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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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    rotated the images and reloaded this morning, as the forum SW cant handle that.
    070fcf7c98c5fa66f282ae93d2f8d833-huge-lm3914001.jpg



    and again

    828ede0812387ddff8c8818c15587251-huge-lm3914002.jpg


    These formulae ignore the loading of the in-chip resistors on the ladder values, but the current in the main branch will be 1mA or so for 10mA LED, and the ladder is  going to be a couple of hundred uA or so, an error of 10% ish. The middle resistor needs increasing by a similar fraction to compensate.
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  • 0
    rotated the images and reloaded this morning, as the forum SW cant handle that.
    070fcf7c98c5fa66f282ae93d2f8d833-huge-lm3914001.jpg



    and again

    828ede0812387ddff8c8818c15587251-huge-lm3914002.jpg


    These formulae ignore the loading of the in-chip resistors on the ladder values, but the current in the main branch will be 1mA or so for 10mA LED, and the ladder is  going to be a couple of hundred uA or so, an error of 10% ish. The middle resistor needs increasing by a similar fraction to compensate.
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