Rheostat vs Potentiometer: What Actually Separates Them

A rheostat is a two-terminal variable resistor that controls current. A potentiometer is a three-terminal one that controls voltage. That is the whole difference, and it comes down to wiring rather than to the part.

Both are the same basic construction — a resistive element with a sliding contact called a wiper. Wire a potentiometer using only the wiper and one end of the element and it becomes a rheostat. Use all three terminals and it is a voltage divider again.

So the question a datasheet cannot answer for you is which one your circuit needs. That depends on whether you are trying to set a current or a voltage, and the two roles put very different stress on the same part.

The one-line answer

A rheostat goes in series with a load and drops whatever voltage the current and its own resistance produce. A potentiometer goes across a supply, and the wiper taps off a fraction of that voltage.

The practical consequences follow from that. A rheostat carries the full load current through its element and through the wiper contact. A potentiometer carries only the current its own element draws, which is usually small.

Side by side

Where people get it wrong

The arrow on each element in the drawing above is the wiper — the sliding contact that picks off the resistance. On the left it sits in the load path, and on the right it is the output tap.

Wiring a potentiometer with two terminals does not raise its rating. The element and the wiper are the same parts either way. What changes is that the load current now flows through the wiper contact, which is the least robust point in the assembly.

The wiper current limit is a separate specification from the power rating. Bourns, which publishes extensively on this, states that exceeding the maximum wiper current has caused more trimmer problems than any other design error — and it can damage the contact spring even when total dissipation looks acceptable.

Where a datasheet does not state a wiper current, a common working assumption is the current that would dissipate the element's full power rating through the element alone, capped at 100mA.

The full power rating only applies at full element travel. A 10kΩ trimmer set to half its resistance has roughly half the element in the circuit, and heat concentrates in that smaller area. The rating has to be derated accordingly.

This is the mistake that produces a scorched spot rather than an outright failure, which makes it harder to trace.

Both ends of the travel are unusable. Bourns recommends operating between 10% and 90% of the total resistance, because the last percent or two is mechanically unstable and reads erratically.

One end approaches zero ohms. In a rheostat that means the current is limited only by the load. A fixed series resistor sets a floor, and it is cheaper than replacing the element.

And the wiper can lose contact. Tying the unused outer terminal to the wiper gives the circuit a fallback path — the resistance jumps to the full element instead of going open.

The gap between those rows is where most of the failures come from. A signal-grade part used as a power rheostat is cited as the source of most “it failed” stories, and the failure is thermal rather than electrical.

Practical guidance from the manufacturers is to load any of them to only 50% to 70% of rated power, because the rating assumes ideal cooling that a real enclosure does not provide.

What to check when you are buying one

Element material tells you the power class before you read a single number. Wirewound is what real power handling is built on; carbon, cermet and conductive plastic cover the signal and control end.

Four specifications decide whether a part will survive the job, and they are worth asking for explicitly:

• Power rating and the derating basis — at what element usage, and at what ambient.
• Maximum wiper current — a separate limit from the power rating, and frequently absent from summary listings.
• Taper — linear or logarithmic. A rheostat is linear by definition, so a log pot wired as one gives a response curve nobody intended.
• Mechanical life and end-set limits — how many cycles, and how much of the travel is usable.

For anything carrying real current, the honest engineering answer is often that the rheostat is the wrong tool. A motor, a lamp or a heater controlled by series resistance dumps the difference as heat. PWM or a current-mode controller does the same job without burning it.

Where a rheostat still earns its place is in test and calibration work: bench current limiting, resistance substitution, and anywhere a human needs to turn a physical knob.

Frequently Asked Questions

Q1: Can I use a potentiometer as a rheostat?

A: Yes — connect the wiper and one outer terminal only. It becomes a two-terminal variable resistance. Tie the unused terminal to the wiper so the circuit falls back to full element resistance instead of going open if the wiper lifts.

Q2: Does two-terminal wiring increase the power rating?

A: No. The element and wiper are unchanged. The difference is that load current now passes through the wiper contact, which is the mechanically least robust part of the assembly.

Q3: What is the maximum wiper current?

A: A separate specification from the power rating, and one that is often omitted. Where a datasheet does not state it, a common working figure is the current that would dissipate the element's full rated power through the element alone, capped at 100mA.

Q4: Why does my trimmer scorch at one spot?

A: Because the full power rating assumes the whole element is in circuit. Set a 10kΩ trimmer to half resistance and only half the element carries the heat, so the rating must be derated. The damage is localised, which is why it does not look like a normal overload.

Q5: Should I use the full rotation range?

A: No — stay between about 10% and 90% of the travel. The extreme ends are mechanically unstable and read erratically. They are also where the resistance approaches zero, which matters in a rheostat because current is then limited only by the load.

Q6: When is a rheostat the wrong choice?

A: Whenever it is controlling real power. Series resistance turns the unwanted voltage into heat, so a motor, lamp or heater run that way wastes energy and needs a part rated for the dissipation. PWM or a current-mode controller does the same job without the heat.