Im planing on making a 7 segment display out of leds, with each segment made up of three leds. The leds draw 20mA each so each segment will draw 60mA yes? So I cannot use just a BCD decoder as they can only source about 25mA per segment, which is less than the 60mA. So can you suggest a BCD decoder which can supply the required current or tell me how to use transistors with the decoder. If transistor are used will there be any sort of voltage drop across the transistor. BTW i am using 5 volts.
Thanks
Thanks
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You have several options.
(1) Use low current LEDs
(2) Use PNP transistors as switches to drive the LEDs in parallel. As in fig. 1 here:
http://www.rason.org/Projects/transwit/t…
This will work, assuming you are using a 74 series 7-segment driver with negative logic open-collector outputs like the 7446. Use a 3.3k resistor for R1. You probably don't need R2.
(3) Using LEDs in parallel means that each LED needs its own current limiting resistor, otherwise you may find that the brightness of the LEDs is different. That means 21 resistors. A better way might be to wire the LEDs in series with one resistor per chain of 3 LEDs. The only thing is you need a higher supply voltage for the LEDs. Depending on what LEDs you are using it could be up to 12V. But you can easily get that by using a small DC-DC converter. I'd use a 5V to 12V converter for white or blue LEDs, 5V to 9V for other types.
Calculate the resistor by R = ( Vs - (3*Vf) ) / 0.06
Where Vs is the supply voltage and Vf is the forward voltage of the LED from the datasheet.
(1) Use low current LEDs
(2) Use PNP transistors as switches to drive the LEDs in parallel. As in fig. 1 here:
http://www.rason.org/Projects/transwit/t…
This will work, assuming you are using a 74 series 7-segment driver with negative logic open-collector outputs like the 7446. Use a 3.3k resistor for R1. You probably don't need R2.
(3) Using LEDs in parallel means that each LED needs its own current limiting resistor, otherwise you may find that the brightness of the LEDs is different. That means 21 resistors. A better way might be to wire the LEDs in series with one resistor per chain of 3 LEDs. The only thing is you need a higher supply voltage for the LEDs. Depending on what LEDs you are using it could be up to 12V. But you can easily get that by using a small DC-DC converter. I'd use a 5V to 12V converter for white or blue LEDs, 5V to 9V for other types.
Calculate the resistor by R = ( Vs - (3*Vf) ) / 0.06
Where Vs is the supply voltage and Vf is the forward voltage of the LED from the datasheet.