ULN2003 vs UDN2981

Discussion in 'The Projects Forum' started by rubicon99, Apr 19, 2012.

  1. rubicon99

    Thread Starter Member

    Apr 19, 2012
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    I am making an emergency estop system and I used the ULN2003 and a pull up resistor and the circuit works, everytime I input a 0 it outputs a high and vice versa. The problem was that I didn't take into consideration that a load would be attached to the output making the voltage decrease and essentially making a voltage divider. What is a way to get around this? I thought of using the UDN2981 which is the opposite, this way a load won't affect my results, because from my thought is that the UNL2003 is a pull down while the UDN2981 is a pull up. Any ideas?
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,145
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    A diagram of your circuit would help, including any details about the power supply and the load. Maybe you just need an output transistor that can handle your load.
     
  3. John P

    AAC Fanatic!

    Oct 14, 2008
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    You can do this exactly the way you've described. But does your load have to be connected to Gnd and have its high side switched? It's somewhat more common to keep the power applied to the plus side of the load, and switch the minus side to Gnd, which is what a ULN2003 would do. That's just a convention, but if you look at the internal circuit of the ULN2003 versus the UDN2981, you can see that the ULN2003 is quite a bit simpler. However, once the silicon has been designed and put into production, one of them is just as easy to use as the other. Or is there much of a price difference?
     
  4. rubicon99

    Thread Starter Member

    Apr 19, 2012
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    There is a 24 volt input power which I use as the Common on the ULN2003 and the resistance load is around 2.5 KOhms, I need an outout of 24 Volts.
     
  5. panic mode

    Senior Member

    Oct 10, 2011
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    digram in format that everyone can read would be nice., perhaps image or pdf.

    as you have noticed UDN2981 switches positive side while ULN2803 or ULN2003 switch negative. bit this is not the only difference. ULN2803 and ULN2003 use open collector, this means that voltage drop is small when transistor is on (saturation, hence Vce is about 0.1V or so). UDN2981 is not quite the same. for example it is emitter follower and voltage drop can be as large as 2V.

    Now you are mentioning E-Stop circuit. Is this something for your own project or industrial E-Stop? If industrial, what category it is supposed to comply with (B,1,2,3,4)?
     
  6. rubicon99

    Thread Starter Member

    Apr 19, 2012
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    Sorry here is the proper attachment in pdf. If I put the UDN 2981 replacing the ULN2003 and have resistors going low, will that just invert everything? No real standards. Other than I have to have a 24 volt output.
     
  7. panic mode

    Senior Member

    Oct 10, 2011
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    the circuit you posted shows ULN2003 but I understand that you want to replace them with UDNs. that is fine, UDN2981 provides positive voltage when input is positive (turned on from MCU for example). as mentioned before if your supply is 24V, energized output of UDN2981 is goung to be about 1.7..1.8V lower (ie. 22.2 ... 22.3V) which in most cases makes no difference. this should take care of your problem. the output voltage (even though it is reduced by 1.7..1.8V) will be stable even when you add load. of course load may not exceed rated current output and total power dissipation of the chip.
     
  8. rubicon99

    Thread Starter Member

    Apr 19, 2012
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    Is there a way to use the ULN 2003 with the load to achieve 24 volts withou affecting the voltage in my current design?
     
  9. panic mode

    Senior Member

    Oct 10, 2011
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    no matter what you do to switch some voltage, there will be voltage drop.
    question is - how big?

    if you are using sink outputs (such as ULN2003 or ULN2803), the voltage drop across outputs will be only about 0.1V and your load will connect from +24V to output. your load will see voltage:
    24V -0.1V = 23.9V
    reason for low voltage drop is that output transistor is in common emitter mode and in saturation


    if you are going to use source outputs ( such as UDN2981 or UDN 2982) output transistor is in common collector mode. in fact there is cascade of two transistors. this means voltage drop is about 1.8V so voltage across load will be 24V-1.8V=22.2V

    it drops a bit more. when you have several outputs turned on, each and every one of them will have such voltage drop. total power dissipated by the IC is P=1.8V*(Io1+Io2+...Io8).
    where Io is output current for some output.
     
  10. rubicon99

    Thread Starter Member

    Apr 19, 2012
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    So there is a voltage drop. Is there anyway that I can recover this voltage without raising a logic low output from the uln2981. Like an inductor/capacitor combination? Or would a better route be going with a mosfet?
     
  11. rubicon99

    Thread Starter Member

    Apr 19, 2012
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    By the way thank you for the help.
     
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