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I'm driving a blue 7-segment LED display with a TI CD4026BE on a 3.3V circuit.

I cannot find any data sheet for the display part. It has a common cathode pin layout and it's marked FJ5161AB on the side and D5611 A/B on the bottom.

At first I tried putting some conservative resistor values, but then I noticed that the formula would give a Vf (LED drop) pretty close to Vcc. So I tried connecting the LEDs directly to the chip's outputs (3.28V) and the current consumption is 0.67mA per LED.

I know that blue LEDs can drop up to 3.3V. Is this current value low enough that I can get away with no series resistors?

Here's a couple pictures:

enter image description here

enter image description here

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    \$\begingroup\$ Never buy electronic parts without data sheets. Sorry, it's something you MUST learn. \$\endgroup\$
    – Andy aka
    May 16, 2015 at 17:33
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    \$\begingroup\$ Andy, that only really applies to production, not one off hobby projects. \$\endgroup\$
    – Passerby
    May 17, 2015 at 2:08

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A reasonable guess would be that you'll be fine if the LEDs appear bright enough to you.

4000-series CMOS is capable of very little current with a 3.3V supply, so the output transistors are acting like current sources/sinks. Typical is only 1mA at 5V supply.

Of course if you're using 74HC4000 series rather than the datasheet you linked, the situation would be a bit different.

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  • \$\begingroup\$ I'm actually using a CD4026BE. You're right, the CMOS transistors are limiting the current output to 1.2mA per pin. Still that's higher than the 0.67mA my LEDs are draining, so I guess the limit here is in the LEDs. \$\endgroup\$
    – Tobia
    May 16, 2015 at 18:50
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    \$\begingroup\$ It's not one or the other, but the crossing point of two nonlinear curves of voltage vs. current. One curve for the LED and one for the output MOSFET (one subtracted from supply voltage). \$\endgroup\$
    – Spehro Pefhany
    May 16, 2015 at 22:38
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    \$\begingroup\$ Usually LEDs are okay with 15-20mA so you are far from damaging the LEDs. \$\endgroup\$
    – Spehro Pefhany
    May 16, 2015 at 22:44

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