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What happens when you exceed the current rating on a voltage regulator? — Parallax Forums

What happens when you exceed the current rating on a voltage regulator?

Martin_HMartin_H Posts: 4,051
edited 2012-05-20 16:54 in General Discussion
I have a 5 volt regulator in the TO-92 case style which has a 150 ma current rating. While I don't plan to draw more than that, what happens when you do? I'm hoping a graceful failure or the device getting hot rather than instant magic smoke dispersal.

Comments

  • ercoerco Posts: 20,259
    edited 2012-05-17 10:21
    The old standby 7805 linear series claims to have thermal shutdown protection, but they can get plenty hot in normal use. Which regulator RU using?
  • Mike GreenMike Green Posts: 23,101
    edited 2012-05-17 10:59
    All single chip voltage regulators have overcurrent protection and thermal shutdown. Their datasheets usually provide graphs showing how the device responds to heat. Usually, they can get quite hot to the touch at the point where the shutdown process kicks in. Some regulators are not protected against reverse voltages (although many are) and all regulators will let the magic smoke out (and maybe some flames too) if subjected to high enough voltage spikes with enough energy available. Believe the datasheet when it says "Absolute Maximum".
  • Martin_HMartin_H Posts: 4,051
    edited 2012-05-17 11:20
    OK thanks.

    I am using the TL750L05CLP from TI and I found the data sheet http://www.ti.com/lit/ds/symlink/tl750l05.pdf and it does have over current thermal protection. I plan on putting 9 volts in from a battery, so I won't go over voltage. The microcontroller definitely won't go over the current budget, but I will be connecting a few sensors (IR LED wheel encoders, a Ping, and an IR line following module). Which is why I was concerned. My servos will have a separate power supply, so no worries there.
  • RobotWorkshopRobotWorkshop Posts: 2,307
    edited 2012-05-17 11:29
    150mA seems pretty tiny when you start adding up all the extra's that you want to power from it. I'd prefer to have a little more of a safety margin on that part and not try to work it at the limit by default. There should be plenty of small 5V regulators that can easily put out up to 500mA which may be a better option.

    Robert
  • RDL2004RDL2004 Posts: 2,554
    edited 2012-05-17 12:10
    Usually, they can get quite hot to the touch at the point where the shutdown process kicks in.

    Absolutely. They always get way hotter than I like before the thermal shut down triggers.
  • tobdectobdec Posts: 267
    edited 2012-05-17 12:45
    I just bought a surplus of 5amp regulators to use on all my projects...that way I never have to worry no matter what im doing.
  • ercoerco Posts: 20,259
    edited 2012-05-17 16:46
    Q: What happens when you exceed the current rating on a voltage regulator?


    A: http://forums.parallax.com/attachment.php?attachmentid=92496&d=1336959728
  • Martin_HMartin_H Posts: 4,051
    edited 2012-05-17 19:38
    That is definitely a case of catastrophic magic desmoking.
  • Mike GreenMike Green Posts: 23,101
    edited 2012-05-17 20:07
    @tobdec - Remember that a 5A regulator will supply up to 5A into a short circuit (as long as the unregulated supply can provide that). Make sure to have a fast acting fuse in the circuit. You might have a large heatsink as well if you plan to actually use much over 1A. Even a 2V drop across a regulator at 5A will produce 10W of heat that has to be dissipated.

    @erco - Unfair. That's a picture of a transistor (or maybe half a transistor), not a regulator that has current and thermal limit protection.
  • tobdectobdec Posts: 267
    edited 2012-05-17 20:14
    Yup Mike ive been to that play before.....I use fuses on like everything!
  • ercoerco Posts: 20,259
    edited 2012-05-17 20:21
    Mike Green wrote: »
    @erco - Unfair. That's a picture of a transistor (or maybe half a transistor), not a regulator that has current and thermal limit protection.

    erco does not play fairly with others...
  • pik33pik33 Posts: 2,398
    edited 2012-05-19 02:06
    In the ancient times I accidentally powered an electronic circiit directly from 220V AC.

    These were 6 diodes and some transistors in this circuit. After this accident, all transistors have 2 legs only, Third legs disappeared in magical way.

    All 6 diodes exploded and emitted a lot of red light. These were not LEDs,

    A lot of magic smoke emerges...
  • Peter KG6LSEPeter KG6LSE Posts: 1,383
    edited 2012-05-19 18:56
    Yea a NPN .. a TIP120 . a VERY common device too..


    Peter
  • kwinnkwinn Posts: 8,697
    edited 2012-05-19 23:12
    Peter KG6LSE wrote: »
    Yea a NPN .. a TIP120 . a VERY common device too..


    Peter

    Which is often used in regulators so erco has a point. If the 3 terminal regulators did not have built in over current/temperature protection they might end up like that.
  • TtailspinTtailspin Posts: 1,326
    edited 2012-05-20 07:59
    No sir, that is a PNP TIP125... :smile:
  • Peter KG6LSEPeter KG6LSE Posts: 1,383
    edited 2012-05-20 16:54
    Ttailspin wrote: »
    No sir, that is a PNP TIP125... :smile:


    At least some one got my humor :)

    Martin desmoking Eh ..


    Worse I have seen is some one plugged a LM7805 in breadboard to a line cord and plugged it in .
    He had tantalum caps on the input and output that became very fast moving projectiles .
    What was there was in a cycle new forms of carbon and rare Gasses .. And one VERY shook up Freshman.



    Peter
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