Before learning about potentiometer vs rheostat, we need to know about what a resistor is. Resistor is known as its function “to provide resistance”. We can use either fixed resistor or variable resistor. Just as mentioned, the fixed resistor provides fixed resistance, while the variable resistor provides variable resistance. This resistance will resist or block the flowing current in the circuit and produce voltage drop. These will be calculated using basic Ohm’s Law.
Potentiometer or “pot” is one type of variable resistor. This component has three terminals and operated by an analogue slider or rotary wiper. Since potentiometers don’t generate power, we call them passive elements or passive devices.
Potentiometer
When we use a variable resistor as a potentiometer, we connect both ends to input source(pin a) and ground(pin c) while the wiper is connected with output(pin b) side to produce signal. The output signal itself has linear value depending on voltage range applied to one end to other end (Vin to 0V). The value will be linear according to wiper position on the movement range.
Looking at the circuit above we can conclude that the potentiometer acts as a voltage divider.
Potentiometer is a common variable resistor used for many purposes. A potentiometer or pot for short, has three terminals with sliding contact, rotary slider or wiper. Using the slider or wiper we can produce various resistance within the potentiometer’s range.
Potentiometer is formed from potential and meter, means that we can measure the potential value of it. This three-terminal component uses voltage division principle and an adjustable voltage divider circuit in a single component.
The equation will be:
where:
Rac = Rab + Rbc. Hence, the voltage Vout can be increased or decreased by using the slider or wiper towards c or a, respectively.
There are three types of potentiometer as listed below before we moving on to the potentiometer vs rheostat matter:
Rotary potentiometer
This potentiometer has an angular wiper with the usual three terminals. The resistance will vary depending on the angular movement. This component will produce linear resistance with the wiper motion. You can find this type of volume control of audio device, amplifier, or even ON-OFF switches.
Slider potentiometer
Also known as slider-pot, is used to provide variable resistance by moving the linear movement wiper. Just as a rotary potentiometer, this type also has linear resistance with the wiper motion. You can find this type of audio mixer, equalizer, etc.
Trimmer Potentiometer
This potentiometer has three terminals but the wiper is different from other types. It has a square shape usually with blue color with an angular wiper in the middle of the box. We can adjust the resistance by rotating its wiper using a screwdriver or something with the same shape. This type can’t be adjusted easily like a rotary or slider potentiometer.
Variable Resistor for Variable Resistance
If we use the variable resistor to provide variable resistance we only need to connect two of three terminals to the circuit. Remember the configuration because we will see it again when we reach potentiometer vs rheostat. The wiper will be connected to one of the end terminals, pin A or pin C. The example can be seen below:
We can adjust the resistance simply with moving the wider up or down/ left or right. Sometimes it is wise to connect the unused terminal and the wiper to make it a closed-circuit.
Looking at the circuit above we can say that a variable resistor is a two-terminal component. This component when used to control high current in a motor is known as Rheostat. We will discuss this matter later along with potentiometer vs rheostat.
Read also : transformer as a matching device
Rheostat
Rheostat is used to control electric current flowing through it from varying resistance by moving the wiper. The Rheostat is derived from Greek words ‘rheos’ and ‘statis’ means current controlling device.
The electric current depends on the value of applied voltage and resistance in the circuit. The higher the resistance means the less current in the circuit. Unfortunately, rheostat can’t block electric current completely.
Structure of Rheostat
Rheostat’s structure is similar to the potentiometer, and has three terminals: pin A, pin B, and pin C. However we only use two terminals for rheostat: pin A and B, or pin B and C.
Unlike the rotary-pot or slider-pot which can be made from carbon, the rheostats are mostly made wirewound. Usually, rheostat is made from an insulating ceramic core wired by Nichrome wire. The core acts as an insulator material to the heat.
Below is the slider rheostat shows the easier illustration about how it works.
The resistance of a rheostat depends on the length of the resistive track:
- If we use pin A and pin B: the minimum resistance is achieved when the wiper is near to pin A.
- If we use pin B and pin C: the minimum resistance is achieved when the wiper is near to pin C.
Potentiometer vs Rheostat
Now we have reached our main topic: potentiometer vs rheostat. When observed, both have the same structure but for a potentiometer we use three terminals while a rheostat only needs two terminals. The other difference is the power rating. The wirewound rheostat has a large power rating hence it has wider application for higher current application as its main advantage. For more differences, we can observe the table below:
| Potentiometer | Rheostat | |
| Number of connections | Three terminals | Two terminals |
| Circuit connections | Parallel with source
| Series with load
|
| Functions | Controlling voltage | Controlling current |
| Symbols |  |  |
| Controls | Voltage drop across the load | Current to load |
| When to use | Controlling voltage to a high impedance load | Controlling current to a low impedance load |
| Applications | Tuning, volume control | Dimmers, motor control |
| Suitable for | Low power circuit | High power circuit |
