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Hot regulators

FlyingFishFingerFlyingFishFinger Posts: 461
edited 2005-08-26 18:59 in BASIC Stamp
Hello!
My 5V 7805 regulator is getting very hot even though it's just powering the Stamp (that works). It's regulating from a large (7 Ah) 12V SLA. My 6V regulator for servos does the same thing, but they have a high current draw, so that may be explained.
does anyone know if this is normal for large batteries or if something is wrong?
Rafael

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-08-26 16:00
    Hello,

    ·· The simplest way to explain the heat is this...The higher the voltage going into the regulator, the more heat the regulator will expend regulating the voltage down to 5V.· Current draw affects it as well on an almost equal curve, but again, the 12V input will definately make a 7805 warm (toasty).· A heatsink (Even a clip-on) will usually keep it within check until you start drawing excessive current.



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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-08-26 16:02
    A linear regular such as the 7805 does regulation by dumping excess voltage, it dumps this voltage as heat. Supplying it with 12V means your dumping more voltage than the regulator is supplying. To combat the excess heat, you may want to do double regulation by first regulating the 12V down to 9V which is then fed to the 5V and 6V regulators. Or you can use a switching regulator which is more efficient (meaning your battery will last longer) and creates the voltage by rapidly switching the 12V so the average voltage is near your target voltage. Also I would use an DMM to check the current flowing from the 7805 to the stamp to make sure the expected currrent is drawn, if it is higher than expected there may be a wiring problem.

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  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2005-08-26 16:12
    OK, thanks.
    For the 6V regulator, the servos under load may be drawing several amps of current for a few seconds at a time, so I don't think double regulators would do the trick. The regulator is the tin-can bolt-down 7806 type. Can this handle the current?
    Rafael
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-08-26 16:15
    Paul,

    ·· I agree with your assessment on the loss in the regulator, however I respectfully disagree on daisy chaining two regulators to reduce heat.· For one you're adding more loss to the system.· In all honesty I don't remember back that far, but I was once given what seemed like a really good explanation on why not to do this by a fellow engineer at the time (Who was way smarter than I).· But to that end I have never seen a schematic employing such a scheme, probably for the reason lost to my decaying memory...· =)

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2005-08-26 16:16
    And what kind of switching regulators might do the job and where could I get them?
    Rafael
  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-08-26 16:32
    Chris, I know some of the newer regulators can have oscillation problems when daisy chaining since the input capacitor of the second stage is the output capoacitor of the second stage and each has thier own desired value and capacitor behaviour. I thought the older ones would be a little more forgiving, but I have not tried this myself. So it is probably better not to daisy chain as a general principle.

    Rafeal, Im in my lunch break now, Ill hop onto digikey and see whats availible in switching regulators.

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  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-08-26 16:38
    Paul,

    ·· I would rather have had a leg to stand on when posting this, and I asked another engineer just now about it, and he also thinks that is a design no-no, but doesn't recall why either.· I will find the information, but I wanted to save any possible problems related to the design.

    [noparse][[/noparse]EDIT] - Okay, it was as I suspected, the original issues I was warned about were related to transients causing oscillations/feedback problems.· Usable design requirements would include using the right capacitors such that the ESR (series resistance) prevents this.· That would require the person designing/building such a system to understand these concepts.· Myself I avoid it, but if it's to be suggested, then I would recommend a complete solution rather than concept, just because of design specifics.· =)· (Whew!)


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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com


    Post Edited (Chris Savage (Parallax)) : 8/26/2005 4:45:16 PM GMT
  • NewzedNewzed Posts: 2,503
    edited 2005-08-26 16:42
    Rafael, instead of fighting the regulator problem, wht not just use a Darlington transistor and a relay to switch the power.· The 5 volt JS1 relay, an SPDT, will handle 5 amps.· Very inexpensive relay.

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    Sid Weaver
    Do you have a Stamp Tester yet?
    http://hometown.aol.com/newzed/index.html

    ·
  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-08-26 16:56
    Ok here are a few:

    LM2574·5V, 500mA $1.86
    LM2575 5V, 1A $2.79
    LM2576 5V, 3A $2.80

    Similarly there is the LM series 259(4,5,6) whichave current limiting and thermal protection, they are a couple dollars more.

    these are just a few, to look at the entire selection, goto www.digikey.com search "switching regulator", click 5V in voltage dialog, click apply filter, then click "View Page". Look for the ones that say step down either in the description or in the features.


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  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-08-26 17:49
    Paul,

    ·· Thanks for the information...I wasn't aware there was a 3A Switcher in that small of a package.· I wonder if there is a TO-220/5 package available in something similar?· I seem to recall Dr. Tracy Allen posting once about a TO-220 switcher...Although as I said, memory is failing these days and I could be wrong.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Beau SchwabeBeau Schwabe Posts: 6,557
    edited 2005-08-26 17:51
    Chris,
    ·
    About the only way I can think of to run two regulators together is in a Cascaded style or to make use of a 78xx and a 79xx regulator.
    ·
    I wouldn't trust the Cascaded method outside of the 5V supply reference to ground, because the input voltage (12V) can effect the
    other outputs (6V and 11V).· If you are looking for a "ball park" voltage reference this would be ok I guess.
    ·
    Using the not so popular 79xx regulator with a 78xx regulator would provide a more stable output voltage, however your GND reference
    is lost and must "float".· Other circuits that reference GND MUST use the second GND (GND2).
    ·



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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
    CASCODE.GIF
    1073 x 937 - 24K
    POSNEG.GIF
    1293 x 975 - 29K
  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-08-26 18:13
    Chris Savage (Parallax) said...

    Paul,

    ·· Thanks for the information...I wasn't aware there was a 3A Switcher in that small of a package.· I wonder if there is a TO-220/5 package available in something similar?· I seem to recall Dr. Tracy Allen posting once about a TO-220 switcher...Although as I said, memory is failing these days and I could be wrong.

    Ayup there are a few:

    5V, 3A, TO-220/5

    http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=181260&Row=344294&Site=US
    http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=181260&Row=344446&Site=US
    http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=181260&Row=344601&Site=US
    http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=181260&Row=345079&Site=US

    5V, 3A, TO-220/7

    http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=181260&Row=344490&Site=US
    http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=181260&Row=344662&Site=US
    http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=181260&Row=345059&Site=US
    http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=181260&Row=345107&Site=US

    cheapest of the lot is the 4th and 5th link of the TO-220/5 for $2.80 and $2.04 respectively, the rest are > $4.

    gotta love digikey's parametric searching, makes limiting down >2k possible regulators to choose from down to a managable number.

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    Post Edited (Paul Baker) : 8/26/2005 6:15:41 PM GMT
  • Robert KubichekRobert Kubichek Posts: 343
    edited 2005-08-26 18:17
    Try here, it is a TO-220 form 2 amp max switcher from TI.

    focus.ti.com/docs/prod/folders/print/pt78ht205.html

    The pt78ht105 is a 1.5 amp switcher

    Bob N9LVU scool.gif
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-08-26 18:59
    Paul,

    ·· A fountain of information!· =)

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
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