7447 IC is wierd!

Discussion in 'General Electronics Chat' started by Nathan Hale, Nov 20, 2013.

  1. Nathan Hale

    Thread Starter Active Member

    Oct 28, 2011
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    Hi guys , hope all is well with everyone. So i just started to learn about ICs that convert a BCD nicely into a 7 segment display . I have learnt about the 4511 and 7447. the 4511 is cmos. when i looked at the truth table for the 7447 IC i found that everywhere the LEDs should be "high" they are on "low". It seems that this IC does exactly the opposite of what you would want it to do. I am sure the guys who made this IC knew what they were doing, but my question is why are the outputs exactly opposite of what you would expect. For example here is the table to display the number 1.

    Decimal LT RBI BI/RBO D C B A a b c d e f g
    1 H X H L L L H H L L H H H H
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    There is nothing wrong with the 7447. It was designed to sink current when using common anode 7-segments LED. This is called ACTIVE-LOW output.

    If you wish ACTIVE-HIGH outputs you can use a 7448.
     
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  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    If I am reading it right. When you connect the LED, the leg that is normally connected to ground should be connected to the 7447, the leg that is normally connected to voltage source should be connected to voltage source.

    Here is what is going to happen. 7447 keeps some voltage on the pins where LED is connected. The difference in voltage between 7447 pin and the voltage source is too small to light up the LED, so there is no voltage drop across the LED (or the drop is very small) and no current flow. When you want to light up the LED, you let the pin on 7447 to go low, this makes the pin a "ground" for the voltage source, now there is a voltage drop across the LED and there is flow of current, the LED lights up.
     
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  4. Nathan Hale

    Thread Starter Active Member

    Oct 28, 2011
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    2 major IC selling websites (jameco and nte ) say they dont have such an IC called a "7448"
     
  5. MrChips

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    The 7448 is obsolete. You can get it on ebay.
    If you're kind enough to send me your address I can send you some.
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    http://www.ti.com/product/sn7448
    SN7448
    (OBSOLETE) BCD-To-Seven-Segment Decoders/Drivers


    Well, that explains it.
     
  7. WBahn

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    Mar 31, 2012
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    And, to bring things full circuit, the reason that TTL uses predominantly active LO drives is that (particularly the early) TTL families has significantly strong current-sinking capabilities than they had current-sourcing capabilities. If you just checked the output voltage of unloaded 5V TTL signals when they were HI it was not uncommon to see them in the 3.5V range. This is adequate for asserting a logic HI to other TTL logic chips, but it is not very good for driving other things. On the other hand, when outputting a logic LO, the output is seldom more than a few hundred millivolts above ground, which is a much more reliable drive signal (not to mention the higher current capacity).
     
  8. WBahn

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    Mar 31, 2012
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    The 7448 is not nearly as common as the 7447. There's a reason for that. So if you can design your system to be compatible with the 7447, do so.
     
  9. Nathan Hale

    Thread Starter Active Member

    Oct 28, 2011
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    thank you for ur offer but i just found out that nte makes a 7447 that well....works the way i think it should work. the out puts are high when i think they should be high. thank u again.

    http://www.nteinc.com/specs/7400to7499/pdf/nte7447.pdf
     
  10. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    I might have some 7448 or 7447 chips laying around let me see though...
     
  11. WBahn

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    Uh... look again. Those are open-collector outputs. When they say ON they mean that the output transistor is turned on which connects the output pin to ground. In other words, these are active-LO outputs, just like every other 7447.
     
  12. MrChips

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    Oct 2, 2009
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    Before you go looking for obsolete ICs, show us your circuit diagram and we can offer a solution.
     
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I don't understand why you think the NTE7447 works like a 7448. The datasheet uses the words ON and OFF in the truth table but when it says ON, it means 'active'. Read the datasheet carefully...

    ========= from the first paragraph in the datasheet ==========
    The NTE7447 is a BCD−to−Seven−Segment Decoder/Driver in a 16−Lead plastic DIP type package that features active−low outputs designed for driving common−anode VLEDs or incandescent indicators directly.
     
  14. Nathan Hale

    Thread Starter Active Member

    Oct 28, 2011
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    Well i am trying to make a 4 bit counter attached to a 7 segment display so i can see the numbers on the display. The circuit is on page 328 (pdf format) of the "digital " section available through this website.
     
  15. Nathan Hale

    Thread Starter Active Member

    Oct 28, 2011
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    can you please be more specific as to which pins u r talking about on the 7 seg display? thank you
     
  16. shteii01

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    Feb 19, 2010
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    For 7447 chip you will need common anode 7 segment display.

    On the 7 seg. display you will have one pin where you apply positive voltage. There will be 7 pins that you connect to the 7 pins on 7447 chip. You may need to limit the current that goes through each LED of the 7 seg. display, check the datasheet for the voltage drop across the LED and maximum current through LED, we will use these to calculate the resistor you will need.
     
    Last edited: Nov 21, 2013
  17. Nathan Hale

    Thread Starter Active Member

    Oct 28, 2011
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    4774 chip??? u mean 7447???
     
  18. MrChips

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    Here is circuit on how to wire a 7447 decoder/driver to a common anode 7-segment LED display.

    [​IMG]

    The 1kΩ current limiting resistors shown will work for high efficient LEDs.
    You can try values between 220Ω and 1kΩ to get the brightness that suits you.
     
  19. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Oops. Sorry. Yes, 7447.

    Here is pick that shows exactly what you have to do:

    [​IMG]



    Notice the one pin (anode pin) connect to positive supply (+5V). The 7 pins that would normally connected to ground, are connected to the 7447 chip. Also notice the resistors that they use to limit the current through the LED that make the display. One advice I read elsewhere is that instead of using 7 resistors for each led, you can put one resistor between positive supply and the anode pin, and that will limit the current for all the LED segments.
     
  20. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    MrChips, I have a question.
    Can they use one resistor, between the voltage source and anode pin of the 7 seg. display, instead of seven resistors between cathode pins and 7447 chip pins?
     
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