Can't solve total resistance for parallel circuit

[Langgin19]

I v/r
Ohms law
I have measured 100ma through circuit using ammeter getting 100ma but my math keeps coming up with it= 338ma
This is a parallel circuit with
12.70v battery

Connecting 14 LEDs total

4 resistors each 150ohm
4 strings of LEDs running series-parallel
S1= 2.2x4 LEDs / 150ohm
S2= 2.2x4 LEDs /150ohm
S3= 3.2x3 LEDs / 150ohm
S4= 3.2x3 LEDs /150ohm
My math says 37.5ohms for R-req
338ma
Somethings not right please help

 

[Ian Rogers]

Each LED string = 12.7v-8.8v = 3.9v... 3.9v / 150 ohm... that is 25mA... Four of them... Make 100mA in my book..

 

[Langgin19]

No they are a mix

Vf 2.2 reds ------2 strings


Vf 3.2 whites -------2 strings

Multimeter reads

12.70 v battery

100ma

Each white led 3.0 to 3.1

Each red led 2.18 to 2.25v

What I'm trying to do is fig out using ohms Lea which amps divided for each string

 

[Langgin19]

 

[Ian Rogers]

As your photo.. Each string is series connected so after calculating the vf of the three / four LED's you have @ 4v.. 4v / 150 ohms is around the 25mA.... The results you obtained seem to be right!! 4 x 25mA is 100mA...

 

[Ian Rogers]

Just been thinking!!!! Are you are totalling the mA across each LED? There are 4 circuits, each circuit consumes 25mA regardless of the amount of LED's.... Each string ( including the 150 ohm resistor ) totals 508 ohm... All four together total resistance of 127 ohms or a total power of 1.27 Watts

 

[alec_t]

Each red string: I=(12.7-4*2.2)/150=26mA.
Each white string: I=(12.7-3*3.2)/150=21mA.
For the 4 strings in parallel, total current = 2*26+2*21=94mA.
Combined 'effective resistance' = 12.7/0.094 = 135Ω.

Bear in mind that a LED is a non-linear device so its resistance is current-dependent.

 

[Langgin19]

Ok one how did you get 508ohms total i thought when you have resistors in parallel then you 1/r1+1/r2+1/r3+1/r4= 37.5ohms

What would be the math if I only new the 12.7 v battery and the 150 ohm resistors or is there no way to figure that out?

 

[Ian Rogers]

508 ohms is the total resistance in the led string + the 150 ohm resistor.. Each LED has internal resistance.. (2.2v / 25mA = 88 ohm ) ..
Remember these figures are the ideal!!


i in a circuit is always the same.. So you have 12.7v and 150 ohm...measure the V across the resistor ( will be about 4v )
4v / 150 = 26mA.. Once you have the current..... Jobs done..