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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
    Aha the cascadable bar  graph chip from nat semi - a venerable piece of silicon indeed. Know it well.


    If you are using the internal refernce to also set the full scale voltage of the graphs, as well as the  LED current, you need to realise in the app note circuit the ratio of the resistors sets the voltage, and the current through them sets the LED brightness, as it is the  load on the reference voltage regualtor that sets the LED brightness.


    The high and low voltages for the comparators can either both or neither be derived from the reference regulator by resistive division.

    Can you put up a sketch of what you have so we can talk about things with names - R1 R2 or whatever, or it will be confusing and probably wrong.

    I'm off to lunch now but back in ~ 90-mins, and I can look the chip datasheets up then so I have the right pin nos and voltages.


Reply
  • 0
    Aha the cascadable bar  graph chip from nat semi - a venerable piece of silicon indeed. Know it well.


    If you are using the internal refernce to also set the full scale voltage of the graphs, as well as the  LED current, you need to realise in the app note circuit the ratio of the resistors sets the voltage, and the current through them sets the LED brightness, as it is the  load on the reference voltage regualtor that sets the LED brightness.


    The high and low voltages for the comparators can either both or neither be derived from the reference regulator by resistive division.

    Can you put up a sketch of what you have so we can talk about things with names - R1 R2 or whatever, or it will be confusing and probably wrong.

    I'm off to lunch now but back in ~ 90-mins, and I can look the chip datasheets up then so I have the right pin nos and voltages.


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