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Transcript
P6B Resistors in series Two or more resistors in a circuit increase the circuit’s resistance. When components are connected in series, their total resistance is the sum of their individual resistances: RT = R1 + R2 + R3 Where: The potential divider A potential divider is used to produce the required voltage in a circuit. Two fixed resistors can be used as a potential divider. In the circuit diagram for a potential divider: the two resistors are shown as R1 and R2 RT is the total resistance R1, R2 and R3 are the individual resistances the input voltage is shown as Vin the output voltage is shown as Vout Note that it does not matter how many components are connected in series. For example, the total resistance of two 5 Ω (ohm) resistors in series is 10 Ω (5 Ω + 5 Ω). The total resistance given by these three resistors is 2 + 1 + 3 = 6 Ω If you increase the number of lamps in a series circuit, the total resistance will increase and less current will flow. The output voltage depends on the relative resistances of the two resistors. For example: if the resistance of R1 and R2 is the same, Vout is ½ of Vin if the resistance of R2 is double the resistance of R1, Vout is ⅔ of Vin if the resistance of R2 is three times the resistance of R1, Vout is ¾ of Vin One of the fixed resistors can be replaced by a variable resistor. This produces a potential divider that allows the output voltage to be varied, even if the input voltage stays the same. LDRs and thermistors You should be able to recognise the circuit symbols for the thermistor and the LDR (light-dependent resistor), and know how the resistance of these components can be changed. The thermistor Thermistors are used as temperature sensors, for example, in fire alarms. Their resistance decreases as the temperature increases: at low temperatures, the resistance of a thermistor is high - and little current can flow through it Take care not to confuse the symbol for a thermistor with the symbol for a variable resistor (which contains an arrow rather than a bent line). The LDR LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases: in the dark and at low light levels, the resistance of an LDR is high - and little current can flow through it in bright light, the resistance of an LDR is low - and more current can flow through it LDRs are also useful for controlling how long the shutter should remain open on a digital camera. Changes in the resistance are measured and, if the light level is low, the shutter stays open for longer than if the light level is high. Resistors in parallel The equation below shows how to calculate the total resistance for three resistors in parallel: 1/R = 1/R + 1/R + 1/R T 1 2 3 For example, what is the total resistance when three 6 Ω resistors are connected in parallel? 1/R T at high temperatures, the resistance of a thermistor is low - and more current can flow through it = 1/6 + 1/6 + 1/6 = 1/R3 The value of 1/RT = 1/2, so the value of RT is 2 Ω. Remember that when components are connected in series, their total resistance is the sum of their individual resistances (RT = R1 + R2 + R3). There are two general points to note: Vout is approximately the same as Vin when R2 is much greater than R1 Therefore if these three 6 Ω resistors were connected in series, the total resistance would be 6 + 6 + 6 = 18 Ω Vout is approximately zero when R1 is much greater than R2 The potential divider You need to be able to calculate the value of Vout if you are given the values of Vin, R1 and R2 for a potential divider. You can calculate Vout using this equation: Vout = R2 / R1 + R2 x Vin For example, what is the output voltage when the input voltage is 12 V, R1 = 2 Ω and R2 is 1 Ω? Vout = 1/ 2 + 1 x 12 = 1/ 3 x 12 = 4 Ω The fixed resistor R2 can be replaced by a thermistor or an LDR. If a thermistor is used, Vout depends on the temperature. If a thermistor is used, Vout depends on the light levels.