SME
Embedded Systems · Problem type
An LED’s resistor on a pin
Use this for “A 3.3 V pin drives an LED with a 2 V drop at 10 mA. What resistor goes in series?”
Diagram
VR = VDD − VF = 3.3 V − 2 V
VR = 1.3 V
I
=
VR
R
=
1.3
V
130
Ω
=
10
mA
The LED takes its forward drop and lights while current flows the way it points; the resistor sets that current.
This is an example. Type your own values to start a new problem.
VR (V)
=
VDD (V)
−
VF (V)
R (Ω)
=
VR (V)
÷
I (mA)
Iₘₐₓ
Pin current rating
example
PR
Resistor power
calculated
Formulas
VR = VDD − VF
1.3 = 3.3 − 2
R
=
VR
I
130
=
1.3
0.01
PR
=
VR
×
I
0.013 = 1.3 × 0.01
Formulas are worked in A, W. The step-by-step shows the conversions.
Assumptions
•
The pin drives high at VDD, and the LED drops a fixed forward voltage VF once it is on.
•
The resistor takes what is left, VDD − VF, and sets the one current through both parts.
•
The current must stay within the pin’s rating Iₘₐₓ from its datasheet, often about 20 mA.
Step-by-step
WE KNOW
VDD = 3.3 V, VF = 2 V, I = 10 mA, Iₘₐₓ = 20 mA
FIND
resistor voltage (VR), series resistance (R), resistor power (PR)
Step 1 · Convert to the formula’s units
The formulas don’t work directly in the units you chose, so convert first (to A, W).
I = 10 mA = 0.01 A (1 A = 1000 mA)
Iₘₐₓ = 20 mA = 0.02 A (1 A = 1000 mA)
Step 2 · Find resistor voltage (VR)
Use VR = VDD − VF
Kirchhoff’s voltage law: the LED takes its drop and the resistor takes the rest.
VR = 3.3 − 2
VR = 1.3 V
Step 3 · Find series resistance (R)
Use R = VR ÷ I
Ohm’s law for the resistor: its voltage over the current it must pass.
R
=
1.3
0.01
R
=
130
Ω
Step 4 · Find resistor power (PR)
Use PR = VR × I
The resistor turns its voltage times its current into heat; pick a part rated above it.
PR = 1.3 × 0.01
PR = 0.013 W
Step 5 · Convert the answers
Change each answer back to the unit you chose.
PR = 0.013 W = 13 mW (1 W = 1000 mW)
Check
Put the numbers back in. Both sides should match.
1.3 = 3.3 − 2 ✓
130
=
1.3
0.01
✓
0.013 = 1.3 × 0.01 ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
Related lessons
Microcontrollers
Main lesson
A DAC’s output voltage
Use this for “An 8-bit DAC with a 5 V reference gets the code 200. What voltage comes out?”
A divider down to 3.3 V
Use this for “A 5 V signal reaches a 3.3 V pin through 2.2 kΩ over 3.3 kΩ. What voltage does the pin see?”
Setting and clearing a register bit
Use this for “An 8-bit register holds 0x41. Set bit 3, then clear bit 6. What does it hold?”