Voltage Divider Calculator Widget

Add a voltage divider calculator to your website. Makers enter the input voltage and resistor values such as 10k or 4k7 to get the output voltage, or ask for the R2 that gives a target voltage and the nearest standard resistor.

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<iframe src="https://a2z.tools/embed/w/voltage-divider-calculator" title="Voltage Divider Calculator by A2Z Tools" width="100%" height="590" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

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How it works

The widget uses the divider engine and E-series tables of the A2Z Voltage Divider Calculator. Resistances can be typed in electronics shorthand (4k7, 10k, 2.2M, 220R), the RKM notation of IEC 60062. With a load resistance the load is treated as being in parallel with R2, which is why a loaded divider always gives less than the unloaded formula, and the widget shows the error the load causes. In R2 mode it solves for the exact resistor, accounts for the load if one is given, and picks the nearest value in the IEC 60063 E24 series with the output voltage that value really gives.

Calculation method

  • Vout = Vin x R2 / (R1 + R2)
  • Loaded: R2' = R2 x RL / (R2 + RL), Vout = Vin x R2' / (R1 + R2')
  • Solving for R2: R2' = R1 x Vout / (Vin - Vout); with a load R2 = 1 / (1/R2' - 1/RL)
  • Thevenin source resistance = R1 x R2 / (R1 + R2); nearest standard value from the IEC 60063 E24 series; Vout shown to 4 decimals

Worked examples

12 V down to an ADC

Inputs: Vin 12 V, R1 10k, R2 4k7, no load

Result: Vout 3.8367 V; divider current 816.3 uA; Thevenin resistance 3.197 kohm

12 x 4.7 / 14.7 = 3.8367 V.

Resistor for 3.3 V from 5 V

Inputs: R2 mode, Vin 5 V, R1 10k, target 3.3 V

Result: R2 = 19.41 kohm; nearest E24 20 kohm gives 3.3333 V

10k x 3.3 / 1.7 = 19.41k.

Limitations

  • Ideal resistors: tolerance, temperature drift and the ADC's own sampling current are not modelled beyond a fixed load resistance.
  • Suggests E24 values only, not E48 or E96 precision series.

Where publishers use it

  • Arduino and Raspberry Pi tutorials scaling 5 V or 12 V signals down to an ADC input
  • Battery-monitor and sensor projects on maker blogs
  • Electronics courses explaining loading effects
  • Component retailers' application notes

Questions

Why does my measured output differ from the formula?

Anything connected to the output - a meter, an ADC input, another circuit - draws current and acts like a resistor in parallel with R2. Enter its resistance as the load to see the real output.

Can I power a circuit from a voltage divider?

Only a very light, constant load. The output voltage falls as the load current rises; a regulator is the right way to supply power.

How do I type resistor values?

Plain ohms (4700) or shorthand: 4.7k, 4k7, 2M2, 220R. A lower-case m means milliohms and an upper-case M means megohms.

Which standard value does it suggest?

The nearest value in the E24 series, the IEC 60063 series with 24 values per decade associated with 5% parts, together with the output voltage that value gives.

Sources

  1. IEC 60063:2015 Preferred number series for resistors and capacitors - International Electrotechnical Commission . E24 series values
  2. IEC 60062:2016 Marking codes for resistors and capacitors - International Electrotechnical Commission . RKM shorthand such as 4k7 and 2M2

Cite or recommend this tool

If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.

A2Z Tools Voltage Divider Calculator
https://a2z.tools/voltage-divider-calculator

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