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7.8mA

  • firethorn Tuesday, February 4, 2014 5:41 PM Reply
    7.8mA seems pretty pointless?
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  • sheepish Top 10 Forum Poster Tuesday, February 4, 2014 6:40 PM Reply
    "It's not how big it is but how you..." Oh, never mind.
    Remembering 30 years.
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  • gasbag11 Top 10 Forum Poster Tuesday, February 4, 2014 8:19 PM Reply
    "It's not how big it is but how you..."

    LOL. I'm sure that originated from some guy with an inadequate....... "integrated circuit". :D
    NO! I don't know why DX screwed up the site so badly.
    Ask them.
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  • ebpDoug Wednesday, February 5, 2014 12:59 AM Reply
    Why is 7.8mA "pointless"? It's a 74HC family shift register.
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  • desolder Top 10 Forum Poster Wednesday, February 5, 2014 5:13 AM Reply
    Piggyback all 5 of them and you can get a whopping 39mA!

    But seriously... if you need higher current then buffer the outputs with some power transistors.
    What's up with the scrubbed LED driver IC markings?
    What are they trying to hide?

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  • firethorn Wednesday, February 5, 2014 7:44 AM Reply
    Has anyone actually read the datasheet of the "original"?

    The way I'm using mine draws about 20-30mA per out.

    You'll also find all over the inteweb, circuits that definitely expect more than a measly 7.8mA from a 595N.
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  • ebpDoug Wednesday, February 5, 2014 8:00 AM Reply
    What exactly do you mean by "original?" That part has been on the market for probably 20 years. I would have to look it up, but it is probably a CMOS version of a TTL part that has been around for at least 30 years.
    595N?? - looks to me like a partial part number. Something derived from the original for a special purpose? N is a common suffix to denote a dual inline package.

    [edit] Just checked. It is definitely a CMOS version of the 74LS595 or 74S595. the SN74LS595N is in my 1985 TI TTL data book.
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    post edited by ebpDoug on 2/5/2014 at 8:10 AM
  • firethorn Wednesday, February 5, 2014 8:42 AM Reply
    There is a reason I put double quotes around "original".

    595N is a known but not ubiquitous shortening of the popular SN74HC595N name.

    You should read the datasheet and then comment on whether or not using this chip with its 7.8mA ouput current limit will break many designs that can be found on the interweb.
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  • TxCharlie Wednesday, February 5, 2014 8:50 AM Reply
    The outputs of the 74HC595N shift register (and most other TTL and CMOS logic circuits) are intended to only drive other chips of the same type. When I designed circuits with the old TTL circuits back in the 1970's and 1980's, the going standard was that each output should be able to drive at least 5 inputs at full speed. More than 5 inputs required a Buffer/Driver chip to drive them.

    The only TTL chips that can output a significant current are a few Driver - type chips, but a Power MOSFET or transistor is required to operate a relay or anything like that. The 74L595N is the low-power version - Its output is even weaker.

    That 65 cent chip probably cost over $10 - $15 each when it came out in the 1970's, which would be at least $50 USD in 1970 dollars!!! In those days, electronic hobbyists would buy "grab bags" of sub-standard chips and transistors, to hopefully sort out the ones that work well enough to do the job.

    It was either use "untested" chips or spend $400 building a circuit to make a digital clock or something. Just the numeric LED's would cost $100 in fully tested prime parts, so we'd have to buy grab bags of substandard LED displays, too. Generally they were rejected because a segment was weak or mechanically damaged.

    Grab bags were interesting - They often contained parts that you had no idea what they were! Many of the military overruns just had a serial number on them. There was a company named "Poly Paks" in Michigan which put out a catalog with hundreds of electronic parts grab bags. I probably spent $5000 in grab bags, and have most of the leftovers in my closet!
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    post edited by TxCharlie on 2/5/2014 at 8:51 AM
  • firethorn Wednesday, February 5, 2014 9:01 AM Reply
    That's quite interesting. But alas that is how it goes. And there are still grab bags available ;).

    In any case, I'm going to close the main issue here by saying, of course there are still uses for this chip. But if you're just buying parts off an instructable list (especially if you're building an 8x8 led matrix with an arduino and these SN74HC595Ns [wink ebpDoug]), this probably isn't for you. And it isn't for me.

    7.8mA be warned.
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