Calculate LED voltage, resistor, current, or source voltage from three inputs in a series circuit using VLED = VS − I×R for LED circuits.

LED Voltage Calculator

Enter any 3 values to calculate the missing variable

Note: This calculator uses the simple series-circuit relationship VLED = VS − I·R where R is the external series (current‑limiting) resistor. LEDs are non‑ohmic devices; forward voltage depends on LED type, current, and temperature.

Transformer Voltage Formula

The transformer voltage relationship is based on the turns ratio between the primary and secondary windings. For an ideal transformer, voltage changes in the same proportion as the number of windings.

V_s = (N_s/N_p) * V_p
  • Vs = transformer output voltage, or secondary voltage
  • Vp = source voltage, or primary voltage
  • Ns = number of secondary windings
  • Np = number of primary windings

The calculator can rearrange this same transformer equation depending on which value you leave blank.

V_p = (N_p/N_s) * V_s
N_p = (V_p/V_s) * N_s
N_s = (V_s/V_p) * N_p

If you leave transformer voltage blank, the calculator solves for secondary voltage. If you leave source voltage blank, it solves for primary voltage. If you leave either winding field blank, it solves for the required number of windings using the voltage ratio.

Voltages can be entered in volts or kilovolts. The calculator converts voltage values to volts internally before applying the formula, then converts the answer back to the unit you selected.

Common Transformer Turns Ratio Results

The turns ratio tells you whether the transformer steps voltage up, steps voltage down, or keeps it the same.

Relationship Turns Ratio Voltage Result Transformer Type
Ns > Np Greater than 1 Secondary voltage is higher than source voltage Step-up transformer
Ns < Np Less than 1 Secondary voltage is lower than source voltage Step-down transformer
Ns = Np Equal to 1 Secondary voltage equals source voltage Isolation transformer

Voltage Unit Conversion Reference

Unit Equivalent in Volts Example
1 V 1 V 240 V = 240 V
1 kV 1000 V 2.4 kV = 2400 V

Example Problems

Example 1: Find transformer voltage

You have a source voltage of 120 V, 500 primary windings, and 100 secondary windings.

V_s = (N_s/N_p) * V_p
V_s = (100/500) * 120 = 24 V

The transformer voltage is 24 V.

Example 2: Find primary windings

You have a 240 V source, a 24 V transformer voltage, and 100 secondary windings.

N_p = (V_p/V_s) * N_s
N_p = (240/24) * 100 = 1000

The transformer needs 1000 primary windings.

FAQ

What is source voltage in a transformer?

Source voltage is the voltage applied to the primary side of the transformer. In the calculator, this is treated as the primary voltage, Vp.

What is transformer voltage in this calculator?

Transformer voltage means the secondary-side voltage produced by the transformer. It is calculated from the source voltage and the ratio of secondary windings to primary windings.

Why must the winding values be greater than zero?

A transformer voltage ratio cannot be calculated with zero windings because the formula divides by the number of primary or secondary turns. A zero value would make the ratio invalid and does not represent a working transformer winding.