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LED to light when battery drops below 3.5V

Discussion in 'Power Electronics' started by JPU, Nov 2, 2012.

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  1. JPU

    JPU

    281
    1
    May 19, 2012
    Hi All

    I spent a couple of hours last night reading the TINA manual regarding adding components,,,,,I am none the wiser!:confused:


    I altered my circuit and used the second version of the 2n7000 and I now get the same results as you.:D

    The components turned up this afternoon and I have built the first circuit using the 2n7000. It does the job perfectly and is very reliable. There is some slight flickering/dimming of the LED as expected but this is not a problem.

    I had also ordered the MCP101-270hi/to. The component also works a treat and when connected to the 2n7000 it gives a distinct ON/OFF when the voltage passes 2.6V. (OFF when > 2.65V)

    However when I introduced my voltage divider to try and get the component to work when the voltage drops below 3.5V it all goes wrong and hopefully you guys can help.

    The voltage divider should have an output of <=2.6V when the battery is at <=3.5V. This should then cause the MCP101 to switch the reset to high >2V. However the voltage divider does not produce the voltage I expected when connected to the MCP101 as shown in the circuit schematic eg

    voltage into divider (V) V out of divider (V)
    3.0 2.0
    3.6 1.76
    4.0 1.7
    5.0 1.6
    8.3 2.7

    I have to turn the input voltage upto 8.2V at which point my voltage divider then produces 2.65V and the circuit then switches reset to High.

    I am sure this is all down to the load on the circuit altering the voltage divider but are you able to explain what I have to do to counter this problem. (if this is the problem,:confused:)

    I am sorry if my question and explanation of the problem is not clear. (You know when your confused when asking the question is confusing you:p). I did think of a video to explain but then I thought you may know immediately where I am going wrong.

    Justin
     

    Attached Files:

  2. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
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    May 8, 2012
    The trip point for the MCP101-270 is 2.625V 'typical'. This series of chips were not designed for use with a voltage divider.

    Chris
     
  3. JPU

    JPU

    281
    1
    May 19, 2012
    Ah, that explains that then!
     
  4. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    The data sheet specs the operating current at 45uA (Typical) @ Vdd = 5.5V with no load on the output pin. They don't provide a graph of operating current at other voltages, so we can only guess what it would be. There is a work around but it's low grade because it'll force you to insure that the voltage divider draws substantially more than the chip. This makes little sense because we want the current drain by the voltage monitor to be nearly invisible to the battery until the LED fires.

    That said, if you want to experiment, make R1 = 22K and R2 = 5.6K. With these values and a variable supply the output should go high when Vbat <= 3.5V or close to it.

    Chris
     
  5. JPU

    JPU

    281
    1
    May 19, 2012
    Hi Chris

    Thanks for your suggestion.

    That does work! (please ignore the email you were sent with my original post).

    Will there be significant current drawn by this circuit from the battery when >3.5V and when <3.5V with an LED lit (220ohm resistor). In short, when connected to the buck and the two LEDs mentioned in the other thread,will this circuit make much difference to the overall battery life?

    Regards

    Justin
     
    Last edited: Nov 7, 2012
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Good, because your pre-edited post made no sense, which is par for the course because today NOTHING makes any sense.

    Chris
     
  7. JPU

    JPU

    281
    1
    May 19, 2012
    It could be worse, just look at what happened in our election!

    Justin
     
  8. JPU

    JPU

    281
    1
    May 19, 2012
    Chris

    Will there be significant current drawn by this circuit from the battery when >3.5V and when <3.5V with an LED lit (220ohm resistor). In short, when connected to the buck and the two LEDs mentioned in the other thread,will this circuit make much difference to the overall battery life?

    How would this circuit compare to your earlier circuit using the 2n7000. Which would be best?

    Justin
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    The takers now outnumber the makers. Hey, I'm surprised you knew what I was referring to.

    Alcohol is not helping. :(
    Chris.
     
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Compared to what your high current LEDs pull,,,, no. Look at the plot.

    https://www.electronicspoint.com/led-light-battery-drops-below-3-5v-t253854p2.html#post1504478

    Chris
     
  11. JPU

    JPU

    281
    1
    May 19, 2012
    Hi chris

    I meant compared to the earlier circuit you posted for me using the 2n7000.

    Which would be the best circuit to incorporate into the headlight that one or the MCP101 circuit.

    I think you might say neither are perfect but I thought I might ask. I have searched the net for a suitable voltage supervisor in the right type of package that I can hand solder but I have had no luck.

    FYI: I live in a street where my wife and I work and two other houses are owned by working families! There are 21 houses on my street,,,mostly on social benefit or income support. The takers are everywhere and they will keep taking until there is nothing left to take and that ain't far off!

    Friday night isn't far off..;)

    Justin
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    My 2N7000 circuit is iffy, as it has no temperature compensation, where the MCP101 does. I just don't like using it with a resistive voltage divider.

    Let me see if I can sniff out an appropriate chip for you.

    Anyone else have opinions on this?

    Chris
     
  13. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
  14. JPU

    JPU

    281
    1
    May 19, 2012
    Hi Chris, That component looks just right. I have had a search on the net for a supplier as my usual suppliers Farnell and Rs don't stock the value I need.

    Ill keep looking.

    Thanks for researching it for me.

    Justin
     
  15. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    Rohm has a pretty good sample program if you want to deal with a face to face with you local sales rep... As least that has been my experience, but I'm also local to their base of operations so it might just be a 'local' thing...
     
  16. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    CC, do you have an email contact for them?

    Chris
     
  17. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    It's been several years now but I believe I just used the contact form on the page...

    https://www.rohm.com/web/global/contactus

    Within a few days a sales rep was calling me at odd hours (kinda like he was part time and doing it after work) trying to setup a face to face to give me the sample in person... I never actually followed through as the part I wanted to sample was about $1 and not worth my time to deal with a face to face vs just purchasing it... But, if it was a non-stocked part it would probably be worth the face to face, and the face to face will almost always get you a foot in the door to purchase a few more 'samples' from that rep direct instead of making the minimum order count...
     
  18. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    I guess Justin has nothing to loose by following your link. He'll probably be safe from salesmen, ...being in S. Wales. ;)
     
  19. JPU

    JPU

    281
    1
    May 19, 2012
    I did phone the Rohm office in the UK when I got home from work today, but no answer. Ill try again on Monday. RS-online do stock a version, BD45275G-TR.. I have noticed that values around a 2.5 - 2.9 are readily available. I am not sure why? perhaps you guys have an idea why? Most of the voltage supervisors available (in stock) seem to range from 1.5-2.8V and then 4.0V onwards?

    Just
     
  20. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    Yeah, it certainly might be easier to get samples in some areas, I'm only about 20 minutes away from their Chicago (area) sales offices so I'm sure when they saw a local asking for samples they pushed a little harder for the face to face...
     
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