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I have a device absorbing up to 12 amps that is powered by a 3S 12.6V lipo battery. Every time I have to start this device, I need to plug-in the battery and this is quite annoying. I would like to keep the two connected and have a MOSFET which does the job. But I don't know where to start.

Can anyone suggest me a possible circuit layout and eventually a MOSFET to do that?

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  • \$\begingroup\$ @Big6 's FEt would work but dissipates a lot of power and needs a substantial heatsink. Eith eg this or this FET will dissipate under 1.5 Watts at 12A and need a minimal heatsink (PCB copper probably OK). In your case it's not an issue but for higher voltages a clamp zener from gate to source keep Vgs within limits. \$\endgroup\$ – Russell McMahon Nov 11 '19 at 2:11
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You could try something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

You'd need to operate the switch to allow current to flow to your load device. When the switch is open, no current will flow to the load, when the switch is closed, the PMOS will allow current flow to the load. Notice the high value for the pull-up resistor (1M\$\Omega\$)—it really could be something lower (eg. 47k\$\Omega\$, 100k\$\Omega\$, etc), but since you're using a battery pack, you probably want to be more efficient.

The PMOS has to be capable of handling the whole load current (>12A). This MOSFET, as an example, can handle continuous current up to 18.7A. You may still have to add a heatsink to avoid overheating it.

EDIT: As suggested by @RussellMcMahon, better transistor options are available (lower RDS(on) but the approach still holds.

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  • \$\begingroup\$ At this point, why not just use a mechanical switch? Then again, a switch that can handle 12A DC might be a bit much... Hm. \$\endgroup\$ – Hearth Nov 11 '19 at 1:08
  • \$\begingroup\$ @Hearth you're correct, a simple switch is going to work but the high current was what got me thinking about a MOSFET. A lot easier to find MOSFETs that can handle current above that . Also since the OP mentioned MOSFET specifically, I wanted to give an answer in that context. But your point still holds, may be worth the effort to just drop a SPST switch there and call it good. \$\endgroup\$ – Big6 Nov 11 '19 at 1:26
  • \$\begingroup\$ Appreciate that whoever downvoted, point out what's wrong with the approach I suggested. Thanks. \$\endgroup\$ – Big6 Nov 11 '19 at 3:42
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    \$\begingroup\$ Upvoted to compensate for the downvote. \$\endgroup\$ – winny Nov 11 '19 at 6:31
  • \$\begingroup\$ Thanks for your suggestion. I asked for a MOSFET and not a mech switch because my intention is to eventually switch on/off that MOSFET by using an RFID unit. The MOSFET will power up only if the output of the RFID is true. I have no idea about this would work but at least this is the idea behind. I have to combine all the pieces together ... your suggestions are a good start \$\endgroup\$ – Emanuel Nov 11 '19 at 21:17

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