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Topic: 7-segment display driver, external resistors needed? (Read 444 times) previous topic - next topic

jensvanhoof

Hi all,

I'm trying to get an 7-segment LED display working. For now, I've done it with a PCF8574 expander on I2C, and this works well. The problem is that I forgot to add a resistor array to limit the current to the segments so I don't expect the LEDs to last long. So far for my thought 'why not take an I/O expander in stead of a BCD-to 7segment driver..'

I was looking at some drivers now, like this one:
http://be.farnell.com/texas-instruments/cd4055bm/logic-bcd-7seg-decoder-dvr-16soic/dp/1753424

My question is: are external resistors required for each LED in such a setup? I would think that the manufacturer of those would already build resistors in there, since those drivers are specifically to be used with 7-segment displays. But since some of them have a wide range of supported voltage (3V to 18V in this case), I suspect them not to be included. Am I right about this?

Would there be any other advantage to use such a driver in stead of, say, that PCF8574? Or does anybody knows a driver that has  resistors build in for, say 5V? (220ohm?). I'm trying to limit my components count, that's why :-)

Thanks a lot!
Jens

Chagrin

TLC5916 or TLC5925 are shift registers that implement current limiting (set by a single resistor for all channels on the chip).

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