Drive Current Sufficiency


+ Reply to Thread
Results 1 to 9 of 9
  1. #1
    Join Date
    Dec 2012
    Location
    Türkiye
    Posts
    103

    Default Drive Current Sufficiency

    Hello Everyone ;

    The question in my mind might be quite simple for some experienced users maybe . Here it is .. Since I read the datasheet of the 18f87k22 I want to use all the pins (except MCLR of course )of the mcu to drive relays .. So to do this instead transistors I'll use ULN2003A . But as far as I read the datasheet the source/sink currents for some ports supply less current (around 2 ma) then we are used to from other PIC MCUs classic 25 ma source current . So I checked the input current vs input voltage graphics of ULN2003a and it says max 1.25 ma input current is required to drive an input of the uln2003a .As seen it is quite close to the specified 2 ma max source current , so can I possibly encounter problems in the future ? I ask these questions because I don't have neither the mcu nor the uln2003 so I can't make an experiment on this matter .. Your valuable experiences are required .
    Regards
    - Ipsa scientia potestas est -

  2. #2
    Join Date
    Oct 2005
    Location
    Sweden
    Posts
    3,413


    Did you find this post helpful? Yes | No

    Default Re: Drive Current Sufficiency

    I'm pretty sure you'll be fine. You have a margin of 60% even under worst conditions. What collector current are you designing for and do you have a specific ULN2803 in mind (several manufacturers makes them, they may not all be equal).

  3. #3
    Join Date
    Dec 2012
    Location
    Türkiye
    Posts
    103


    Did you find this post helpful? Yes | No

    Default Re: Drive Current Sufficiency

    Henrik Thanks for being my guardian savior angel again . The relays have 1028 ohm coil resistance so that returns a 11.67 ma collector current for per relay at 12V .ULN2803 is not an option for this project since we doesn't have a chance for the component stock .
    - Ipsa scientia potestas est -

  4. #4
    Join Date
    Dec 2012
    Location
    Türkiye
    Posts
    103


    Did you find this post helpful? Yes | No

    Default Re: Drive Current Sufficiency

    Also do you think this MCU's weak current supplying ports can handle and succesfully drive the A/B/C/INH signal inputs of CD4051 and A/B/C/D/INH signal inputs of 4514 ?
    - Ipsa scientia potestas est -

  5. #5
    Join Date
    Oct 2005
    Location
    Sweden
    Posts
    3,413


    Did you find this post helpful? Yes | No

    Default Re: Drive Current Sufficiency

    I think perhaps we've both misinterpreted the datasheet a little bit.
    Look at table 6.5 in this Texas Instruments datasheet. There we can see that indeed the maximum input current is 1.35mA at 3.85V but a little bit further down we can see that an input current of 250uA will still produce a collector current of 100mA while only dropping a maximum of 1.1V.

    Input current on CMOS series logic chips seems to be in the uA range so no problems there.

  6. #6
    Join Date
    Dec 2012
    Location
    Türkiye
    Posts
    103


    Did you find this post helpful? Yes | No

    Default Re: Drive Current Sufficiency

    Don't you think the current at 3.85 V proportionally should increase at 5V drive voltage..? Secondly do you have any idea with my other question ?
    • Also do you think this MCU's weak current supplying ports can handle and succesfully drive the A/B/C/INH signal inputs of CD4051 and A/B/C/D/INH signal inputs of 4514 ?


    - Ipsa scientia potestas est -

  7. #7
    Join Date
    Dec 2012
    Location
    Türkiye
    Posts
    103


    Did you find this post helpful? Yes | No

    Default Re: Drive Current Sufficiency

    I'm sorry I've been a victim of a mobile phone browsing because I haven't seen the line with the CMOS sentence
    - Ipsa scientia potestas est -

  8. #8
    Join Date
    Oct 2005
    Location
    Sweden
    Posts
    3,413


    Did you find this post helpful? Yes | No

    Default Re: Drive Current Sufficiency

    Don't you think the current at 3.85 V proportionally should increase at 5V drive voltage..? Secondly do you have any idea with my other question ?
    It might, if it was allowed to but it can't since the output can only source 2mA. If you're worried, why not measure it? Add an external series resistor if you want to be sure that the circuit can't ever pull more 2mA.

    Secondly do you have any idea with my other question ?
    Please read my previous reply:
    Input current on CMOS series logic chips seems to be in the uA range so no problems there.

  9. #9
    Join Date
    Dec 2012
    Location
    Türkiye
    Posts
    103


    Did you find this post helpful? Yes | No

    Default Re: Drive Current Sufficiency

    As I said , I'll design the PCB according to the technical specs of all these ICs so measuring it will be possible only when the PCBs arrive .. But so far you've been very helpful to all my questions digging me ...

    Thanks so much again
    - Ipsa scientia potestas est -

Similar Threads

  1. Replies: 23
    Last Post: - 25th June 2011, 23:27
  2. Can I drive a 5v LED without a current resistor?
    By Fredrick in forum Schematics
    Replies: 31
    Last Post: - 11th September 2009, 14:42
  3. Differential Drive Programming
    By shaiqbashir in forum mel PIC BASIC Pro
    Replies: 14
    Last Post: - 30th December 2006, 14:03
  4. drive relay?
    By serdar_ozcan in forum mel PIC BASIC Pro
    Replies: 1
    Last Post: - 26th November 2005, 11:01

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts