Engineering
Voltage Divider (Vout) & LED current-limiting series resistor formulas
Calculate resistive divider output voltage Vout = Vin * R2/(R1+R2), solve R2 for ADC scaling, and size LED series resistors with Wattage checks.
Two foundational electronics bench formulas: ADC dividers & LED current limiting
An ideal unloaded divider with a12 V supply,20 kΩ upper resistor and 10 kΩ lower resistor produces 4 V. Loading the output changes the effective lower resistance. This is a basic circuit calculation, not an automotive-input protection design.
An assumed 5 V supply,2 V LED forward drop and 20 mA target gives 150 Ω and 0.06 W nominal dissipation. Supply and forward-voltage variation, resistor tolerance, current limits and manufacturer derating curves determine a practical part choice.
ADC acquisition behavior, input impedance, supply transients and clamp/protection limits are outside this model. A nominal divider output alone does not establish a safe ADC interface.
| Circuit Calculation | Formula | Sample Result |
|---|---|---|
| Divider Output Voltage (12V, 20k/10k) | =A2 * (C2 / (B2 + C2)) | 4.00 V |
| LED Series Resistor (5V, Vf=2V, 20mA) | =(D2 - E2) / (F2 / 1000) | 150 Ω |
| LED Resistor Power Dissipation (Watts) | =(D2 - E2) * (F2 / 1000) | 0.06 W (60 mW) |
Voltage Divider output Vout and LED series resistor (Ohms) formulas
Works with any consistent resistance unit (Ohms or kOhms) since R2 / (R1 + R2) is a dimensionless ratio.
=IF(OR(COUNT(A2:C2)<>3,A2<0,B2<=0,C2<=0),"Invalid divider inputs",ROUND(A2 * (C2 / (B2 + C2)), 4))Required LED current-limiting resistor in Ohms (from mA target current F2)
Divides F2 by 1000 to convert milliamps (mA) into Amperes.
=IF(OR(COUNT(D2:F2)<>3,D2<=E2,E2<0,F2<=0),"Invalid LED inputs",ROUND((D2 - E2) / (F2 / 1000), 2))Try the example
Use the original values or change them to test your understanding.