Electronic Components Found in Medical Devices

Types of Electronic Components

• Resistors

• Capacitors

• Inductors

• Diodes

• Transistors

• Integrated Circuits

Resistors definition + symbol (photo + 2)

Limit or control the flow of electric current in a circuit

fx.: thermistors, photoresistors/LDRs

Resistors Unit

[Ω] = ohms

mains cable 0.1Ω/m, human arm 1000Ω

Resistors Types

• wire-wound

• carbon film

• metal oxide

• variable resistors - LDRs (light dependent resistors)

• Thermistors

Wire-Wound Resistors (3)

• Constructed by winding a metal wire (nichrome) around a ceramic or insulating core

• high power handling

• precise

Carbon Film Resistors (2)

• Made by depositing a thin layer of carbon on a ceramic substrate

• Good stability

Metal Oxide Resistors (3)

• Composed of a metal oxide film

• High-temperature stability and reliability

• Used in high-power applications bc/ thermal properties

Variable Resistors (2)

Allow adjustment of resistance

fx.: LDRs (Light Dependent Resistors)

LDRs (Light Dependent Resistors) (3)

• made from semiconductor materials that change resistance based on light intensity

  • semiconductor based

• used as light sensors

Thermistors (2)

• Semiconductor-based resistors whose resistance changes significantly with temperature

• Used as temperature sensors

Uses of resistors (2)

• set currents and voltages in circuits

• thermistors and LDRs: used as light and temperature sensors

Power and Voltage Ratings of a resistor (2)

• Maximum values that resistors can handle without damage.

• Important for ensuring safe operation in circuits.

Carbon Film Resistor Colour Code (6)

• Colour Code: A system of colored bands on resistors indicating their resistance value, tolerance, and reliability.

• Typically consists of 4 or 5 bands

  • the first two or three represent significant digits

  • the next indicates the multiplier

  • the last band shows tolerance

    • uncertainty in the value

Capacitors symbol + unit + extra note (photo +2)

[F] = Farad

The patient-earth system has a capacitance of approximately 100 pF (pico = 10^-12)

Capacitors uses (3)

• Store and release electrical energy

• Timing Circuits

  • create time delays or oscillations w/ resistors

• smooth voltages by removing low unwanted frequencies from signals

Types of Capacitors (Based on Dielectric) (3)

• Paper Capacitors

• Air Capacitors

• Mica Capacitors etc.

Paper Capacitors (3)

• Use paper as the dielectric material

• Stable

• Reliable

Air Capacitors (2)

• Use air as the dielectric

• Found in tuning circuits due to their variable capacitance

Mica Capacitors (3)

• Utilize mica as the dielectric

• High precision and stability

• Used in high-frequency applications

Inductors symbol and unit

[H] = Henry

Inductors uses (2)

• Store energy in a magnetic field when current flows through them

• Smooth voltages by removing high unwanted frequencies from signals

Diodes definition + symbol

Allow current to flow in one direction only

Diodes uses

• Rectification

• LEDs as Indicators

• Photodiodes as Light Sensors

• Zener Diodes in Regulated Power Supplies

Rectification

Convert alternating current (AC) to direct current (DC) in power supplies

LEDs as Indicators

Provide visual feedback in devices (e.g., power indicators)

Photodiodes as Light Sensors

Detect light levels for applications like automatic lighting and camera exposure

Zener Diodes in Regulated Power Supplies

Maintain a stable output voltage despite variations in input voltage or load conditions

Types of Diodes

• Generic Diodes

• LEDs (Light Emitting Diodes)

• Photodiodes

• Zener Diodes

Generic Diodes

Standard diodes that allow current to flow in one direction only; used for rectification

LEDs (Light Emitting Diodes)

Emit light when current passes through; commonly used as indicators in electronic devices

Photodiodes

Function as light sensors; generate current or voltage when exposed to light

Zener Diodes

Allow current to flow in the reverse direction when a specific voltage (Zener voltage) is reached; used for voltage regulation in power supplies.

Transistors symbols

it has 3 terminals

Transistors Uses (3)

• very fast switching

• amplifiers

• filter circuits

Integrated Circuits (ICs) (2)

• Complex circuits with multiple components in a single package

• consist of thousands of miniature resistors, diodes, transistors and capacitors

Integrated Circuits Uses (7)

• amplifiers

• filters

• RAM, ROM

• counters

• timers

• comparators

• microprocessors

Semiconductors (9)

fx.: Silicon (Si) and Germanium (Ge)

• Found in resistors, diodes, transistors, and integrated circuits

• Act as insulators in their natural state

• Become conductors when energy is applied

  • heat, light E

• Conductivity increases with more energy exposure

• Used as energy sensors for measuring heat and light energy

  • Heat E: thermometers

  • Light E: spectrophotometers, digital microscopes

Power Supplies

• Mains voltage 240V AC but most instruments need few volts

• constant DC

• every electronic medical device which is connected to the mains supply needs to have a ‘power supply’

Power supplies consist of

• step-down transformer

• 4 diodes for full-wave rectificatio

• smoothing capacitors and inductors

• zener diodes for voltage regulation (essential when voltage supply is erratic)