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A potentiometer uses all three terminals as an adjustable voltage divider — the wiper taps off a fraction of the input voltage. A rheostat uses just two terminals as a variable resistor in series with a load to control current. The taper (how resistance varies with rotation) is linear for most control and sensing, or logarithmic for audio volume, which matches how we hear loudness.
| Quantity | Formula |
|---|---|
| Ideal wiper voltage | Vout = Vin × fraction |
| Loaded wiper voltage | Vout = Vin × (Rlower∥RL)/(Rupper+Rlower∥RL) |
| Rheostat resistance | R = Rtotal × fraction |
| Rheostat current | I = V / (R + Rload) |
Watch the loading effect: if the load resistance isn't much larger than the pot, it pulls the wiper voltage down and makes the response non-linear. Keep RL >> Rtotal for a faithful divider.
A potentiometer uses all three terminals as a voltage divider to tap off a fraction of a voltage; a rheostat uses two terminals as a variable resistor to control current in series with a load.
How the resistance changes as you turn the pot. Linear taper changes evenly; logarithmic (audio) taper changes slowly then quickly, matching how we perceive sound level.
For volume and other controls perceived logarithmically. A linear pot would make most of the useful volume change happen in a small part of the rotation.
When the load resistance is not much bigger than the pot, it draws current from the wiper, lowering the output voltage and bending the once-linear response into a curve.
Make the load resistance at least ten times the pot's total resistance, or buffer the wiper with a high-impedance op-amp follower.
A value about ten times smaller than the load it drives, so loading is negligible, but large enough to keep current and power low. 10 kΩ is a common general-purpose choice.
Yes — connect the wiper and one end terminal (leave the third unconnected) and it acts as a two-terminal variable resistor.
For a linear pot with no load, yes. With a log taper or a significant load, the wiper voltage is a non-linear function of position.
Small pots are rated a fraction of a watt. As a rheostat carrying current, check I²R against the rating — the low-resistance end can get hot.
A small, screwdriver-adjusted pot for one-time calibration or trimming inside a circuit, not for frequent user adjustment.
It probably uses a linear pot where a log taper is needed, or the load is changing its response. A log-taper pot fixes the perceived unevenness.
At one end the output is 0 V (or full resistance), at the other it is the full input (or zero resistance). Real pots may have a small end resistance rather than exactly zero.
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