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Voltage regulator question — Parallax Forums

Voltage regulator question

george miyagigeorge miyagi Posts: 48
edited 2005-07-11 03:54 in BASIC Stamp
Hi
I've noticed sometimes the voltage tegulator on my super carrier gets hot, and sometimes VERY hot. Does this mean i've got a bad connection somewhere?
tx

Comments

  • dandreaedandreae Posts: 1,375
    edited 2005-07-01 15:36
    If your regulator is getting so hot· that you can't touch it?··This means·you have a short or that you are drawing way more current than the regulator can handle.

    Dave

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    Dave Andreae

    Tech Support
    dandreae@parallax.com
    Http://www.parallax.com

    ·
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-07-01 16:13
    It also depends on the wall-wart you're using to drive the board. If you're driving the thing at 25 volts, or even 12 volts, it can get warm.

    Oh, and note that the 7805 (or 3840-5) does have·thermal shutdown protection -- so if it really gets too warm for the chip, it will turn off for a while.· If you're really concerned, you can add a TO-220 heat-sink from Radio-shack which should help a lot.
    ·
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2005-07-05 10:32
    Just wondering...

    Is Japan is a 220-240volt country?

    Maybe you are using a 110-120volt rated wall wart·might be outputing double the voltage due to double the input.



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    G. Herzog in Taiwan
  • OakGraphicsOakGraphics Posts: 202
    edited 2005-07-05 10:45
    Since we are on the voltage regulator subject - I was wondering about improper use of a 7805.
    I have an input voltage of 24VDC that is a 8.9watt power supply. I want to use a 7805 to give me 5vdc - 1 amp or less to run a project. I am assuming I am stressing out the 7805 by attaching it to such a large watt power supply. What would be my solution to bring 5volt from the 24vdc I have available? I am assuming I would simply put a high watt rating resistor to help tame some of the current. Or is there a beefier 7805 that I should consider? I think when I first tried using the 7805 it went into shutdown pretty much instantly. Of course I can just use a wall wart 9volt 1Amp along with my project, but thought 'might as well use the supplied current' if possible. (shrug)
  • nick bernardnick bernard Posts: 329
    edited 2005-07-05 13:30
    i had a simular question and G
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-07-05 15:11
    OakGraphics said...(trimmed)
    I am assuming I am stressing out the 7805 by attaching it to such a large watt power supply.· I think when I first tried using the 7805 it went into shutdown pretty much instantly.
    Hello,

    ··· Remember that, while the 7805 can handle an Vin of around 30 volts, the higher the voltage, the more the 7805 has to disaapate as heat to regulate it down to 5 volts.· It's not about wattage, per se.· But if it went into thermal shut down that quick, my guess is that either you did not have adequate heat sinking on the device, or that your supply is unregulated and actually is higher that 24 volts without a load on it.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • KenMKenM Posts: 657
    edited 2005-07-05 16:40
    Japan is 100v 50 Hz and 110v 60 Hz in some areas.
    Kramer said...

    Just wondering...

    Is Japan is a 220-240volt country?

    Maybe you are using a 110-120volt rated wall wart·might be outputing double the voltage due to double the input.



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    Ken
  • OakGraphicsOakGraphics Posts: 202
    edited 2005-07-05 18:27
    nick bernard said...
    i had a simular question and G
  • george miyagigeorge miyagi Posts: 48
    edited 2005-07-08 07:10
    Someone suggested to me putting two 7805s in parallel, and getting a power adaptor higher than the 1.2A one that came with my BS kit, to solve this issue.
    1. what's the theory behind this?
    2. and would this work?
    3. is the heat sink the metal 'casing' surrounding the 7805 on the SuperCarrier?
    tx
  • Bruce BatesBruce Bates Posts: 3,045
    edited 2005-07-08 08:07
    George -

    First, the 7805 is an obsolete part, so I'd recommend against using it. The voltage regulator on the Super Carrier Board I just recently purchased from Parallax has an LM2940 on it, just as a matter of information.

    The topic of parallelling voltage regulators has come up before, and as I remember, the general advice seems to be against doing so. Equally balancing the load between the two voltage regulators seems to be the problem. It would seem more appropriate to just replace the 1A voltage regulator with one of a higher current capacity, although one must be careful to evaluate the size of the heat sink in such as case, to make sure it is of adequate size to dissapate the heat which may be created by the higher capacity regulator.

    Alternatively, one could just build/buy a suitable regulated power supply or add a separate regulator and just tie the negative terminals together. This way the grounds are common, but the power provided is independent. Obviously the voltages should be the same, if a common voltage is desired.

    Regards,

    Bruce Bates
  • nick bernardnick bernard Posts: 329
    edited 2005-07-08 13:18
    why couldnt you put 2 diodes on each of the regulators to form a single bus?

    +5 ----|>---|
    +~4.5V
    +5 ----|>---|

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    If I lived back in the wild west days, instead of carrying a six-gun in my holster, I'd carry a soldering iron. That way, if some smart-aleck cowboy said something like "Hey, look. He's carrying a soldering iron!" and started laughing, and everybody else started laughing, I could just say, "That's right, it's a soldering iron. The soldering iron of justice." Then everybody would get real quiet and ashamed, because they had made fun of the soldering iron of justice, and I could probably hit them up for a free drink. - Jack Handy
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-07-08 13:33
    I think there's an app note somewhere which can tell you how to properly use two 7805's in parallel. If you use them improperly (like Bean's suggestion above) then the one with the slightly higher voltage setting will supply all the current (until it shuts down from thermal overload). With Bean's diode above, the second one will then supply all the current until it shuts down.

    It turns out, where you plug in the feedback line is critically important.

    Also, "obsolete" does not describe the 7805 -- there's LOTS of them out there, and I assume they're still being manufactured.· The 2940 is a "low drop-out regulator", so it can work with lower input voltages before it stops regulating, which is awfully nice when you're using a battery.
  • nick bernardnick bernard Posts: 329
    edited 2005-07-08 13:39
    Bean?

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    If I lived back in the wild west days, instead of carrying a six-gun in my holster, I'd carry a soldering iron. That way, if some smart-aleck cowboy said something like "Hey, look. He's carrying a soldering iron!" and started laughing, and everybody else started laughing, I could just say, "That's right, it's a soldering iron. The soldering iron of justice." Then everybody would get real quiet and ashamed, because they had made fun of the soldering iron of justice, and I could probably hit them up for a free drink. - Jack Handy
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-07-08 14:15
    Hello,

    ·· I agree that the 7805 is not obsolete.· I also agree that it should not be used in parallel with another one.· There are higher-capacity devices available.· We have such a regulator on our Professional Development Board.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Bruce BatesBruce Bates Posts: 3,045
    edited 2005-07-08 14:33
    Gents -

    Perhaps "obsolete" is too strong a word, but it's certainly a discontinued part, as you can read quite clearly here, from what I believe is the original manufacturer, or one of them at least:
    http://www.national.com/pf/LM/LM7805C.html

    Regards,

    Bruce Bates
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-07-08 14:46
    Okay,

    ·· Perhaps this has gotten a little off the beaten path.· The 7805 is probably right in there with the 555 timer.· Many manufacturers has discontinued the part, and many hobbyists continue to use it.· The 7805, like the 555 is not hard to find currently.· Until it is, it could still be easily used.

    ·· Now, there are better regulators.· There are some nice switching regulators in TO-220 from what I have seen on these forums.· But the main point was to use the right part for the job.· And if you have to use 2 of a part in parallel to do somthing, it's probably not the right part.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Ryan ClarkeRyan Clarke Posts: 738
    edited 2005-07-08 18:08
    The fact that you can still get a 7805 from Digikey, Radioshack, etc. renders it very much not obsolete to me. [noparse]:)[/noparse]

    And with resistors every 7805 is also a variable output voltage regulator [noparse];)[/noparse] (all voltage regulators are adjustable!)

    Chris has a good point about the 555. I still use those too!

    Ryan
  • george miyagigeorge miyagi Posts: 48
    edited 2005-07-11 03:31
    would something like this work?
    powersupply.jpg
    700 x 500 - 21K
  • Beau SchwabeBeau Schwabe Posts: 6,557
    edited 2005-07-11 03:54
    If you look at the specs of any flavor of regulator, there is usually a circuit "modification" for a high current version of the regulator with an external transistor.

    See...

    http://pdf.alldatasheet.com/datasheet-pdf/view/82833/FAIRCHILD/LM7805.html

    ...on page 23 figure 12

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.

    Post Edited (Beau Schwabe (Parallax)) : 7/11/2005 3:58:51 AM GMT
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