Varistor selection

Hi newsgroup,

I have a problem with a varistor selection.

I have a coil with DC current supply that is used inside a steel piece as an electromagnet. The varistor is inside a rectifier that gives the power to the coil and I don't know for sure if the varistor is suitable for the application.

The coil is at 110 Vdc and 2,2 A with 2500 turns and 0,9 mm diameter and I don't know the inductance as I don't have the B-h curve of the steel.

The varistor datasheet is as follows: Varistor voltage: 820 V Max. work voltage: 670 Vdc Energy(10x1000us): 110J Rated power: 0,4W Max. clamping voltage: 1355V and 25 A

I have another rectifier with this other varistor: Varistor voltage: 680 V Max. work voltage: 560 Vdc Energy(10x1000us): 45J Rated power: 0,4W Max. clamping voltage: 1120V and 25 A

I don't know if anyone of these two varistors are suitable and if anyone can help me I would appreciate it.

Thank you

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This is one of those magnetizing benchtop magnets, right?

I always found it amusing they put that in there... the varistor never conducts, whatsoever -- there's no way it possibly could -- it's driven with positive current during startup and operation, and clamped by the diode bridge during shutdown.

If there's an AC-only mode, the varistor would be best placed across the switch contacts (or two, one for each, if it's a double-pole switch).


Seven Transistor Labs 
Electrical Engineering Consultation 
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Tim Williams

If the varistor is connected across the coil it should conduct momentarily when power is switched off. But the varistor voltage seems excessively high.


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If the electromagnet is connected directly to the rectifiers then as Tim Williams said the rectifier diodes will catch the electromagnet back-emf. However I wonder if the varistor helps catch spikes due to transformer leakage inductance at switch off - in other words it protects the electromagnet wire insulation ?


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