Potential Dividers Electric Circuits ☞

Resistors in Series Consider a simple circuit with two resistors connected in series. The potential across Vin is divided in the ratio of the resistances. ☜ ☞

☜ ☞ Ratio of Resistances In the circuit shown, R1 has the following share of the total resistance: Thus 3/8 of Vin is shared across R1 The potential across R1 is: ☜ ☞

Potential Divider Circuit Potential dividers divide up the voltage within a circuit, so that parts of a circuit only receive the voltage they require. Vin is divided such that parts of a circuit only receive the voltage they require, Vout. Vout in this circuit is given by: ☜ ☞

Potential Divider - Question Calculate the voltage output, Vout, of this potential divider circuit. ☜ ☞ Click Here for the Answer

Variable Output The voltage output, Vout, of a potential divider can be varied by replacing a fixed resistor with a variable resistor. In the example shown, Vout can range from 0→15.5V, depending on the value of R2. If R2 = 0, Vout = 0V If R2 = 100, Vout = 15.5V ☜ ☞

Thermistors A thermistor is a resistor whose value is controlled by temperature. This is known as a negative temperature coefficient (ntc) thermistor. Thermistors are especially useful in potential divider circuits. ☜ ☞

Temperature Controlled Circuit A thermistor is used to control Vout depending on the surrounding temperature. When temperature is high, the resistance of the thermistor is low. The thermistor has a small share of Vin. Vout is small. This circuit would be good to automatically switch a heater on or off. ☜ ☞

Light Dependent Resistor LDRs allow a circuit to be controlled by light. Resistance and Light Level are inversely proportional. ☜ ☞

Light Controlled Circuit An LDR is used to control Vout depending on the surrounding light levels. When light level is low, the resistance of the LDR is high. The LDR has a large share of Vin. Vout is large. This circuit would be good to automatically switch on a light when it gets dark. ☜ ☞

Sensing Circuits - Question What happens to Vout as the light level decreases to complete darkness in this circuit? ☜ ☞ Click Here for the Answer

Strain gauges are useful for circuits measuring small length changes A strain gauge is a resistor whose value varies depending on the tension or compression applied to it. Strain gauges are useful for circuits measuring small length changes ☜ ☞

Potentiometer A potentiometer is a three-terminal resistor with a sliding or rotating contact, usually called a Wiper. Diagram Symbol ☜ ☞

☜ Uses of Potentiometer There are two possible uses of a potentiometer: When three terminals are used, it forms an adjustable voltage divider. When two terminals are used, it acts as a variable resistor (or rheostat). ☜

Potential Divider - Answer Click Here to Return to Slides

Equation Variety - Answer Question When light level very low (dark), the resistance of the LDR is very high. The LDR has a large share of Vin. The fixed resistor has a very small share of Vin. Vout is very small. Click Here to Return to Slides