Resistor Basics 4

We've looked at fixed value resistors, but what if we want to change the resistence without having to swap out the resistor? Let's take a look at some of the resistors that allow their values to be changed.

Manually Adjusted Resistors

Variable resistors allow value of the resistance to be changed usually by turning a knob or moving a slider on the unit. This in turn moves a contact wiper along a track, lengthening or shortening the electrical path, which adjusts the resistance accordingly.

Rheostats are variable resistors that typically comprise of a resistive coil winding and a moving wiper connected to an adjustment knob. As the knob is turned, the wiper bypasses more of the winding, shortening path for the current to flow, decreasing the resistance. Rheostats may also be wound along a length, using a slider to make the adjustment.

Potentiometers are a special kind of variable resistor that have an additional connection, acting as a voltage divider. If the additional connection is not used, then the potentiometer acts as a variable resistor.

Common potentiometers typically comprise of a carbon track with a contact wiper, and can be linear (using a slider) or axial (using a knob).

Potentiometers and variable resistors are often confused, however, a Potentiometer can be used as a variable resistor if the third connection is not used.

Electronically Adjusted Variable Resistors are also available in the form of a digital resistor. Steps up or down in the resistance value are made by the digital resistor depending on the control that its receiving from either an I2C signal, or from an up/down signal pulse.

Schematic Symbol Description Circuit Schematic Reference
Standard Variable Resistor Symbol Variable Resistor - Standard Symbol R or VR
Obsolete Variable Resistor Symbol Variable Resistor - Discontinued, but may still appear in circuit diagrams. R or VR
Standard Potentiometer Symbol Potentiometer - Standard Symbol R
Obsolete Potentiometer Symbol Potentiometer - Discontinued, but may still appear in circuit diagrams. R

 

Environmentally adjusted Resistors

Some resistors have their resistance value altered depending on external stimuli.

A Thermistor is a special type of variable resistor where the resistance value changes according to the temperatures it is exposed to.

Schematic Symbol Description Circuit Schematic Reference
Standard Thermistor Symbol Thermistor - Standard Symbol R
 Obsolete Thermistor Symbol Thermistor - Discontinued, but may still appear in circuit diagrams.  R

 

A Photoresistor (Light Dependant Resistor or LDR) is another special type of variable resistor that changes value according to how much light it is exposed to.

Schematic Symbol Description Circuit Schematic Reference
Standard Photoresistor Symbol Photoresistor - Standard Symbol R
Obsolete Photoresistor Symbol Photoresistor - Discontinued, but may still appear in circuit diagrams. R

 

Electrically adjusted Resistors

Some resistors can have their resistance value changed by electrical input:

A Varistor is a resistor that changes value as current is increased. These are typically used for circuit protection, where a spike in current will cause the resistance to increase, counteracting the effect.

Schematic Symbol Description Circuit Schematic Reference
Standard Varistor Symbol Varistor - Standard Symbol R
Obsolete Varistor Symbol Varistor - Discontinued, but may still appear in circuit diagrams. R

 

A Digital Potentiometer allows a digital input to control the resistance within a specified range. These are often used with microcontrollers where a variable resistance is required from within the microcontroller's programme, for instance, dimming lights. These are also known as DACs or Digital to Analogue Converters.

Schematic Symbol Description Circuit Schematic Reference
Standard Digital Potentiometer Symbol Digital potentiometer - Standard Symbol R
Obsolete Digital Potentiometer Symbol Digital potentiometer - Discontinued, but may still appear in circuit diagrams. R

NOTE: The symbol for a digital potentiometer is the same as a manually operated one, as the overall functionality is the same. However, in some circuit diagrams, the digital potentiometer may be shown as part of a module with an additional input given for the selection control.

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