LEDs on output of 74HC00 - Page 1
Forum section: electronics / beginners

author: Chai
author_rank: Regular Contributor
timestamp: September 07, 2017, 08:05:26 pm
content: Does this mean I could attach an LED to each Y direct to gnd and expect 4mA without damage? Does the loading require a resistor in series?74HC00 datasheet:http://www.ti.com/lit/ds/symlink/sn74hc00.pdf

author: Zero999
author_rank: Super Contributor
timestamp: September 07, 2017, 08:29:27 pm
content: Quote from: Chai on September 07, 2017, 08:05:26 pmThe short answer is no. It just means the output voltage will be at least 3.7V, when the output current is 4mA, indicating a maximum resistance of R = (4.5 - 3.7)/0.004 = 200Ohm. The typical figure shows 4.3V, when the output current is 4mA, which gives an output resistance of R = (4.5 - 4.3)/0.004 = 50Ohm. Again that's a typical figure, it could be lower.Note, that this is with a supply voltage of 4.5V. If one runs through the above calculations again, they'll get higher resistances at lower supply voltages, and lower resistances as the supply voltage is increased.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 07, 2017, 09:19:22 pm
content: A LED is about a 20mA load, so I don't see that driving anything at 4mA.

author: Chai
author_rank: Regular Contributor
timestamp: September 07, 2017, 09:43:41 pm
content: @hero999 -- thanks. so at the top of the datasheet this just means you should limit output to -5mA to get good operation at 5V VCC?@Quarlo -- they light fine as low as 3mA but they're dim.

author: langwadt
author_rank: Super Contributor
timestamp: September 07, 2017, 10:34:12 pm
content: Quote from: Quarlo Klobrigney on September 07, 2017, 09:19:22 pmyour granddads LEDs maybemodern LEDs are often plenty bright at a few mA

author: Zero999
author_rank: Super Contributor
timestamp: September 07, 2017, 10:57:39 pm
content: Quote from: Chai on September 07, 2017, 09:43:41 pmIf the current output is greater than 4mA, then the output voltage might be outside the range required to give a valid logic level. This means if an output is driving too higher current, it might not be able to reliably drive another logic input. The maximum current output rating is 25mA. The total maximum current in/out of all of the output pins in the IC is 50mA.

author: Chai
author_rank: Regular Contributor
timestamp: September 07, 2017, 11:39:43 pm
content: Quote from: Hero999 on September 07, 2017, 10:57:39 pmGot it.Final question, are the guys who write these 7x00 series datasheet all allergic to just putting a table explaining the pinout?

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 08, 2017, 12:24:58 am
content: "your granddads LEDs maybemodern LEDs are often plenty bright at a few mA"Not my granddad's LED, he had no electricity until the 70's!But I stand corrected, there they are with 5mA and such. Who knew!

author: Audioguru
author_rank: Super Contributor
timestamp: September 08, 2017, 02:49:55 am
content: HC Cmos output currentis massiveif the output transistor is allowed to have a substantial voltage drop. Its typical output current is only 4mA when it has a 5V supply and an output voltage drop of only about 0.2V. When its output is shorted then the output current will try to be typically 51mA. Driving a 1.8V red LED with no resistor the current is typically 47mA which will zap the LED. BUT the maximum allowed output current is only 25mA so you must use a series resistor to limit the current.Here are graphs showing 4.5V and 6V supplies and the output current at various output voltage drops:

author: Chai
author_rank: Regular Contributor
timestamp: September 08, 2017, 04:44:48 am
content: Quote from: Audioguru on September 08, 2017, 02:49:55 amI did try a red led direct to ground and I got around 45mA @ 5vcc. Where did those graphs come from?

author: Zero999
author_rank: Super Contributor
timestamp: September 08, 2017, 08:15:22 am
content: Quote from: Chai on September 07, 2017, 11:39:43 pmProbably here. Look at page 34 or 35, depending on whether you go by the PDF page number or not.https://assets.nexperia.com/documents/user-manual/HCT_USER_GUIDE.pdfQuote from: Chai on September 07, 2017, 11:39:43 pmThe pin out is quite clear, in my opinion.Perhaps this data sheet is more clear:https://mil.ufl.edu/3701/docs/74HC00_ST.pdf

author: TomS_
author_rank: Frequent Contributor
timestamp: September 08, 2017, 12:26:22 pm
content: Quote from: Chai on September 07, 2017, 11:39:43 pmPage 3?

author: wraper
author_rank: Unknown
timestamp: September 08, 2017, 12:46:56 pm
content: Quote from: Quarlo Klobrigney on September 07, 2017, 09:19:22 pm20mA is MAXIMUM rated current for most of the small LEDs. You should not drive them at such current if expect high reliability. 4mA is more that enough to achieve high brightness with modern ultrabright LEDs.

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2017, 07:47:34 pm
content: The fact that a 4 milliamp output drive is a feature of HC/HCT logic means that you can reliably draw that much current continuously but you will still require a current limiting resistor. Somewhere between 220 and 470 ohms is typical but high efficiency low current LEDs could use 1 kilohm.Most indicator LED types have reliably worked on 4 milliamps almost since they were made available and some were intended to be driven on only 1 milliamp.Quote from: Chai on September 07, 2017, 11:39:43 pmSome manufacturers produce much better datasheets than others.

author: Chai
author_rank: Regular Contributor
timestamp: September 08, 2017, 08:05:41 pm
content: Quote from: TomS_ on September 08, 2017, 12:26:22 pmOoh yea. This nand pdf is OK butsome aren't. I was just generally frustrated.

author: Zero999
author_rank: Super Contributor
timestamp: September 08, 2017, 09:17:21 pm
content: Quote from: Chai on September 08, 2017, 08:05:41 pmPages 1, 2 & 3?Are you struggling with interpreting it? Is it that the power supply pins are only shown on the first page and not on the schematics? That's fairly standard on logic component data sheets. The 74 series normally uses the bottomrightleftpin for GND and the top right for Vcc.

author: TomS_
author_rank: Frequent Contributor
timestamp: September 16, 2017, 05:48:49 pm
content: Quote from: Chai on September 08, 2017, 08:05:41 pmI find the NXP datasheets to be generally pretty good. I use them a lot as a reference.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 16, 2017, 08:53:34 pm
content: Quote from: Hero999 on September 08, 2017, 09:17:21 pmAre you quite sure about that?

author: Zero999
author_rank: Super Contributor
timestamp: September 16, 2017, 10:55:41 pm
content: Quote from: alsetalokin4017 on September 16, 2017, 08:53:34 pmTypographical error. Fixed.

author: EEVblog
author_rank: Unknown
timestamp: September 16, 2017, 11:24:52 pm
content: Quote from: Hero999 on September 07, 2017, 08:29:27 pmErr, the answer is yes. You can easily drive a LED direct from the output of a HC chip at 4mA with a dropper resistor.Yes the output voltage will drop, but who cares unless you are driving other logic with the output.

author: EEVblog
author_rank: Unknown
timestamp: September 16, 2017, 11:28:15 pm
content: Quote from: wraper on September 08, 2017, 12:46:56 pmEven old LEDs worked fine on a few mA or two.This digital clock I did using 1980's vintage LED displays worked on around 1mA per segment IIRC.

author: Zero999
author_rank: Super Contributor
timestamp: September 16, 2017, 11:31:31 pm
content: Quote from: EEVblog on September 16, 2017, 11:24:52 pmNo, I didn't say that. Please read the original poster's first post again!Quote from: Chai on September 07, 2017, 08:05:26 pmI thought I'd leave it up to the original poster to figure out that a separate series resistor is required.