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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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Reply to
Patxi
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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).

Tim

-- Seven Transistor Labs Electrical Engineering Consultation Website:

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Reply to
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.

NT

Reply to
meow2222

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 ?

piglet

Reply to
piglet

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