# Seven-segment display seems to draw little current

I bought a 1.8-inch seven-segment common cathode LED display from AliExpress. So no datasheet available.

I used a 1 kΩ resistor and a 5 V source, as to put a max 5 mA in the LED. I measured 1.6 V for the decimal dot, and 3.5 V for each segment. The measured current draw for one segment was 1.5 mA.

Now, I expected each LED to be quite dim at 1.5 mA, but it was the opposite, I could put a 4 kΩ resistor and still has a usable display.

I intended to drive the display using an ATmega328P and a MAX7219 (which would have been OK up to 500 mA per segment).

Does this measurement seem correct, and if so, I can drop the MAX7219 and drive the segment directly (with current limiting resistors) with the ATmega?

• A modern single digit is easy to drive directly from a micro. If you need to drive multiple digits it gets more difficult. The digit drivers will be that DC segment current * n * 8 for n digits. – Spehro Pefhany Aug 21 '18 at 12:09
• @SpehroPefhany I need to drive 2 digits, but I have plenty of spare IO on my atmega328p ;) I could use this as an opportunity to multiplex. – mathieu Aug 21 '18 at 12:13
• I've found that modern LEDs are a lot brighter than I think. I use 10k at 5V when breadboarding stuff and it's still plenty bright as an indicator (indoors, anyway). Except for yellow ones for some reason. – immibis Aug 22 '18 at 2:40
• Note that max current draw for Microcontrollers typically is two specs: per pin and total current limit. And, typically, total != npins*max_per_pin – crasic Aug 22 '18 at 6:12