2

For my capstone project I am building an outlet which can be controlled using WiFi signals.

I am having trouble finding a suitable relay for this device. The relay I first used was a latching relay (399-11009-1-ND).. but it only had a maximum switching power of 125 VA. I am unsure of how this VA value was determined at the maximum switching current is 2A and the maximum switching voltage is 220 VAC. By multiplying the two I expected a value of 440 VA. Why is the VA value 125 VA instead of 440 VA?

https://www.digikey.com/product-detail/en/kemet/EE2-3SNU-L/399-11009-1-ND/4506461

user134923
  • 21
  • 2

2 Answers2

8

The maximum voltage and maximum current are not allowed to occur in the same application.

If you are running the contacts at the 2A maximum current you are limited to 62.5V to ensure you don't exceed the 125VA rating.

If you are running the contacts at 220V AC you must limit the current to 125/220 = 0.56A.

The rationale behind this is that the arc energy that limits the life of the contacts depends upon both the current and the voltage.

Often the DC rating will be far less than the AC rating as a DC arc can be self-sustaining. That relay doesn't seem too bad, often on manually operated switches the DC voltage may be limited to only 30V but 250V AC. With AC the arc that is formed upon opening of the contacts will be quenched when the voltage goes to zero every half-cycle.

Kevin White
  • 32,097
  • 1
  • 47
  • 74
5

This sort of graph generally applies to contacts, transformers, motors, power-semiconductors and power inverters etc: -

enter image description here

Picture taken from here, a website about power inverters.

The main part of the graph is a curve where the product of V and I is a constant but, at extremes, there are end-stops that represent the maximum voltage or current of the device.

Like I said earlier, it's a commonly used graph for power (or VA) in many electrical components and parts.

Andy aka
  • 434,556
  • 28
  • 351
  • 777