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I would like to control a pump. The pump rating is 12 volt 60watts. The pump mfg says to use a 12 volt power supply of 5 - 12 amps but AT LEAST 5 amps. This pump has a pressure turn off at 116 psi and the mfg states that the pump will not make maximum pressure with less than 5 amps.

So the question becomes, to be able to switch this pump on and off based on some sensor reading what would be considered best practice: transistor, MOSFET, or relay?

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  • Transistor - Big voltage drop between C-E @ 5A (depends on type but it will be about 2V). Power dissipitation is voltage drop times current so about 24Watts. You must be careful with inductive load - induced high voltage must be supressed. Usual way is using anti-parallel fast diode over load.

  • Mosfet - Voltage drop can be really small, so it might be possible to use it without heatsink. Big Gate capacity needs strong driver to charge and discharge. If not, slow switching causes huge losses (again voltage drop times current). For inductive load it's same as for transistor.

  • Relay - It must be selected carefully as switch-off causes electric arc (= burning contacts) and because of DC current it's burning for much longer time than AC.

  • If you're going to use a transistor make sure you use it as a high side switch – Doodle Aug 10 '16 at 8:49
  • @Hayman Could you please explain reasons for this? Now it's like "because someone said so" – KIIV Aug 10 '16 at 9:36
  • In most cases you can just use it either side. However as it is a reasonably sized pump I'd probably remove the power from the device rather than the ground from the device (just personal preference/intuition). That's only because of this specific circumstance however. Also a couple of other things, you can get low Vce(sat) transistors even at higher currents. A Mosfet with a drain current of 5A is going to likely need more voltage on the gate than an arduino can supply. The best choice here is a relay used with a low side NPN transsistor – Doodle Aug 10 '16 at 10:12
  • I know I'm being a bit wordy now, sorry about that. Here's a good article about possible issues with low side switching – Doodle Aug 10 '16 at 10:15
  • I'm not 100% sure, but could a flyback diode help with the relay situation? – Len Aug 10 '16 at 14:04
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The problem with a MOSFET is that at 5A you might find that the arduino output pin can't deliver enough voltage to the gate of the MOSFET to get it to fully turn on.

For the transistor, it is possible to use a transistor however it is probably best to construct it as a high side switch as opposed to a low side switch. This is to deal with potential problems that may arise with disconnecting the ground from the pump whilst running.
This is a good article which explains why low side switching can be bad

The best possible solution in this circumstance would be to use a relay connected to a NPN transistor which switches the low side of the relay. This isolates the delicate arduino electronics from the pump.

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