0
\$\begingroup\$

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

\$\endgroup\$
  • 2
    \$\begingroup\$ Never buy electronic parts without data sheets. Sorry, it's something you MUST learn. \$\endgroup\$ – Andy aka May 16 '15 at 17:33
  • 1
    \$\begingroup\$ Andy, that only really applies to production, not one off hobby projects. \$\endgroup\$ – Passerby May 17 '15 at 2:08
1
\$\begingroup\$

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.

\$\endgroup\$
  • \$\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 '15 at 18:50
  • 1
    \$\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 '15 at 22:38
  • 1
    \$\begingroup\$ Usually LEDs are okay with 15-20mA so you are far from damaging the LEDs. \$\endgroup\$ – Spehro Pefhany May 16 '15 at 22:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.