Understanding Relays

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RELAYS WITH DE-SPIKING RESISTORS
High ohm resistors are sometimes used instead of diodes. A resistor is more durable than a diode and can suppress voltage spikes similar to a diode, but the resistor will allow current to flow through it whenever the relay is on. Therefore resistance of the resistor must be fairly high (about 600 ohms) in order to prevent too much current flow in the circuit. High ohm resistors are not quite as efficient at suppressing a voltage spike as diodes.