Sorry to drag up a dead post…
I’m not an EE and I’m having trouble figuring out the math on the resistor calculation. I’ve tried the circuit described in the post, and it works but my math doesn’t agree with the physical results. Please help me find the error in my logic.
I start with two resistors in series, the current limiting resistor and the heater and a 5 volt source. Applying ohms law, the equation is 5 = 0.12*(R + 4.5). Solve for R and you get approximately 37 ohms.
From here I want to find the voltage drop across the current limiting resistor so I apply ohms law again: V = 0.12*37 = 4.44 volts, which only leaves (5-4.44) or 0.56 volts for the heater.
I used a 33 ohm resistor and I measured the resistance on the heater to 4.3 ohms but with those resistors and a 5 volt source, I should see 130 mA but there’s only 120…
What gives?