# MOSFET vs. relay power efficiency

Newbie question here. I am putting together a remote switching box for power circuits aboard a yacht. The design is for a central Raspberry pi to send orders to a few boxes, each controlling a number of circuits distributed through the vessel. Each box will contain an Arduino communicating with the RPi using XBee.

Each circuit will have a typical draw of a couple A with a max of 15 A. Typical switching frequency will be very low with some circuits staying almost always on and others almost always off.

My question is for the actual switching, should I choose relays or MOSFETs? My question is specifically around power conservation which is a primary concern on a sailing vessel. Given the high heat output of the MOSFET, am I correct in assuming these are wasteful components? More so than a relay?

• Relays, mosfets, IGBTs and SSRs can all have potentially negligible power consumption. Relays have moving parts, so have much lower cycle lives and also make noise. They have subjectively the smallest resistance and can maintain their state or not when power is lost, depending what you want. At 5-15A mosfets should be fine. They only heat up (waste power) when switching too slowly or conducting too much. – K H Sep 4 '18 at 5:39
• If you're talking about switching low voltage DC (like 12V for example) MOSFETS with less than 1mOhm of on resistance are available. That's around 200mW of dissipation and pretty competitive with a relay given the better life and reliability. – John D Sep 4 '18 at 5:46
• Given the high heat output of the mosfet A MOSFET used as switch is only inefficient (has high heat output) if you use it incorrectly. If used correctly MOSFETs can be very efficient and can switch with very low losses. As a newbie, watch out with these assumptions /statements because often they're simply not true and only vented by this who don't have a clue. Don't become one of them! – Bimpelrekkie Sep 4 '18 at 6:22

Your choice depends on a lot of things:

• Are you switching AC or DC?
• What is your switching voltage? (5/3.3V from a microcontroller? 12V from a battery? You mentioned and Arduino/RPi/Xbee but what exactly is doing the controlling, and is there any other power sources available?)
• Switching speed
• Thickness of your wire (how much power have you already lost from just copper losses?)

Efficiency is much of a muchness, and while relays might "seem" more efficient because they are a galvanic contact, that is definitely not the case. Just now with a quick google I found a relay with 100mR of resistance rated for 15A (here). This gives a power loss of:

$P = I^2 \times R = 15^2 \times 0.1 = 22.5W$

On the contrary, I also just now found a MOSFET with an on resistance of 5mR (here). This gives a power loss of:

$P = I^2 \times R = 15^2 \times 0.005 = 1.125W$

Yes - you can choose a higher-rated relay and that will have a lower contact resistance, and probably you will find one even more efficienct than the MOSFET. But that will cost more. Is cost a concern? The list of factors goes on.

Just goes to show you can't assume anything. If you give more details of your project I can help further and edit my answer, but for now it's very open-ended and frankly, depends on a lot of factors you haven't mentioned.