LED Connection to OC Output Connection Card

[AZWildk4t]

Can someone clarify where the resistor should be placed when using the OC Output Connection Card? http://www.opencockpits.com/catalog/outputs-connection-p-69.html?cPath=21_27

According to the OC USB Expansion Master Manual Revision 2.1, i find it confusing. See screenshot. The first diagram between the positive connection and the anode (+) side of the LED. However, in the second diagram they show the resister on negative side, connected to the cathode (-).

Which is correct? Currently I have it where I have a single resistor connected to the GND and the cathode of all LEDs strung together. Then each anode is connected to the terminal connectors P3-40.

[iwik]

Hi,
Doesn't matter which lead the resistor is connected to. The resistor just limits the current thru the diode.

Les

[AZWildk4t]

Thanks Les. With SIOC I was able to verify each LED on my MIP is working. However, when I turn on all the LEDs at once, I noticed several of them do not light up, specifically the green LEDs for the Speedbrake Arm, Nose Gear Down, Left Gear Down, and Right Gear Down. So I turned all the LEDs off again and turned on Speedbrake Arm. Then I added 1 light at a time. As I did this, I noticed the Speedbrake Arm led slowly go dimmer and dimmer. I am puzzled by this.

Thanks,
Steve

[Zlatko]

Hi Steve,
As Les told you, it doesn't matter if you connect resistor to anode or cathode. But you have to connect one resistor for each led. Don't just tie all anodes or cathodes to a single resistor. All led's have their individual forward voltage, even they are of a single colour and they most certainly have different forward voltage if they are different colours.
The formula to calculate necessary resistance is very basic:
Resistor ohms = ( V1 - V2 ) / Amperage of the Led.
V1 is the voltage you powering the led with. V2 is the forward voltage for the led. There is info out there what is led forward voltage for each colour : https://www.google.bg/search?q=led+forward+voltage+by+color&es_sm=93&tbm=isch&imgil=mUHfeQl0egY0SM%253A%253BFOpq0lx5tlqVGM%253Bhttp%25253A%25252F%25252Fdangerousprototypes.com%25252Fdocs%25252FBasic_Light_Emitting_Diode_guide&source=iu&pf=m&fir=mUHfeQl0egY0SM%253A%252CFOpq0lx5tlqVGM%252C_&biw=1280&bih=655&usg=__VwK6Sfy_JTaSLvMVs0YZrNa5Qaw%3D&ved=0CDwQyjc&ei=NdifVam3BsOXsAGRorfgDg#imgrc=4CyFU6eK6ZcqYM%3A&usg=__VwK6Sfy_JTaSLvMVs0YZrNa5Qaw%3D

Amperage of the led is usually 20mA , but best is to verify with the seller of you led's.

Let's imagine you powering red led at 12 volts . Red led forward voltage according to table is 2.1 v. and amperage is 20mA. Then the resistor you need would be ( 12 - 2.1 ) / 0.02 Amps = 500 Ohm resistor or above.
Now lets imagine you powering a blue or white led and their forward voltage is 3.2 v . Then resistor used should be:
( 12 v - 3.2 ) / 0.02 Amps = 440 Ohm or above. If you power it with same resistor as per Red led , blue led would be dim . Things get worst if you power them with 5v . Or if you connecting all anodes or all cathodes to a single resistor - then while some of the led's are open and draw a lot of current and might burn, others would be dim or not lit at all ( as you notice ). Just tie each led with it's own resistor and you should be all-right.
If that does't help , then check what is the amperage of your power supply and if it limited or "unlimited" compared to your total consumption.

[AZWildk4t]

Thanks Zlatko. I have everything tied to a single resistor. It really makes a mess with a resistor to each LED. I'm now curious where most builders put the resistor - to the OC Output Card terminal connect or directly behind the Led. Guess it doesn't matter cuz it's a wiring mess anyways.

Steve

[Zlatko]

I personally solder the resistor directly to led's anode ( 4 seconds max not to damage led ). Then - resistor ( led's anode ) to card. Cathodes tied together - to common Gnd.
And some heat shrink tubes are probably your best friend to prevent shortages- they absolutely love to wrap around solder joints, when you heat them up .