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As with all ferromagnetic materials, in a relay a hysteresis is also present. It appears because when the coil is initially charged the magnetic field builds up, and cannot collapse instantly when prompted. Even if that effect is diode dampened it is still present in a certain degree. At higher frequencies the coil sees the current through it as DC, because the coil doesn't have enough time for its field to collapse, so it stays always energised (just like charging a capacitor, but exactly the opposite) while you cannot charge a capacitor instantly, you cannot discharge a coil instantly. Remember coils and capacitors are complete antipodes in their way of operation. In coils the current lags behind the voltage, and in capacitors the voltage lags behind the current. Anyway, this is of less importance, but at higher frequencies the coil makes the relay's contact plates latch onto a single position and renders it static, because the static magnetic field created by the coil doesn't change, or collapse. This is one of the reasons why relays are only suitable for low frequencies. Even the fastest relays, can't switch faster than 200-300Hz, and even then their reliability is questionable at best. Moreover at higher frequencies, you risk the contacts welding shut, rendering the relay inoperable
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