Key points about diodes
- a diode allows current in one direction
- diodes regulate voltage, convert AC to DC, and protect circuits
- a forward-biased diode conducts current at 0.6v
- reverse bias blocks current flow
- inductive loads generate back A spike in voltage generated when the current is interrupted, opposite to the original current, potentially damaging circuitry. when switched off
- a reverse-biased diode protects against back EMF
What are Semi-conductor diodes?
A semiconductor diode is an electronic component that allows current to flow in only one direction, acting like a one-way valve for electricity. It is commonly used for voltage regulation, converting an electrical circuit where the voltage and current regularly change in direction and magnitude to , and for protecting circuits from reverse voltage.
Circuit symbol: the diode symbol consists of a triangle pointing towards a line. The triangle represents the anode, and the line represents the cathode.
Forward-biased diode: an ideal diode conducts electricity when a voltage of 0.6V is applied in the forward direction. This means current flows from the anode (positive) to the cathode (negative).
Reverse bias: in reverse bias, the anode is connected to the negative side, and the cathode to the positive side, preventing current flow.
How diodes protect against back EMF
- Back EMF: when an an electrical device that uses coils or windings to create a magnetic field and can cause power surges when turned on or off like a relay coil, solenoid, or motor is turned off, it can generate a high voltage spike (back electromotive force) that can damage other components.
- Reverse-biased diode: placing a diode in parallel with the inductive load, but in reverse bias, protects against back EMF. The diode allows the current generated by the back EMF to circulate through the coil, preventing damage.
A circuit with a diode protecting against back EMF
Can you describe how the circuit works?
This circuit uses a relay to control a 24V lamp using a lower 5V control circuit. When the PTM (push to make) switch is pressed, the program of the microcontroller tells the base leg of the transistor to switch on, activating the relay which then powers the 24V lamp.
The diode's role is crucial - when the A type of switch that when energized creates a magnetic field to switch the relay's contacts, controlling the flow of electricity in a circuit. (relay SPST) is switched off, the collapsing magnetic field creates a sudden voltage spike in the opposite direction (back EMF). The diode provides a safe path for this reverse current to flow back through the coil, protecting the transistor from damage. Think of it like a safety valve that opens only when needed.
Without this diode, the back EMF voltage spike could be high enough to destroy the transistor.
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