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Just wondering, why are LM78XX & LM79XX series still made when LM317T and LM337T are available.

Discussion in 'General Electronics Chat' started by Ed Bray, Mar 14, 2017.

  1. Ed Bray

    Thread Starter Member

    Feb 1, 2017
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    Pretty much as the title.

    Why would you buy all of the LM78xx and LM79xx series when you can pretty much cover all the options with a LM317T and a LM337T. Are they not a one size fits all?

    With the correct selection of components they can act as fixed voltage regulators and their range is not restricted to a specific value so should they be considered as useful stock items which means only carrying a pair of ICs rather than a multitude.

    Just wondering.
     
  2. Ramussons

    Well-Known Member

    May 3, 2013
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    You need 2 additional resistors and need to trim at least 1 to get the required voltage. 78xx and 79xx give you that voltage straightaway.
     
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  3. Bordodynov

    Well-Known Member

    May 20, 2015
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    It is more convenient to have a stabilizer with a fixed voltage. Do not need two precision resistors. You need less space on the board. There are more advanced microcircuits of stabilizers.
     
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  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The LT78XX/79XX series has no minimum load requirement. 10 mA is wasted on the 317/337.
     
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  5. Alec_t

    Expert

    Sep 17, 2013
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    Equipment repairs are simpler if the exact replacement IC is available.
     
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  6. OBW0549

    Distinguished Member

    Mar 2, 2015
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    I think neither the LM78xx/79xx nor the LM317/337 are used much in new designs, as they all have significant dropout voltages (1-2 volts for most parts) as well as being wasteful of current: the LM117/337 have a minimum load current (5-10 mA), and the LM78xx/79xx have quiescent (i.e., no-load) currents of around 5-8 mA. Low power and micropower low-dropout (LDO) regulators are often preferred for applications in which the entire system may draw only a few milliamps.
     
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  7. Reloadron

    Distinguished Member

    Jan 15, 2015
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    The LM78 and LM79 series are fixed regulators and the latter are adjustable.

    Ron
     
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  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Different strokes for different folks. And so on and so on and scooby dooby doo.

    There is no such thing as the best part or circuit. There are devices and schemes with more advantages than disadvantages over others in some particular situation.
     
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  9. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Actually, nearly no one would design a 78xx or 317 into any new design for production equipment. Both parts are essentially obsolete.

    Original manufacturers (and third parties) continue to produce parts for legacy designs as long as the volumes (and price) warrant it. Over time, these volumes will decrease and the price will necessarily rise. At some point, the cost of continuing to use the part will exceed the cost to redesign (or discontinue) the legacy product using lower cost modern parts.

    At that point, the 78xx and 317 parts will no longer be available from manufacturers -- and the supply will only be available from surplus parts vendors and ebay.
     
    Last edited: Mar 14, 2017
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  10. Ed Bray

    Thread Starter Member

    Feb 1, 2017
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    Thanks folks its very interesting, I was just looking at my IC Box and I have a load of various 78xx and 79xx as well as 317T and 337Ts, and obviously for a hobbyist like myself it takes up valuable space. I just wondered what are the advantages of the fixed regs versus the adj regs (when used as fixed).
     
  11. dl324

    AAC Fanatic!

    Mar 30, 2015
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    They can't possibly be taking up *that* much space.

    By the definitions given, I have many obsolete parts in my stock. I won't be getting rid of them any time soon because I also have many pieces of old test equipment that use those "obsolete" parts...
     
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  12. OBW0549

    Distinguished Member

    Mar 2, 2015
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    Don't throw out chips; get more space. :D
     
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  13. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Personally I can't remember the last time I used an adjustable regulator when I did not need adjustability, but I am a minimalist and always seek to use the very fewest lowest cost parts.

    Component cost/count trade-offs can have some surprising trade offs. Once when I needed a high side driver I discovered using a low dropout regulator with an enable input that was cheaper and less components than the single transistor (plus resistor) I had been using.
     
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  14. hp1729

    Well-Known Member

    Nov 23, 2015
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    LM317 - $0.39 plus resistors
    LM7805 - $0.29
     
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  15. Tonyr1084

    Distinguished Member

    Sep 24, 2015
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    Both series regulators waste a lot of current converted to heat. Buck converters are much better design considerations. If battery life is being considered - a buck converter is going to out perform those older regulators - of which I have many too. They just sit in my static foam.
     
  16. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Ah, but the fixed regulators "waste" around 10 mA-ish through the ground leg (old memory).

    ak
     
    Last edited: Mar 14, 2017
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  17. dendad

    Distinguished Member

    Feb 20, 2016
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    Unless you REALLY need a linear reg, (or an adjustable one), and some times you do, a "drop in " switcher is the way to go now i think. Yes, they are more expensive, but the overall power saving is worth while, and no heat sink is required. I was using the Recom regs, like R-785.0-0.5 and R-78HB12-0.5 for up to 72V in, but now there are a number of other suppliers, like Traco TSR 1-2450, and a few others. These are worth looking at. They can now be found for under $10 each.
    In a number of my products I use a switcher from 24V to 5V then a 3.3V Low Drop Out linear regulator for better noise specs.
     
    Last edited: Mar 14, 2017
  18. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Static current and minimum load current are not the same thing. With a 500 mA guaranteed load, the setting resistor current can be pretty low. The static current out of a 317 adjustment pin is only 50 uA typ., 100 uA max.

    ak
     
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  19. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Early in my career I designed a high performance low-noise deuterium lamp driver using a "floating" lm317hv as a current source.

    The performance was better than the typical opamp circuits of the day, and the ultimate performance was better (and far less costly) than the lamp manufacturer's reference design.

    [​IMG]
     
    Last edited: Mar 14, 2017
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  20. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Hey! That's my board!

    [​IMG]

    Circa 1994.
     
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