Electronics > Beginners
74hc595 module , Why no resistors?
typematrix:
hi
These modules are commonly available on eBay, "4 bit LED tube".
They consists of two 74hc595 Daisy chained, driving a seven segment module 3641BS
http://www.dipmicro.com/?datasheet=NFD-3641.pdf
Why are no resistors on the control lines to the seven segment LEDS?
thinkfat:
--- Quote from: typematrix on June 07, 2019, 02:36:39 pm ---hi
These modules are commonly available on eBay, "4 bit LED tube".
They consists of two 74hc595 Daisy chained, driving a seven segment module 3641BS
http://www.dipmicro.com/?datasheet=NFD-3641.pdf
Why are no resistors on the control lines to the seven segment LEDS?
--- End quote ---
Apparently the current limiting is through the maximum sourcing capability of the segment driver. And clearly above the package sink limit of the poor 595 that selects the digits. No good style. Recommend not to buy.
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Kjelt:
You regulate the average current by multiplexing the digits, you should never let the 595 display full the time then the leds are destroyed.
This is often used since many leds can handle for instance a current of 5 times nominal with a DC of 10%. However I recommend another board though with the max7219 which is made for this purpose.
Dabbot:
Those poor chips. Not even some bypass caps to deal with all that current thrashing back and forth. :(
ebclr:
This is a very common tech used by the Chinese. I always guessed what a #%%$, But works
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