5. Select a Resistor to Drive an LED
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NabilMohamed
Jun 15 2026
Chosen DigiKey Part Number: RMCF1210FT453RCT-ND
Selection Approach:
- Computed desired voltage drop across the resistor:
- (12V = V_r + V_led = V_r + 2 => V_r = 10V)
- Computed nominal resistor resistance (R_ideal = 454.545454... Ohms):
- R_nominal = V_r / I_r = 10 [V] / 0.022 [A] ~= 454.54 [Ohms]
- It seems unlikely that one would be able to find a resistor having a resistance of exactly 454.545454... [Ohms].
- Selected an actual resistance value R_acc to filter for (R_acc == 453 [Ohms])
- Would it be better to overestimate or underestimate the nominal resistance R_nominal? If we go for a slightly higher resistance, then we would be getting a slight underestimate of desired current going through the LED. On the other hand, a lower chosen resistance would mean a little more than desired current going through the LED. It seems that it would be better for the LED to be slightly brighter, than to be slightly less bright than we expect. A rationale for this would be that if the LED is supposed to signal something to the user, the user would be less likely to miss the signal if it was slightly brighter than expected, than if it were less so. In either case, we would want to pick a resistance as close as possible to R_nominal. On the website, we opted to filter for resistors with an actual resistance R_acc = 453 [Ohms].
- Computed lower bound on resistor power rating (P_r > 0.44W)
- What current I_r would be going through the resistor? What power P_acc would the resistor consequently be dissipating? After identifying P_acc, we can establish a lower bound on the resistor power rating P_r: Namely, that P_r > 2*P_acc.
- I_r: With a 453 [Ohm] resistor, the current going through it would be V_r / I_r = 10[V] / 453 [Ohms] = 0.022075.. [A] (slightly more than the desired 0.022 [A]).
- P_acc: P_acc = 10 [V] * 0.022075 ... [A] = 0.22 [W] (used saved current value in calculator)
- => P_r > 0.22 * 2 = 0.44 [W]
- I tried to look for resistors with power rating > 0.44 [W], and settled on one with a power rating of 0.5 [W].
Another constraint to consider are the packaging dimensions: should be 1210 imperial, i.e., the SMD (thick film) resistor should be of length and width 0.12" x 0.10" (approximately)). I was not sure how to choose between the three packaging options (tape and reel, cut-tape, digi-reel) so opted for one at 'random' (cut-tape, for no particularly rigorous reason).
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