I am working on a DIY drone, and I (preferably) need to be able to disconnect my ESCs from the battery via the transmitter (a Raspberry Pi). After looking for solutions for a while, I concluded FET switches are my best bet to automate this. However, I am not certain how to determine the requirements for the switches (I have no engineering knowledge, I am learning on the fly). Considering that the maximum current draw for the motors is 12A, the battery is 11.5V, and the RPI's pins for control are 3.3V and can take a maximum draw of 16mA, are my assumptions correct:

  1. Switches need to have Vds>11.5V, and Vgs<2.5V
  2. The draw on the gate can be neglected because it is ~0

Also, what considerations should be made about the S->D current draw? And am I missing other factors?

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    \$\begingroup\$ In RC applications, the goal is to use the lowest possible \$R_{DSon}\$ to minimise heating which is \$P=I^2R\$. You probably want to use high-side switch which can be done either with PMOS and a small BJT (simpler) or with NMOS and self-oscillating bootstrap driver (cooler). \$\endgroup\$ – venny Sep 18 '14 at 14:51
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    \$\begingroup\$ "Learning on the fly" -- I like that :-) \$\endgroup\$ – Dave Tweed Sep 18 '14 at 14:54
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    \$\begingroup\$ Why do you need to disconnect the ESCs from the battery? WHat is it you are trying to achieve? For example, why would it not be sufficient to switch the ESCs off? \$\endgroup\$ – gbulmer Sep 18 '14 at 15:15
  • \$\begingroup\$ @venny thanks for your suggestions I will research both of them :) \$\endgroup\$ – Mitio Sep 18 '14 at 15:16
  • \$\begingroup\$ @gbulmer It is indeed sufficient, I am simply looking for a way to automate the process. \$\endgroup\$ – Mitio Sep 18 '14 at 15:19

Generally, it is a good idea to over-specify in such a situation, especially as FETs don't quickly get unreasonably bulky or expensive at these power levels.

Specifically you will need to consider that most ESCs have capacitors at their battery inputs which can draw quite a lot of current when they are connected (that's why you can sometimes see sparks when connecting an ESC).

You are right about the gate current, as long as you don't continuously toggle the FET, the gate current is negligible. Also, as you stated, the transistor needs to be in saturation/ cutoff at the GPIO high and low voltages, respectively.

However, you've completely overlooked a very important specification of FETs: RdsON. This is the drain-source resistance when the FET is in saturated mode. For an application like yours where switching is only occasionally performed this is the main cause for power dissipation and thus heating, which can effectively destroy your switch if it cannot dissipate the heat quickly enough.

  • \$\begingroup\$ Thanks for your fast reply hasep, I will add this to my considerations. \$\endgroup\$ – Mitio Sep 18 '14 at 15:38

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