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LM7805 Overheats VERY quickly, unloaded.. HELP!

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  • Shawn W. McClintock
    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
    Message 1 of 6 , Mar 2, 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.
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    • Dangermouse
      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
      Message 2 of 6 , Mar 2, 2002
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        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

        ---
        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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      • 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 3 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 4 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
             


            To unsubscribe from this group, send an email to:
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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 5 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

              ---
              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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              To unsubscribe from this group, send an email to:
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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 6 of 6 , Mar 3, 2002
              • 0 Attachment
                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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