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LM 3914

ebee
Hello,

anyone into electronics to help me (I have some suspects).


I am having trouble with LM 3914/5/6

I get the current program of LEDs by making path to ground 0V from Pin 7.

Pins 7 & 8 give use approx 1.25 volt ref

It`s the pin 6 & pin 4 Rhi and Rlo bits in order to get values of other than the supply lines.

i.e if we put in two resistors R1 & R2 from pin 7 to 0V and calculate the voltage at the junction then we could use this as a hi or low I think but what about the other (to get a hi and a lo) .


Then methinks factor in the internal comparitor divider chain into these figures to see if a significant variation occours .


I initially want to prog my LEDs to say 10 or maybe 15mA current and the hi and low volt comparisons say 1.8v to 4.0v.

Thanks
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  • 0
    OK,

    R3 + R4 must drop 1mA to 0V therefore R3 = 1.25V/).001A = 1K25

    Therefore R2 + R3 + R4 - 1K8 to make Rlo 1.8V at 1mA

    Therefore R2 = 1K8 - 1K25 = 550R

    R1 therefore needs to lift 4.0v - 1.8v = 2.20v  at 1mA therefoe 2K20

    If R3 = 1K25 then R4 can be zero (Why did I put it in?)

    I`m assuming that once we set the current on R3 then the same current runs on R2 and R1(with the chain of 10K in parallel).


    finally 10K in parallel with 2K20 gives 1K803 so we need to up R1 to recalculate this 2.20 volt drop.


    PS I`d done this before I got your reply Mapj so I`ll have a look at that shortly thanks


Reply
  • 0
    OK,

    R3 + R4 must drop 1mA to 0V therefore R3 = 1.25V/).001A = 1K25

    Therefore R2 + R3 + R4 - 1K8 to make Rlo 1.8V at 1mA

    Therefore R2 = 1K8 - 1K25 = 550R

    R1 therefore needs to lift 4.0v - 1.8v = 2.20v  at 1mA therefoe 2K20

    If R3 = 1K25 then R4 can be zero (Why did I put it in?)

    I`m assuming that once we set the current on R3 then the same current runs on R2 and R1(with the chain of 10K in parallel).


    finally 10K in parallel with 2K20 gives 1K803 so we need to up R1 to recalculate this 2.20 volt drop.


    PS I`d done this before I got your reply Mapj so I`ll have a look at that shortly thanks


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