Using BJT's with a 595 shift register not outputting expected current.

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hi all,

currently in process of muxing tlc5940 , shift register controlling matrix of rgb led's. means each led need approx 20ma of current, , when mux across 8 inputs sn74hc595, 25ma total per pin sourced. issue can fixed fet or bjt. testing 2n3904, bc517 (darlington pair), high power bjt don't have product # right , few different high-power mosfets.

the issue none of these supplying high current led's!

what i'm thinking vbe isn't high enough, vb = 3.3v coming teensy 3.1, , ve needs 2 3.2 volts depending on color! when 3.2v required, vbe wont able match that, needs 0.7v saturated , on, or else linkage current low.

any suggestions on can fix see high current pass through npn transistor?


thanks!

swap led , r1


Arduino Forum > Using Arduino > LEDs and Multiplexing > Using BJT's with a 595 shift register not outputting expected current.


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