Life Time of Relay

Posted by 德明 邱 on

The life time of relay when driving tungsten lamp or ac-dc

The characteristic of tungsten lamp and ac-dc

    The resistance of a tungsten in cold temperature is 0.1 to 0.05 times of that in lighted status. 

    Almost all tungsten lamp has inductance. the inductance of some model is very big, because the coiled structure of the filament.

    Ac-dc is a switching power supply. It has huge capacitance and big inductance. There will be big surging current when power supplied.

Relay driving load

    When relay's contactor points are connected or released, there will be fire discharging between contactor points. Within relay's specification, 100000 times of on/of is no problem for high quality relay. The special materials of the contactor point can bear the dischaging till 100000 times.    

Example of a/c

     An accepted example is air conditioner.

     A 30 ampere relay can drive the max power rate compressor is 1000 to 1250W, because it has inductance.

     1250W / 220Vac = 5.68 ampere. Much less than 30A!

     If drive bigger compressor, we need drive it by a contactor. That is: the relay drives the contactor, the contactor drives the compressor.

     Otherwise the life time of the relay will be short.

Relay driving tungsten lamp

     The resistance of a tungsten lamp at cold status is 1/15 of the value of hot status.

     An detail example: Relay specification is 10A@250Vac. The max power rate of tungsten lamp should be less than 10 / 15 * 220Vac (10 / 15 * 110Vac) =  146.7W (73.3W).

     10A relay drive bigger tungsten lamp will be burnt immediately.

Relay driving ac-dc

     In some application, the use have to drive ac-dc, because they make use low voltage loads, such as 12V humdifier.

     Use as small power rate ac-dc as possible, since the bigger ac-dc, the bigger surging current.

I must drive a big tungsten lamp, or ac-dc

     Ok. Make  use of a big enough contactor, drive with semiconductor driver, instead of relay.

     The products with semiconductor drivers:

        ZL-7802A, it can drive 5A@220Vac(R3) tungsten lamp, and accept big surging current.

        ZL-7802AL, it can drive 17A@12VDC(R1) tungsten lamp, and accpet big surging current.

        ZL-7901A with SSR, any power rate depends on the power rate of SSR.


Victor Qiu






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