johnmobley
New Member
I am very familiar with calculating resistor values and finding current through an LED under normal conditions. However, I have a circuit I am working on that appears to be out of the normal... Here is my situation, I have groups of 4 LEDs in series (Warm White at 3.0V) attached to 12V. Under normal circumstances a resistor would just not be necessary. When testing the circuit in the area it is going to be installed, I found that the LEDs were to bright for the area. I then added a 1.2K resistor in series with the LEDs.
This is where I am now getting confused. I am trying to calculate the current through this circuit and am failing miserably. I am sure I am missing something simple...
So here is what I have tried...
Standard formula for calculation of resistance for LED (modified to calculate current)
(12-12)/1200 = 0, I know that there is current so that is not right...
Knowing that all 12V are being used across the resistor and LEDs, I tried Ohms Law next
12/1200 = .01 A or 10mA
Here is where it gets really confusing for me, when I hook up my meter and measure the actual current through the circuit I find that it is only pulling 1mA. Interestingly enough all of my calculations for the other parts of the circuit are off the same amount, for example:
1 custom 7-segment LED completely lit up 140mA (calculated), 14mA (actual)
All 6 custom 7-segment LEDs completely lit up 840mA (caculated), 84mA (actual)
So long story short, I know my actual current pull but I would like to better understand how to properly calculate current in these uncommon situations. Any help would be appreciated.
This is where I am now getting confused. I am trying to calculate the current through this circuit and am failing miserably. I am sure I am missing something simple...
So here is what I have tried...
Standard formula for calculation of resistance for LED (modified to calculate current)
(12-12)/1200 = 0, I know that there is current so that is not right...
Knowing that all 12V are being used across the resistor and LEDs, I tried Ohms Law next
12/1200 = .01 A or 10mA
Here is where it gets really confusing for me, when I hook up my meter and measure the actual current through the circuit I find that it is only pulling 1mA. Interestingly enough all of my calculations for the other parts of the circuit are off the same amount, for example:
1 custom 7-segment LED completely lit up 140mA (calculated), 14mA (actual)
All 6 custom 7-segment LEDs completely lit up 840mA (caculated), 84mA (actual)
So long story short, I know my actual current pull but I would like to better understand how to properly calculate current in these uncommon situations. Any help would be appreciated.