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LM7812 Linear Regulator Failure

Anand Kumar90
Prodigy 120 points

Hi everyone,

    I have derived 12VDC from 28VDC using LM7812 linear voltage regulator. The circuit works fine for 2 months and after that it is failing. Output and GND are permanent short. If I replace the IC, again works for 2 -3 months and then again same failure. I just wanted to understand, what could be the potential root cause for Output and GND shot for a 7812 linear voltage regulator. I do not know, how internal architecture been configured?

Only a Quad Opamp is connected as load to the regulator, current will not consume more than 10 -20 mA. A 2.2uF capacitor is placed at input. Can anyone tell me root cause of failure ?

  • 0
    TI__Genius 13700 points
    Hello,

    The fact that the part is operating as expected for a number of months before it fails suggests to me that there is some other system issue in the system that simply takes a toll on the LM7812 over time.

    I have a few questions.

    Can you provide a schematic?
    Are there ESD concerns?
    Is the input a stable 28V or does it vary greatly?
    What is the typical environment that this part is in?
    Does the input to this part get turned off ever during operation?
    Which package are you using?
  • 0
    Prodigy 120 points

    Please share the full schematic, or at least exact schematic and parts used to generate the "28VDC" input. Is this 28VDC source actually output of an AC-to-DC rectifier?

    We use LM78XX series regulators in tens of thousands of devices. Failures occur extremely rarely; the root cause is often unexpected over-voltage at the DC input. Assuming AC rectification, the reverse voltage of input diodes can easily be exceeded when the external AC input encounters or a surge. The diodes effectively short and/or send much higher voltage than usual to the LM78XX regulator input.

    For example, an AC transformer connected to a battery backup outlet of a UPS will receive non-sinusoidal AC input during a power failure. I've observed the smoothed, pre-regulated DC level rise by about 50% under such a condition. Usually an extended event will blow the AC transformer fuse or PTC. At the right voltage conditions, the transformer will survive and the regulator will simply receive more voltage than it's rated. A simple solution here is to utilize diodes with larger reverse voltage (eliminating breakdown conditions).

    tl;dr - Carefully review input conditions and component choices. Input must be exceeding 28VDC at rare moments.