Let’s first start with a quick reminder by explaining a power supply.

The aim of a power supply is to transform an alternating current into a direct current and generally to lower it.

If you only wish to adjust the alternating current, I encourage you to visit this blog article How to choose your transformer?.

To make the right choice concerning power supplies, you need to have good knowledge of the different consumers power downstream.

You just need to apply the following equation:

Example:

You want a power supply 230vac – 24vdc.

Downstream consumers will be:

• 2 contactors (30W)
• 1 automation system (5W)
• 1 Ethernet Switch (3W)

The equation to apply is then:

P = 30+30+5+3 = 68W

U = 24V

I becomes the unknown that will allow us to size the supply.

P = U x I → I = P / U

I = 68 / 24 = 2.83A

The supply that you need is the one which provides 5A max as the DPS-1-120-24 from EASI-Spare for example.