A rheostat uses a movable contact and part of a resistive element as a two-terminal variable resistor. Placed in series with a load, increasing active rheostat resistance increases total circuit resistance, reduces current and changes how the source voltage and power are divided between rheostat and load.
Rheostat or Resistance Coils Interactive Calculator
Enter supply voltage, load resistance and rheostat settings to calculate current, power dissipated in the rheostat and voltage across the load. The circuit assumes a steady DC source and a fixed resistive load.
Equation Used
- The wiper selects active resistance linearly from zero to the entered maximum.
- The load is purely resistive and in series with the rheostat.
- The DC source voltage is ideal and constant.
- Contact resistance, wiring, inductance, temperature, tolerance, transients and ratings are excluded.
The companion figure repeats the same contact position and active winding section. Moving dots highlight that section; they do not measure current or charge speed.
How a rheostat controls current
A two-terminal rheostat uses the movable contact and one end of a resistive winding. Moving the contact changes the length of winding in the circuit. For the linear winding assumed here, half the travel selects half the maximum resistance.
The rheostat and load are in series, so both carry the same current. Increasing rheostat resistance reduces current and the voltage across a fixed resistive load.
Hiscox entry 723 describes another arrangement: a switch connects one or more resistance coils in series. Such a coil bank gives discrete settings. This calculator instead illustrates a sliding contact on one winding.
Using the results
Use this calculation to explore a resistor-controlled DC circuit or a laboratory resistance setting. The four outputs show resistance, circuit current, heat dissipation rate in watts and load voltage.
For component selection, compare both current and dissipation with the exact rheostat specifications at the selected position. The result does not calculate temperature rise or account for contact resistance. A motor, lamp or other changing load may need a more detailed model than a fixed resistance.
Ideal DC series-circuit relationships
For maximum rheostat resistance Rmax and selected position fraction w, active resistance is Rr = wRmax. With resistive load RL, total resistance is Rr + RL and current is I = V/(Rr + RL).
Rheostat heat is Pr = I²Rr. Load voltage is VL = IRL. These ideal steady-DC equations do not establish allowable current or power.
Worked series-circuit example
With 25 V DC, a 12 Ω resistive load, a 45 Ω maximum rheostat and 50% selected position, active rheostat resistance is 22.5 Ω. Ideal series current is about 0.725 A, rheostat heat is about 11.8 W and load voltage is about 8.7 V.
The selected component would still need identified current, wiper, power, temperature and duty ratings.
Power, ratings and the drawing
The same current passes through the active winding and the wiper. A short active section can concentrate heat in only part of the element, so its allowable dissipation may be lower than the full-element rating. Check the manufacturer’s current, wiper and position-dependent power limits.
The power output is heat generation in watts, not a predicted temperature. Cooling, enclosure, ambient conditions and operating time affect the actual temperature.
The drawing indicates the contact position and active section, with simplified winding and source symbols. It is an explanatory illustration rather than an installation wiring diagram. The dots move at a fixed display rate, and winding color changes in discrete drawn segments even though resistance is calculated continuously from position.
Rheostat calculator questions
Why enter load resistance?
The rheostat and load carry the same current. Their resistances add, so the load resistance affects both current and how the supply voltage divides between the two components.
Why is rheostat power zero at zero selected resistance?
The ideal calculation gives I²Rr = 0 when Rr = 0. Current still flows through the load. Real wiper contacts and leads have resistance and current limits that this ideal result does not include.
Does the power result establish a suitable rheostat rating?
No. Check the exact component’s current limit, wiper rating and allowable dissipation at the active resistance and ambient conditions. Full-element power ratings may not apply to a short active section.
Can I use a motor’s winding resistance as the load input?
This calculator treats the load as a fixed resistor. A running motor also has back EMF and may have significant inductive transients, so winding resistance alone does not describe its operating current.
Do the animated dots measure current?
No. Their motion only highlights the active section. Read the current output in amperes; animation speed does not follow current.
How do switched resistance coils differ from a sliding rheostat?
A switch selects discrete coil combinations, giving resistance steps. A sliding contact selects a portion of a winding. The calculator uses a linear contact-position model, not a switched coil bank.
Technical references
Gardner D. Hiscox, Mechanical Movements, Powers, Devices and Appliances, entry 723: switched resistance coils connected in series.
- Bourns, The Potentiometer Handbook — two-terminal rheostat mode, load-current behavior, active-element power and wiper-current limits.
- OpenStax University Physics, Resistors in Series and Parallel — series resistance, common current, voltage drops and resistor power.
- OpenStax Physics, Key Equations — Ohm's law and electric-power identities.
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