Simple LEDs in series question

[jappy]

Please refer to the attached image:


Does the image at the bottom equal the same at the top? Meaning, I have 4 LEDs in series with a 220ohm resistor powered at 12volts (want the LEDs ran at 20ma). Will the image at the bottom match these specs?

Thanks for any insight.

 

[gillestugan]

No, as the current through the resistor is four times higher (4*20mA) in the second image, you need 1/4 of the resistance in the first picture. (55 ohm)

Otherwise you would get only 5mA through each string. (Assuming the leds would maintain the same Vf)

 

[yellow]

take Your 1st line and do that four times, means connect each line at the arrows
each line has to have its individual resistor


PS: I am not sure on the 220 Ohms resistor, what are Your settings? No white led I assume?

 

[frenzee]

No, as the current through the resistor is four times higher (4*20mA) in the second image, you need 1/4 of the resistance in the first picture. (55 ohm)

Otherwise you would get only 5mA through each string. (Assuming the leds would maintain the same Vf)

No, that is not correct, however as "yellow' suggested, each string needs to have it's own resistor to balance the current in each circuit. Furthermore, it would be a good idea to connect the LED-to-LED connection points (as in a matrix) to further balance out the current distribution and to avoid the entire circuit going dead if one LED fails.

 

[gillestugan]

Well, I did write "Assuming the leds would maintain the same Vf" And in that case you need to have 4.4V over the resistor at 80mA which gives 55 ohms.

Now in real world Vf is not stable and there may be imbalance between the leds.
This is why it's best to limit the current in each string separately with one resistor /string of leds. Same resistor value as in first picture, as yellow and frenzee pointed out.

 

[jappy]

Thank you.

PS: I am not sure on the 220 Ohms resistor, what are Your settings? No white led I assume?

As far as the 220 ohm resistor, I used this LED series array wizard And I actually meant for the LEDs to be ran @ 30mA's. Here are my specs:

• 12v source voltage
• 4 red LEDs
• 2v forward voltage
• 30mA forward current

With these specs the wizard suggest a 150ohm resistor. Sound correct?

I plan on making some outdoor LED clusters with the above series of LEDs. To water proof them I am going to pour some epoxy over the final circuit board. Would doing this cause any problems, in terms of heat, with the resistors?