LEDs on output of 74HC00

[Chai]

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

[Zero999]

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

The 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.

[Quarlo Klobrigney]

A LED is about a 20mA load, so I don't see that driving anything at 4mA.

[Chai]

@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.

[langwadt]

A LED is about a 20mA load, so I don't see that driving anything at 4mA.

your granddads LEDs maybe modern LEDs are often plenty bright at a few mA

[Zero999]

@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?

If 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.

[Chai]

@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?

If 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.

Got it.

Final question, are the guys who write these 7x00 series datasheet all allergic to just putting a table explaining the pinout? 

[Quarlo Klobrigney]

"your granddads LEDs maybe modern 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!

[Audioguru]

HC Cmos output current is massive if 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:

[Chai]

HC Cmos output current is massive if 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:

I did try a red led direct to ground and I got around 45mA @ 5vcc. Where did those graphs come from?

[Zero999]

@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?

If 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.

Got it.

Final question, are the guys who write these 7x00 series datasheet all allergic to just putting a table explaining the pinout? 

Probably 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.pdf

@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?

If 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.

Got it.

Final question, are the guys who write these 7x00 series datasheet all allergic to just putting a table explaining the pinout? 

The pin out is quite clear, in my opinion.

Perhaps this data sheet is more clear:
https://mil.ufl.edu/3701/docs/74HC00_ST.pdf

[TomS_]

Final question, are the guys who write these 7x00 series datasheet all allergic to just putting a table explaining the pinout? 

Page 3? 

[wraper]

A LED is about a 20mA load, so I don't see that driving anything at 4mA.

20mA 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.

[David Hess]

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.

Final question, are the guys who write these 7x00 series datasheet all allergic to just putting a table explaining the pinout?

Some manufacturers produce much better datasheets than others.

[Chai]

Final question, are the guys who write these 7x00 series datasheet all allergic to just putting a table explaining the pinout? 

Page 3? 

Ooh yea. This nand pdf is OK but some aren't. I was just generally frustrated. 

[Zero999]

Final question, are the guys who write these 7x00 series datasheet all allergic to just putting a table explaining the pinout? 

Page 3? 

Ooh yea. This nand pdf is OK but some aren't. I was just generally frustrated. 

Pages 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 bottom right left pin for GND and the top right for Vcc.

[TomS_]

Final question, are the guys who write these 7x00 series datasheet all allergic to just putting a table explaining the pinout? 

Page 3? 

Ooh yea. This nand pdf is OK but some aren't. I was just generally frustrated. 

I find the NXP datasheets to be generally pretty good. I use them a lot as a reference.

[alsetalokin4017]

Final question, are the guys who write these 7x00 series datasheet all allergic to just putting a table explaining the pinout? 

Page 3? 

Ooh yea. This nand pdf is OK but some aren't. I was just generally frustrated. 

Pages 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 bottom right pin for GND and the top right for Vcc.

Are you quite sure about that?

[Zero999]

Final question, are the guys who write these 7x00 series datasheet all allergic to just putting a table explaining the pinout? 

Page 3? 

Ooh yea. This nand pdf is OK but some aren't. I was just generally frustrated. 

Pages 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 bottom right pin for GND and the top right for Vcc.

Are you quite sure about that?

Typographical error. Fixed.

[EEVblog]

The short answer is no.

Err, 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.

[EEVblog]

20mA 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.

Even 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.

[Zero999]

The short answer is no.

Err, 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.

No, I didn't say that. Please read the original poster's first post again!

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? 

I thought I'd leave it up to the original poster to figure out that a separate series resistor is required.