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RE: [Electronics_101] LM7805 Overheats VERY quickly, unloaded.. HELP!

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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 1 of 6 , Mar 3, 2002
    • 0 Attachment
      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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    • 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 2 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



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