Drawn Current?

Discussion in 'The Projects Forum' started by Ben_C, Jan 26, 2011.

  1. Ben_C

    Thread Starter Active Member

    Oct 19, 2010
    65
    4
    I'm building a temperature sensor where a fan operates when certain temperature limits are met. The question I have is about current on this project.
    Im using a LM7805 volt reg which gives a range of 5mA to 1A. I'm trying to figure out how much current will be drawn when all my components are active. Im using the following with their typical current values:

    PIC16F688: 95mA
    Fan: 220mA
    LCD: 3mA
    DS1820: 1.5mA

    Does this mean that a simple adding of these values give over all drawn current?
    I need to know how much current will be drawn. I've read something about parallel current but I didn't really understand it...
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    The estimate for the PIC seems a bit high.

    If you run the fan on unregulated voltage, you'd save a bit of dissipated power by using a low dropout regulator, chopping 1V off heat generated.

    The energy budget is essentially adding together the known running current, the peak current and duration of peaks, and sleep/quiesent current, with duration of sleep.
     
  3. Ben_C

    Thread Starter Active Member

    Oct 19, 2010
    65
    4
    Thankyou very much :)
    I was discussing with a peers about placing the fan on the unregulated line and it is something that I'm considering, very good information.

    Yeah thats what I'm trying to achieve. I stated in the parameters of my project that I want it to be efficient, thats justified in having the fan only operating when the temperature reaches a certain limit. Peak current will obviously occur when the temperature rises seeing as more compnents will be active, especially the fan. I know the quiescent to be at just 6mA.

    I got the current for the chip off its datsheet, am I looking at the wrong current? :S
     
  4. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    Have a look at the charts in section 14.2 for your voltage and clock speed then add any current that the pins source or sink (unless they are already included in fan, LCD or DS).
    Examples might be base current for transistors, pull-up resistors, etc.
     
  5. Ben_C

    Thread Starter Active Member

    Oct 19, 2010
    65
    4
    Ahh okay I see.
    I've looked at the charts and seeing as im using the internel osc at 4MHz does this mean that the current it draws is just 1mA? :s and then add currents from just the transistors and each component?
    Slightly baffled haha.
     
  6. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    Yes that's it. 0.8mA typical at 5V with the 4Mhz INTOSC
    Your total so far is 225.3 mA, but the fan might draw a bit more current than that as it starts up.
    Have you got a link to the fan information?
     
  7. Ben_C

    Thread Starter Active Member

    Oct 19, 2010
    65
    4
    :D
    I'm going off absolute maximum current it could draw for each component so my maximum is at:

    Fan: 220mA
    LCD: 3mA
    PIC16F688: 1.2mA
    DS1820: 1.5mA

    225.7 :)

    Yeah one of our subjects last year was on motors and start up torque, it needs that little extra energy to get to maximum speed to then settle at a stable speed, I shouldn't see a problem with this though?

    I got the fan from a disassembled laptop, it states 5v at 1.1W. I used P=IV to work current out...
     
  8. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    It might just work. I just tested a 12V PC fan rated 0.21A and it took about half an amp at startup but then dropped to about .15A
    What are you using to switch the fan (type of transistor, MOSFET, etc)
    It's possible that the starting current of the fan could cause voltage drop and resetting of the micro. Using a separate voltage regulator for the fan might help if you do have problems.
     
  9. Ben_C

    Thread Starter Active Member

    Oct 19, 2010
    65
    4
    You're very helpful :)

    The type of transistor I'm using is the BC547 which hopefully is going to work.

    Even if at start up, the regulator should handle this surge of current seeing as it the max value is 1A. I would have put a seperate regulator on as this seems to be a wise idea but I've designed and etched my PCB and drilled the holes already :/
     
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