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I am fairly new to hands-on electrical engineering (I have taken Physics II and have played with computers and electronics all my life, but am only now truly seeking to learn how to safely handle many different electrical appliances) and am involved in a greenhouse automation project.

The greenhouse has 220V AC power. I have a number of submersible liquid pumps that draw 12V, 1.8A DC and have 4A fuses. I've hacked up some old router and cellphone AC/DC adapters that do the trick, but I would like to add a dedicated DC line to the work space for additional motors, pumps, sensors, and other electronics that need DC power.

My question is: if I have an AC/DC adapter that outputs 12V, 16.5A, 203W DC power, would that fry my 4A fuses (as I suspect?) What if I have 8 or 9 1.8A pumps wired in parallel? Could I hook and Arduino Mega into any old DC line, or must I downstep it to the current amperage?

Anindo Ghosh
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sneurlax
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Fuses limit how much current can be used, not how much can be supplied. As long as the devices after the fuses don't use more current than the fuses are rated for, there won't be a problem (with the fuses).

Ignacio Vazquez-Abrams
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It is not clear from the question if each motor has its own 4A fuse, but assuming it does, you should be OK.

A motor briefly takes much more than its rated current when starting; presumably that current drops below 4A before the fuse can blow. Check the fuses, they are probably a "slow blow" variety. It will also take more current when stalled (e.g. trying to pump dead leaves instead of water) - then the fuse may blow to protect the motor.

Then your 16A AC/DC adapter will probably run 8 motors (each with its own fuse) just fine, as long as it doesn't have to start them all at once. What happens then would depend on the adapter : it may current limit to 16A until the motors are running (they will either start slowly, or not at all), or it may shut down immediately to protect itself, or it may blow an internal (16A) fuse, or it may emit smoke...

  • So if there is a brief mains power outage of a second or so whilst all 8 motors are running, the AC adapter may be unable to cope when AC power comes back on. Ditto if someone switches power off and on at the AC adapter. – RedGrittyBrick Aug 10 '13 at 10:06
  • Yes, some possibilities of failure there. One reason why larger motors often have NVR switches. –  Aug 10 '13 at 14:57
  • Thank you for your answer. That was exactly what I was looking for – sneurlax Aug 10 '13 at 15:45