Trouble Wiring a Simple LED with a Micro820 Controller

[Anthony Blanchette-Potvin 0]

Hi everyone,

Recently, I started working with the Micro820 Controller Embedded I/O. My project is simple : when the output _IO_EM_DO_00 is set to true, an LED light turns on. But, even though it's super simple, I can't make it work.

Here's my setup. If you need more information, feel free to ask anything !

[Joe E. 242]

Is your LED wired with the correct polarity?

What is the max mA rating of the LED?  (verify you have enough resistance in series with it; if not, you may have already fried the LED)

Do you know for sure that the LED is good?

It's not clear from my quick glance at the user manual for the 820, but are both of the -DC24 terminals connected together in the controller?  If not, you will need to jumper them together or put the lead from the LED under the same terminal as the wire from the power supply.

 

User manual:

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/2080-um005_-en-e.pdf

 

[Anthony Blanchette-Potvin 0]

1. Anode is wired on DC+ and cathode on DC-. So, I assume that the LED is wired correctly.

2. It's a really basic LED, so, based on a little research, I assumed that the LED needs from 1.5 V to 3.2 V on a 0.20 mA intensity.

3. Based on my calculation...

Nevermind, the LED was fried, thank you !

[panic mode 235]

LEDs are polarized devices like all diodes. When forward biased voltage drop is usually 2-3V and current is usually 5-20mA.

To operate on higher DC voltage al you need is series resistor but 10K will be too high value (LED will likely show some output but very dim). 

If we assume 15mA (0.015A) and 3V for LED, then resistor will be 21V/0.015A = 1400 Ohm. Nearest common value is 1.5k and it should be rated for 0.5W 

 

But keep in mind that reverse biased LED will not survive if it is connected to voltage that exceeds some 6-7V. 

common solution is to either make sure this will never happen or protect LED somehow.

simplest solution is to wire another diode anti-parallel to LED. another option is to use bridge so polarity will not matter.

 

[Anthony Blanchette-Potvin 0]

Just now, panic mode said:

To operate on higher DC voltage al you need is series resistor but 10K will be too high value (LED will likely show some output but very dim). 

Yes. I forgot to mention that I reduced the resistance to 1.2 kΩ and removed the resistor placed after the LED. That said, 10 kΩ was definitely way too much.