Hello all
This is my first post - I have very little knowledge when it come to mA or Voltage, I'm hopping to get schooled on this?
I am working on a little project on my classic Mini - I have converted my heater matrix from the tradition pull cord/wire to using a small stroke 12v actuator, using a DPDT switch to operate the forward/reverse, this work perfectly but need a red LED to show forward and a blue LED to show reverse. I have purchased a TRI-color LED display - RGB (Only need RB) not to sure on the voltage I need? I am going to need 12v supply as its going in the car, the diagram I have states 20mA and the voltage of the LED's is between 2v/3.8v I have a 3v power brick that runs the blue and green but the red is very faint, no idea what I'm going here but I do no what I would like to achieve.. Thanks for any help on this. please see the diagram/attachment

 

just slap 3 x 470 ohm resistors on the 3 cathode leads, one for each LED

The +12 goes to the anode.

 

just slap 3 x 470 ohm resistors on the 3 cathode leads, one for each LED

The +12 goes to the anode.

Thanks Senacell, will the resistors be ok if the car battery is pushing 14,5 volts?

 

You should probably use higher value resistors to make sure the LEDs don’t see more than 20mA. If the red LED has a voltage drop of 2.0V you should use a resistor which drops 12.5V at 20mA. Using Ohms Law that’s a resistance of 12.5/0.02 = 625 ohms. The nearest standard value above this is 680 ohms - always use a higher value to reduce the current. In theory you could use a smaller value for the blue LED but the same value is easy and the perceived brightness of blue LEDs is greater anyway. You may actually decide to use a higher value for the blue LED to match the perceived brightness of red and blue

I’m slightly concerned you may have damaged the red LED by connecting 3.0V across it, forcing more than the specified current.

 

Thank you so much, I have 2 of there's just in case I damaged 1 of them. so if I have this right? I need 2x 680 ohms resistors connected to the red and blue wires and nothing on the black wire? One last thing? will the car's output from the battery be ok with this set up? could push out 14v/15v when running.....

 

You should be OK if you use half watt resistors. Quarter watt resistors is either cutting it close or exceeding the rating. Since the red has a forward voltage drop of 2.0 to 2.4 volts, assume typical 2.0 and assume your charging system is actually pushing 15 volts for a short period of time (typically) (15 - 2.0) ÷ 680Ω = 19mA through the LED. As voltage settles down to typically 13.6 to 13.8 your current should drop to (13.6 - 2.0) ÷ 680Ω = 17mA. At that small difference you won't notice a change in the brightness.

As for the other two colors, 3.5Vf (Voltage Forward - or Forward Voltage) the current through those at startup will likely be around 14.6mA to 17.7mA.

Worst case (15VDC) the wattage will be 15V x 19mA = 285mW (more than 250mW (quarter watt) resistor). Half watt is in order. Quarter watt will burn up.

 

20mA is usually the highest that LED can handle. it is a good idea to not push it to the limit.
i usually try to use some 10mA per LED unless datasheet is available to confirm limits.
btw. apparent brightness does not change much so 8-15mA is still going to look "the same".
i would say use 1k resistors, 1/4W is enough. this will get you current values right in the sweet spot.

 

Thank you all so much, I've been schooled, I will update next week when I put the theory to the test

 

Please post a schematic of your current setup, showing the motor(s), switch contacts, power sources, etc. Without this, it is risky to say how to connect the LEDs for the direction indication you want. The LEDs common-anode connection might make thing a bit messy.

ak

 

I am working on a little project on my classic Mini

Common anode will be a problem with modern vehicles that switch things that have a common ground.

 

20mA is usually the highest that LED can handle.

First; I agree. 20mA should be the upper limit.
Second; I've seen LED's run fine (bright as all get-go) on 30mA. I also recall data sheets stating that 30mA is max current for a 5mm LED.
Not looking to argue. Just my observations in the past.

 

of course... many types exist and specs do differ. since it was not clear what the exact model is, i was purposely offering values on the safe side, something that would work reliably for long time. automotive environment is harsh enough with voltage swings, temperature changes and what not.