wart had enough AC to really goof the regulator.. I think the reason the one blew on the battery is because it was weak. Anyhow, adding a bridge rectifier solved my issues.. sheesh, go fgure.. I just copied the circuit in a device I know to work with that adaptor and the new circuit worked just fine also..
           
          Thanks guys!
          Shawn
           
          -----Original Message-----
          From: Dangermouse [mailto:dangermouse@...]
          Sent: Saturday, March 02, 2002 8:17 PM
          To: Electronics_101@yahoogroups.com
          Subject: Re: [Electronics_101] LM7805 Overheats VERY quickly, unloaded.. HELP!

          I know you said there was nothing on the output but the LED, and that you had checked everything carefully, but you are describing the classic symptoms of an overloaded regulator chip--in other words, the only advice I can offer is that somewhere, in spite of all your testing, there is a short or near-short across the regulated output of the chip.
           
          One other possibility is that the wall-wart's own filter is failing and that the ripple is high enough to "short" across an capacitive load across the output that you've overlooked.
           
          I don't mean to imply that you don't know what you are doing, just that my own experience indicates this.
           
          Good luck.
           
          -DM
           
          ----- Original Message -----
          Sent: Saturday, March 02, 2002 8:28 PM
          Subject: [Electronics_101] LM7805 Overheats VERY quickly, unloaded.. HELP!

          Hey all,
                I got a strange problem, I am unsure of what it is. If anyone can help I
          would appreciate it.

          What is happening is, I am using a TO-220 packaged 7805 to deliver a stable
          +5v for some TTL projects I am working on, I have wired hundreds of these
          and never seen this, but the 7805 overheats within 5 seconds of plugging the
          9VDC @ 1200ma adaptor (wall wart) into the power jack, without a load on the
          output of the regulator other than the LED indicator. I have double checked
          my circuit, everything is wired properly. I thought maybe a bad 7805, so I
          replaced it.. three times.. still all of them overheat and cut off. so I
          thought maybe the 9V wasnt enough (although it should be) so I plugged it
          into a 12V battery (small gel cell) and the one in circuit at that time had
          a very impressive failure (kablooey!!) all of the regulators are on a
          heatsink and nothing shows shorted.. I made the circuit on perf-board the
          first time, and even etched copper for another spectacular failure!

          Crude ASCII schematic follows:

          Pwr Jack (tip +)                     560 Ohm   Red
          --O-------.-----[7805]--------.-----.-[R1]-.-[LED1]-------
          ^         _   In   |  Out     _     |                     |
          |      C1 ^ 3.3uF  |    0.1uF ^ C2  -------X +5v Out      |
          |---------.-------------------.---------------------------
                             | GND
                             ------------------------X GND Out


          the wal wart is very stable, and powers other circuits fine, some using
          7805's, with no problems.. The battery is fully charged, no problems keeping
          an automotive bulb lit. I just dont get it!

          Shawn

          ---
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          Checked by AVG anti-virus system (http://www.grisoft.com).
          Version: 6.0.320 / Virus Database: 179 - Release Date: 1/30/2002



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        • Minerva
          ... Hi, I would check your cap s.. Albert Hey all, I got a strange problem, I am unsure of what it is. If anyone can help I would appreciate it. What is
          Message 4 of 6 , Mar 3, 2002
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            Hey all,
                  I got a strange problem, I am unsure of what it is. If anyone can help I
            would appreciate it.

            What is happening is, I am using a TO-220 packaged 7805 to deliver a stable
            +5v for some TTL projects I am working on, I have wired hundreds of these
            and never seen this, but the 7805 overheats within 5 seconds of plugging the
            9VDC @ 1200ma adaptor (wall wart) into the power jack, without a load on the
            output of the regulator other than the LED indicator. I have double checked
            my circuit, everything is wired properly. I thought maybe a bad 7805, so I
            replaced it.. three times.. still all of them overheat and cut off. so I
            thought maybe the 9V wasnt enough (although it should be) so I plugged it
            into a 12V battery (small gel cell) and the one in circuit at that time had
            a very impressive failure (kablooey!!) all of the regulators are on a
            heatsink and nothing shows shorted.. I made the circuit on perf-board the
            first time, and even etched copper for another spectacular failure!

            Crude ASCII schematic follows:

            Pwr Jack (tip +)                     560 Ohm   Red
            --O-------.-----[7805]--------.-----.-[R1]-.-[LED1]-------
            ^         _   In   |  Out     _     |                     |
            |      C1 ^ 3.3uF  |    0.1uF ^ C2  -------X +5v Out      |
            |---------.-------------------.---------------------------
                               | GND
                               ------------------------X GND Out


            the wal wart is very stable, and powers other circuits fine, some using
            7805's, with no problems.. The battery is fully charged, no problems keeping
            an automotive bulb lit. I just dont get it!

            Shawn

            ---
            Outgoing mail is certified Virus Free.
            Checked by AVG anti-virus system (http://www.grisoft.com).
            Version: 6.0.320 / Virus Database: 179 - Release Date: 1/30/2002



            To unsubscribe from this group, send an email to:
            Electronics_101-unsubscribe@yahoogroups.com



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