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I have a very simple problem to solve and I'm surprised there is no common solution for it.

I need to connect several indicator LEDs to microprocessor outputs and I know I need a serial current-limiting resistor. To save space I'd like to have LED that takes 3.3v without resistor. There is a couple out there rated 3.3v but I understand it is not safe to connect it without resistor even though it is 3.3v rated.

Also they are 20mA, I'd prefer 10mA since they are driven directly by MPU with 200mA total output limit.

Since this is such a common problem, I'm sure it must be solved and I might be just looking into wrong direction. Please enlighten me!

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closed as off-topic by Dmitry Grigoryev, Voltage Spike, DoxyLover, Dave Tweed Sep 30 '17 at 12:10

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    \$\begingroup\$ Just how tight are you on board space?! \$\endgroup\$ – ThreePhaseEel Sep 4 '17 at 0:27
  • \$\begingroup\$ You have created a very difficult requirement from a very simple solution to regulate the current between a voltage source and voltage dependent current sink (LED). If you understood the simple physics, and knew how to select LED sources you could reduce your microprocessor from 3.3 to something like 3.1V !done! Otherwise a tolerance calculation is essential with all series R's of driver and load to meet your explicit specs for min/max current. \$\endgroup\$ – Sunnyskyguy EE75 Sep 4 '17 at 0:41
  • \$\begingroup\$ which uC do you want? \$\endgroup\$ – Sunnyskyguy EE75 Sep 4 '17 at 1:33
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    \$\begingroup\$ LEDs with internal current limiting are available, but AFAIK not in SMD and 10mA. Which microprocessor (and what outputs on it) are you connecting them to? \$\endgroup\$ – Bruce Abbott Sep 4 '17 at 2:25
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    \$\begingroup\$ @m_kramar please ask a specific question, you'll get better answers. Also provide more background on what you intend to do \$\endgroup\$ – Voltage Spike Sep 22 '17 at 6:15
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Understand that the colour, physics and chemistry of the semiconductor determines the "zener-like" forward voltage, Vf and it's tolerance depends on the ESR variations from process controls at the test current used.

The general solution can be simple.

If you choose Red or Yellow with a 3.3V drive signal on a 2V LED, you need a Resistor.

If you choose Blue, white or green, you may not need a resistor.

You must examine the choice of both uC driver and LED to ensure this meets your power budget tolerance.

Let's examine the ATtiny20 which has typical CMOS design RdsOn rated for <=5V enter image description here

Then look at a tiny SMD Green LED.

enter image description here

What current would you expect?

for a high side LED turn on current at 3.3V with no resistor?

Overlapping the a 60 ohm load line from 3.3V , where it intersects w2ith the LED curve determines the resulting current.

Answer?

About 7mA enter image description here

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LEDs have a very steep voltage verses current curve as see in the following plot taken from this page:

enter image description here

Notice how fast the current rises once you forward bias the diode (as seen in the green colored quadrant of the plot). This is why designs include a current limiting resistor in series with the diode. It is true, if you monitor the voltage very carefully you can likely get by with out a current limiting resistor. But using a resistor is the easy & safe way to design the circuit.

Also, different color LEDs operate at slightly different voltages. It would be much easier to design a circuit using different current limiting resistors instead of a circuit using different power supplies, one for each differently colored LED.

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    \$\begingroup\$ By the way, if these are indicator LEDs, then 20mA is an awful lot of current. Most modern indicator LEDs (for about the past decade) now draw less than 10mA. Many only 3mA. \$\endgroup\$ – st2000 Sep 4 '17 at 0:38
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The internal Equivalent Series Resistance of a microcontroller output, as well as the higher forward voltage for a target forward current of some leds (3.3~3.6V Blue, Green, White, etc) means you can directly connect some leds to the output without damage. The output voltage of the pin will fall as the current increases, due to the ESR, and the two will balance out.

A low forward voltage LED, such as red, will not play as nicely.

Frankly, you should just play around with it to see how the voltage and current through the microcontroller pin and led work out. You are really just risking the microcontroller and the led, a few bucks at worst.

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The forward voltage of "bare" LEDs depends on the LED colour. LEDs are not at all fussy about their operating current, as long as you don't exceed their Absolute Maximum rating. LEDs will be dimmer at lower currents, but may still be bright enough for your use at 5 mA or less (I have some green LEDs that I had to run at somewhat under 1 mA to get them dim enough!)

There are some LEDs that include a current-limiting device, so don't need an external resistor. These may be advertised as "5 Volt" or "12 Volt". A "5 Volt" red or yellow LED may be OK at 3.3 volts, as a bare red or yellow LED will want a little less than 2 volts.

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