I suggest reading the data sheet for the CMOS device, and noting what the output voltage and current limits are. NOT ALL DEVICES ARE THE SAME!!

 

Is there a missed chance those seven segment displays do have a single limiting resistor at their common terminal ?

 

Is there a missed chance those seven segment displays do have a single limiting resistor at their common terminal ?

Not unless it's built in to the display itself.

 

Is there a missed chance those seven segment displays do have a single limiting resistor at their common terminal ?

depending on displayed number, different number of segments is active so single resistor would not work. when current limit is done through resistor, one is needed for each segment

 

btw LEDs are not all equal. some of them are VERY bright even though they operate at low current.
i found the hard way that reading datasheets did not really prepare me for that. so bunch of PCBs designed to use KT-0603x or KT-0805x as status indicators were showing the usual specs (max 30mA, test 5mA...). it turned out that 4-5mA per LEDs was pleasant on just about every LED i could find. but those KT-xxx are simply blindingly bright and unbearable to look at. they are better suited for illumination than indication. green, white and blue are really bright, red is not as much. i ended dropping the LED current from 4.7mA to 1mA for red, and 0.5mA for the rest.

 

not sure about 7seg, but there used to be discrete LEDs with inbuilt limiting resistors.
I think they were for 5V circuits. Might have first seen them in a Tandy store, late 1970s!

(quick Google search) Yes they still exist.

 

Is there a missed chance those seven segment displays do have a single limiting resistor at their common terminal ?

As panic mode commented, a single limiting resistor would not work. It would be correct current/brightness if a single segment was driven/illuminated, or a seventh of the current/brightness if you were displaying the number 8.

(or correct for most numbers, but too bright displaying number 1)


P.S. I once made a handheld logic probe with a little 4 digit seven segment display.
I make it display 0, 1, and P by overdriving the segments I wasn't going to use, to burn them out.

 

Here's Roman Black driving a seven-segment display from a PIC, using duty cycle to control current load.

https://www.romanblack.com/led_none.htm

He shows calculations to derive approximate average current for segments, on the basis that the average current should be effective at limiting damage due to heat.

 

The green LED you are using has a specifications curve plotting the current and its forward voltage.
If at 10mA its Vf is 2.0 V; the led can be driven by either 10mA with a limiter resistor or with 2.0V without a limiting resistor from the IC pin.

The supply voltage of the IC being 5.0V needs to drop 3V. Four Si diodes in series to the LED does it. Or two green LED instead of one plus one Si diode does it. (if no Vdrop inside the chip)

+5V------------74HC595-------------LED---------\/\/\/\/\300Ω---------------gnd
or
+5V------------74HC595-----------LED-----|>|----|>|----|>|----|>|----gnd
or
+5V-----------74HC595-------------LED----LED-----|>|--------------gnd

Where gnd can be another 'low' pin of the same IC.

Try it with an adjustable voltage supply : Apply 2.000V to your green LED with no resistor: will drain 10mA.
Or, feed 10.00mA from an adjustable current supply to your green LED : will show 2Vf drop.

So yes, a LED can be driven without current limiting if its Vf is fixed.

Only if you can find a universe where every LED of that type is perfectly manufactured from before-unheard-of-materials that have no temperature dependence at all.

In this universe, with real diodes from real manufacturers made from real materials, if you take ten different diodes and apply 2.000 V to each of them, you will get ten different currents that span a range of values. It is unlikely that any of them will draw 10.00 mA. Also, if you monitor the current over time, you will discover that the current will start at one value and then change as the LED warms up, increasing until thermal equilibrium is reached.

You really need to learn more about LEDs (and semiconductor junctions in general) and what a data sheet does and does not tell you. The curve you refer to is a statistical model for a "typical" device. Real devices operate in the vicinity of that curve. How close they are can be roughly divined from close inspection of the min/max values in the datasheet, if that information is supplied (which it often isn't).

 

One additional detail is that most IC outputs have an effective internal resistance that will limit the current.

 

If you are committed to using a '595 shift register and no extra parts, you could look at other logic families. For example, when an output of the (obsolete and now expensive) 74LS595 is in the high state, it isn't driven directly with a transistor to VCC but pulled high through an internal resistor. One datasheet I looked at said the resistance is typically about 100 ohms, which is too small for your application (to drop 5V to 2V at 10mA you need 300 ohms). You would also want to check the max current spec and whether it could handle that on multiple output pins at once before designing it in.

You could also look at using a lower power supply voltage and/or diodes with larger voltage drop (check out different colors). It might seem that if you set the power supply to match the voltage drop of the LEDs that you wouldn't need a resistor. But as others have mentioned, the voltage drop across the LED is temperature dependent and the current through the LED might change drastically under different conditions.

The ideal way to compensate for that is to use a constant current driver, so the voltage drop across the LED has no effect on the current. Using a series resistor is a middle ground; it reduces the change in current when the LED's voltage drop changes. It does a better job of keeping the current close to the target when you use a higher supply voltage (and larger value resistor), but it also wastes more power and generates more heat. When using a resistor, make sure to design around the worst case numbers or leave some margin (e.g. target 8mA rather than 10mA) to account for the fact that your power supply voltage might run a bit high, the resistor might be at the bottom of its tolerance range, the LED voltage drop might be a the low end of its range, etc.

 

The ideal way to compensate for that is to use a constant current driver, so the voltage drop across the LED has no effect on the current.

Agree entirely, saves components, allows mix of LED colours, and you can get constant current drivers with as many as 52 outputs, so saves ICs as well.