Electrical – 240v ac to 12v dc transformer voltage drop under load

currenttransformervoltage

I have a 12v air pump, on manual it says current is 9A. So I bought this 240ac to 12v dc transformer it has: 12V 150W 12.5A so I thought the transformer can provide enough current.
This is the transformer I bought:
https://www.ebay.com.au/itm/DC-12V-24V-Waterproof-Power-Supply-Transformer-Adapter-LED-Driver-36-250W-/371937953390

But when I plug the air pump to the output of the transformer, the voltage dropps to 3.6V which the air pump couldn't keep working. This is a video I took:
https://obanau-my.sharepoint.com/:v:/g/personal/todd_liang_obansolutions_com_au/EVljWVMZoytGje_Y-86fKGYBdMTBr3bKtOFa4yBB9FQ9GA?e=YPqaZK

The air pump is working perfectly fine if I use a 12v battery so it has to be the transformer issue, so what could be the reason? Why the voltage dropped so much?

Update: here is the pump
enter image description here

Update2
I also bought a 120W transformer from the same seller and oddly enough the 120W transformer is working perfectly fine, only this 150W (which suppose to be more powerful) doesn't work. I reported this issue to the ebay seller and they sent me a replacement. Today I received the new transformer but it has the same issue.

Best Answer

This isn't a transformer, it is a switching power supply.

The "12V 9A" pump is most likely powered by a DC motor, if 9A is the nominal current then it will draw a lot more than 9A when starting. Perhaps 5x more at least.

A 12V battery won't have any problem with this. A 12.5A switching supply will cut off as its overcurrent protection gets triggered. The motor stopping and starting in your video corresponds to the power supply's protection tripping and then resetting after a time delay.

You either need a much more powerful supply (like a 450W PC power supply with most of the power available on the 12V output), or a mains powered air pump, or keep using the 12V battery.