How do you determine correct size varistor on a 24vac coil?

Answer

is this coil a single shot solenoid or is it used as a filter i assume you want to know a good value for a varistor to stop voltage spikes from the reverse EMF which is aprox. 4X the input voltage.

so i would say to use 2X-3X the input Voltage so 48 or 72 Volt MOV (Metal oxide varistor)

Answer

You want the varistor to be several volts greater than the peak voltage of the signal across which it is connected. 24 volts is probably the RMS value of the coils voltage. Your varistor must be selected to be greater than the peak voltage. When the voltage exceeds the clamping voltage of the varistor, it conducts current, preventing the voltage from reaching a dangerous level. If the elevated voltage is maintained for too long a time, the varistor will heat to a point where the metals in it melt and fuse together, causing a short. This also poses the posibility of a fire risk, as the current is now conducting heavily through the varistor. This is what often happens to a varistor in a power strip when lightening strikes nearby. It renders the power strip useless. But the connected devices are usually spared.

First answer by ID1164638891. Last edit by DaytonaHead. Contributor trust: 361 [recommend contributor]. Question popularity: 152 [recommend question]

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