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.

  • \$\begingroup\$ did you connect it correctly? ... you made no mention of connections \$\endgroup\$
    – jsotola
    Mar 13, 2018 at 0:44
  • \$\begingroup\$ Can you post a link to the manual of the pump? Such voltage drops usually result from the load drawing too much current. \$\endgroup\$
    – koalag
    Mar 13, 2018 at 0:45
  • \$\begingroup\$ in the video, it looks like you may be shorting the cables together \$\endgroup\$
    – jsotola
    Mar 13, 2018 at 0:46
  • \$\begingroup\$ No I'm not shorting the cables together, I made sure that. And I did test this many times. \$\endgroup\$
    – Todd Liang
    Mar 13, 2018 at 0:56

1 Answer 1


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.

  • \$\begingroup\$ Thanks for the reply, oddly enough I also bought a 120W transformer from the same seller and that 120W transformer is working perfectly fine. How come this 150W doesn't work... \$\endgroup\$
    – Todd Liang
    Mar 13, 2018 at 0:57
  • \$\begingroup\$ So that could mean the 120W transformer overcurrent protection is broken? \$\endgroup\$
    – Todd Liang
    Mar 13, 2018 at 1:04
  • \$\begingroup\$ It could. Or it could mean that its protection circuit allows a longer overload peak, and that's just enough for the pump to start up. \$\endgroup\$ Mar 13, 2018 at 2:23
  • \$\begingroup\$ @ToddLiang The 'continuous output' watt rating of the unit is not the important specification, it's the response to motor-start surge currents. Shutdown, or voltage dip below nominal, or reset of the power supply logic, are all normal behaviors one expects to see. Some of those normal behaviors make the motor run, some don't. \$\endgroup\$
    – Whit3rd
    Mar 13, 2018 at 3:51
  • \$\begingroup\$ That depends what kind of protection the switching supply uses: the 150W seems to shut down, wait, and retry. If the other one has a current limiter instead, or a slow enough soft start, it could explain why it works. In this case the motor would take a bit more time to spin up than with the battery, it would also have less torque, but that doesn't seem to bother it here. \$\endgroup\$
    – bobflux
    Mar 13, 2018 at 11:07

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