Chapter 3 – Resistive components in circuits

What is the effect of adding resistors in series?

Total resistance increases (R = R1 + R2 + …)

What is the formula for total series resistance?

R = R1 + R2 + R3 + …

What is the effect of adding resistors in parallel?

Total resistance decreases (total is less than smallest resistor)

What is the formula for total parallel resistance?

1/R = 1/R1 + 1/R2 (+ 1/R3 …)

Formula for two resistors in parallel?

R = (R1 × R2) / (R1 + R2)

What are E24 resistor values?

Set of preferred values used for easy selection (e.g. 10, 12, 15, 18, 22, 27, 33, etc.)

What does resistor tolerance mean?

How much above or below the stated value a resistor can be (e.g. ±5%, ±10%)

What is the typical tolerance for metal film resistors?

±1%

What is the typical tolerance for carbon film resistors?

±5%

What is a resistor’s power rating?

Maximum power it can safely dissipate without overheating, e.g., 0.25W, 0.5W, 1W, 5W

How do you work out a resistor’s value using the colour code?

Use four bands: 1st and 2nd digit, multiplier (number of 0s), and tolerance

What does a gold band indicate in resistor colour code?

Tolerance of ±5%

What does a silver band indicate in resistor colour code?

Tolerance of ±10%

If you want 22,000Ω ±5%, what are the colours?

Red, Red, Orange, Gold

What is the value of a resistor with Brown, Black, Orange, Gold bands?

10,000Ω (10k) ±5%

What is a current-limiting resistor?

Resistor placed in series with an output device to prevent excess current

How do you calculate a current-limiting resistor?

R = (Vsupply – Vdevice) / I

If you have a 2.5V, 0.25A lamp on a 6V supply, ideal resistor value?

(6 – 2.5)/0.25 = 14Ω (choose 15Ω from E24 series)

What would you do if your exact resistor value isn’t in E24?

Choose the next highest value to protect the component

What does adding a resistor in series do to brightness of a lamp?

Reduces it (limits current)

What is a potentiometer?

Three-terminal variable resistor, used for variable voltage divider

How is a potentiometer used as a dimmer?

Adjusts resistance, lowers lamp brightness

What is a preset?

Small variable resistor, adjusted once in circuit for calibration

What is a Light Dependent Resistor (LDR)?

Resistor whose resistance decreases as light level increases

What is the symbol for an LDR?

Rectangle with arrows pointing towards it

How does resistance of an LDR change with light?

High resistance in dark, low resistance in bright light

What is an NTC thermistor?

Resistor whose resistance decreases as temperature increases (negative temperature coefficient)

What is the symbol for an NTC thermistor?

Rectangle with a diagonal line and “–t°”

If you want to sense a drop in light (dark detector), where do you place the LDR in a voltage divider?

At the bottom (gives high output voltage when dark)

If you want to sense an increase in light (light detector), where do you place the LDR in a voltage divider?

At the top (gives high output voltage when bright)

What is a voltage divider?

Two resistors in series used to give a specific output voltage from a higher input voltage

What is the voltage divider formula?

Vout = (R2 / (R1 + R2)) × Vin

What is a switch?

Component for making or breaking a circuit manually or automatically

Name types of switch.

Push to make, push to break, toggle, slide, reed, microswitch, tilt

When is a reed switch useful?

Burglar alarm on a window or door (closes when magnet present)

When is a tilt switch useful?

Detecting position or orientation in alarm or safety circuits

Describe what happens if you “load” a voltage divider (connect a load to output)?

Output voltage drops (can’t supply higher current)

What is the rule of thumb for loading voltage dividers?

Load resistance should be at least 10× higher than divider resistance

How do you use a thermistor in a temperature sensing circuit?

As temperature rises, NTC thermistor resistance drops, voltage output changes

If thermistor is top resistor in divider, what happens as temperature increases?

Output voltage decreases

If thermistor is bottom resistor in divider, what happens as temperature increases?

Output voltage increases

Why include variable resistor in a sensing voltage divider?

Allows fine adjustment/calibration of output voltage

What is the symbol for a light emitting diode (LED)?

Diode symbol with two arrows away, showing light emission

Why are resistors always used with LEDs?

To limit current and prevent LED damage

What are two advantages of LEDs over lamps?

More efficient (less energy waste), much longer lifespan, smaller size, more colours

What is the typical forward voltage of a red LED?

About 2.0V

How do you calculate the series resistor for an LED?

R = (Vsupply – Vforward) / I

If you want 10 mA through a 2V LED from 9V supply, what resistor?

(9 – 2)/0.01 = 700Ω (use 750Ω from E24 series)

What happens if you connect an LED backwards?

Little or no current flows; above 5V reverse bias, it may be damaged

What kind of current do most buzzers, lamps and motors need?

Higher mA or A (more than sensors or logic circuits can supply)

What is meant by an output driver?

Circuit to provide the necessary current for output devices

Which output device provides sound?

Buzzer or siren

Which output device provides light?

Lamp or LED

Which output device provides movement?

Motor (rotational) or solenoid (linear)

What is a solenoid?

Component that uses magnetism to create linear (push/pull) motion

Why must lamps and motors not be connected directly to sensing circuits?

They require too much current, will “load” the circuit and may not operate or may damage the circuit

If an LDR has 150 kΩ in the dark and 5 kΩ in light, what happens to Vout in a voltage divider with 15 kΩ fixed resistor on bottom?

Vout drops as light increases (LDR on top: dark = high Vout, light = low Vout)

What must you check before connecting a load to a voltage divider?

Load resistance is much higher than divider resistance, to avoid voltage drop

If a thermistor has 18.4 kΩ at 20°C and 1.1 kΩ at 80°C in divider with 4.7 kΩ, calculate output voltage at 20°C and 80°C.

Use Vout = (4.7 / (18.4 + 4.7)) × Vin for each temp (practice calculation)