Electronics > Beginners
74hc595 module , Why no resistors?
OwO:
Of course a 74HC595 doesn't have built in resistors, it's a generic shift register chip and not specific to driving LEDs. I don't use these modules but instead make my own with proper decoupling and series resistor for every LED, and of course no multiplexing at all (uses 4 74HC595 chips for a 4 digit display).
Peabody:
Maybe this isn't directly relevant, but just a reminder that if a 74HC output can comfortably drive four segments simultaneously, instead of eight, then you could use multiplexing by segment, instead of by digit, with resistors needed only on the common cathodes, and you would still have even brightness on all segments. The attached shows the alternate circuit for two digits using one 74HC4017. The four-digit version would just add two more digits, two more processor pins, and two more resistors, but still use only one 4017. The number of processor pins needed would be equal to the number of digits, plus one. Of course the multiplexing would need to be faster since each segment would be on only 1/8 of the time instead of 1/4 of the time. Unfortunately, they don't make modules that look like this, so you would have to roll your own. If there's any interest, I can link to a video and repo on this method.
David Hess:
--- Quote from: thinkfat on June 08, 2019, 07:47:04 am ---There's a Nexperia data sheet for the 74HC595 which has the following maximum ratings for the chip:
For each of Qn: 35mA, that means the chip can source 35mA per output pin.
Maximum current from the supply and to GND: 70mA (not 200mA)
That means, even with multiplexing taken into account, if you light more than two segments in each of the digits, you're above the maximum ratings for the chip.
--- End quote ---
I am sure it varies significantly between manufacturers if only because of test conditions which are not given. I think I got the 200 milliamp rating from a Hitachi datasheet.
--- Quote from: blueskull on June 08, 2019, 10:02:30 am ---I actually Googled the TM74HC595, nope, it doesn't have built-in output resistor.
--- End quote ---
Sure it does. The output resistance is built into the channel resistance of about 25 ohms at 5 volts.
--- Quote ---So don't buy it. It's not properly designed.
--- End quote ---
I agree although I know it will work albeit with poor reliability compared to a properly designed circuit.
westfw:
--- Quote ---that means the current for each entire digit is limited by the per pin sourcing limit of U6, e.g. 35mA
--- End quote ---
CMOS chips like the xx595 don't *limit* the current, they're just *specified* to be able to provide that much current without damage., and while staying withing the voltage specs for logic outputs.
If you're driving LEDs, you don't much care about in-spec voltages, and CMOS gates seem to be pretty robust WRT not burning out...
james_s:
There are a lot of things that are not good design practice but sometimes you can still get away with them. I wouldn't use a module like this for anything critical but if it's cheap and it works? No real reason not to use it.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version