Gary Schafer wrote:
> A DC relay is wound with enough resistance to withstand the applied voltage.
> An AC relay depends partly on the inductance presented by the coil for its
> impedance to AC. That same coil on DC may not have enough resistance and may
> overheat.
Any AC relay will hold in with some DC current that will not overheat
the coil. DC is more efficient at producing attractive force than AC,
because AC goes through zero twice a cycle. The big difference is
that AC relays automatically draw more current when the armature is in
the unattracted position, so the core produces a surge of additional
force that goes away, when the armature pulls in and increases the
coil inductance. A DC relay draws the same current, regardless of the
armature position.
So if the DC has to be boosted to pull the armature in, and is not
reduced afterward, the coil may overheat. There is a control IC
designed to apply a brief blast of full DC and then PWM the DC to
reduce the average current and some DC relays use a tapped coil, with
the low resistance section used to pull the armature in, with an
auxiliary contact to add the rest of the turns in to lower the current
after the armature moves. Simple DC relays just have the coil
designed to pull the armature in without overheating if that current
is sustained.
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