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Thread: Protection circuit with PolySwitch
face BY : Ruben Jönsson email (remove spam text)


Using a zenerdiode to blow a fuse at over VOLTAGE situations is a common practice when dealing with EX protection. (Maximum current is limited with a resistor together with the voltage rating of the zener).

Whenever using a zenerdiode to blow a fuse You have to make sure that the zener can stand the surge currents.
Decide the maximum input voltage that the circuit can be subjected to. This is the maximum fault input voltage.

Calculate how much current this will give through the zener. Use the nominal cold resistance for the  fuse.

Find out how long it will take the fuse to blow at this current (the fuse datasheet should tell this) and check if the zener will stand the current for this time. The zener datasheet should state a peak surge current diagramme. The more current, the shorter time the zener can stand it. If you find that the zener won't stand this peak surge current You will have to add a resistor in series with the fuse. For EX applications this should be an infallible resistor. A metal film resistor rated for the maximum fault voltage, for example.

You will also have to check that the heat dissipated in the zener when the fuse isn't blowing is no problem. Again, for EX applications, a current of 1.7 * the nominal fuse current rating is used (this does not apply to PolySwitches). Also note that the maximum rated power of the zener is dependent on its temperature. Which means, if You use a 5W zener, that it is perhaps only rated for 5W below 75 degrees C. When the temperature is rising, the power goes down. For the temperature, You also have to take the maximum ambient temperature into account, and any other heat source that could be present inside an enclosure. You also have to make sure that the circuit board can stand this heat.

Regards / Ruben

Ruben Jönsson
AB Liros Electronic
Box 9124, 200 39 Malmö, Sweden
TEL INT +46 40142078
FAX INT +46 40947388

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