74hc595 module , Why no resistors? - Page 1
Forum section: electronics / beginners

author: typematrix
author_rank: Contributor
timestamp: June 07, 2019, 02:36:39 pm
content: hiThese modules are commonly available on eBay, "4 bit LED tube".They consists of two 74hc595 Daisy chained, driving a seven segment module 3641BShttp://www.dipmicro.com/?datasheet=NFD-3641.pdfWhy are no resistors on the control lines to the seven segment LEDS?

author: thinkfat
author_rank: Unknown
timestamp: June 07, 2019, 03:33:04 pm
content: Quote from: typematrix on June 07, 2019, 02:36:39 pmApparently 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.Gesendet von meinem Nokia 6.1 mit Tapatalk

author: Kjelt
author_rank: Super Contributor
timestamp: June 07, 2019, 07:58:22 pm
content: 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.

author: Dabbot
author_rank: Regular Contributor
timestamp: June 08, 2019, 12:47:41 am
content: Those poor chips. Not even some bypass caps to deal with all that current thrashing back and forth.

author: ebclr
author_rank: Super Contributor
timestamp: June 08, 2019, 01:59:50 am
content: This is a very common tech used by the Chinese. I always guessed what a #%%$, But works

author: David Hess
author_rank: Super Contributor
timestamp: June 08, 2019, 02:54:33 am
content: I always use external current limiting on each 74HC595 output when driving LED displays but it might be possible to do without the current limiting resistor. The output resistance of a standard HC output is about 25 ohms at 5 volts but the resistance can be increased by operating at a lower supply voltage. I will have to try it sometime. Note that the voltage drop across the LEDs allows their supply voltage to be higher than the logic supply voltage if necessary so 2.0 volt operation of the 74HC595 should be feasible for maximum output resistance.So what are the limits of using a 74HC595 to drive LEDs? There are two.The first is of course the power dissipated in the output resistance. With external current limiting this is not a problem so I have never studied this limit in detail.The non-obvious limit which is not always given in datasheets (1) is the maximum supply pin current which is typically only 200 milliamps limiting each output to 25 milliamps if all can be on. If this is exceeded or even approached in my experience, (2) then electromigration causes poor operating life and reliability. This means if higher currents are required for best efficiency, external drivers must be used.(1) TI does not list it but some others do.(2) About 4 to 5 milliamps per output is about the maximum which can be sustained reliably in my experience which is fine for non-multiplexed displays but probably not enough for a multiplexed display. 20 milliamps from 8 outputs is only 160 milliamps and below the typical 200 milliamp supply pin limit but still too much for reliable operation. Individual outputs typically have a 35 milliamp absolute maximum output specification.

author: thinkfat
author_rank: Unknown
timestamp: June 08, 2019, 07:47:04 am
content: 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.

author: thinkfat
author_rank: Unknown
timestamp: June 08, 2019, 08:14:55 am
content: Alright, it always helps to check all the involved data sheets.The NFD-3641Bx are "common anode", that means the current for each entire digit is limited by the per pin sourcing limit of U6, e.g. 35mADue to the digit multiplexing, only one digit is ever active at a time. So the maximum current ever flowing is 35mA. This is within the maximum limits of the 595'sIt however means that the brightness of the segments depends on the number of segments lit. If you display a 1, it will be much brighter than an 8.Not good style, but it will work. The multiplexing of the digits will also make sure that you're on average not exceeding the maximum current of the LEDs.Until the micro controller doing the scanning stops. Or if you make a mistake and drive more than two digits simultaneously. Bad, bad style.But cheap.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 08, 2019, 08:46:45 am
content: There could still be resistors hidden under the display on the top side. Depending one the LED color and internal LED resistance (some blue and white LEDs have quite some series resistance internal, it may just work without resistors: With some 3.5 V drop for the LEDs and some 25-50 Ohms ouput resistance for both of the HC595 of an odd brand the current my not be that high : 1.5 V / 50 Ohms = 30 mA for the active digit and thus just below the 35 mA limit. Still the brightness may vary quite a bit and between a 1 and 8 and the LEDs may need the multiplexing.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 08, 2019, 09:39:25 am
content: This is also commonly done in LED signage matrix diaplays. Works fine, nobody dies.The only issue is that LED brightness is somewhat dependent on supply voltage, but for battery powered things you can measure the battery voltage and adjust the multiplex timings to compensate to get reasonably constant brightness over the voltage range of a lipo cell

author: OwO
author_rank: Super Contributor
timestamp: June 08, 2019, 10:34:25 am
content: 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).

author: Peabody
author_rank: Super Contributor
timestamp: June 08, 2019, 02:47:49 pm
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: June 08, 2019, 08:16:20 pm
content: Quote from: thinkfat on June 08, 2019, 07:47:04 amI 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 amSure it does. The output resistance is built into the channel resistance of about 25 ohms at 5 volts.QuoteI agree although I know it will work albeit with poor reliability compared to a properly designed circuit.

author: westfw
author_rank: Super Contributor
timestamp: June 11, 2019, 04:39:17 am
content: QuoteCMOS 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...

author: james_s
author_rank: Super Contributor
timestamp: June 11, 2019, 05:16:36 am
content: 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.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 11, 2019, 08:30:50 am
content: There is no such thing as good or bad design practice, only more or less appropriate to a particular situation.