# Do I use a relay, MOSFET, or just a switch? (5VDC 15A load)

I have a 5VDC 15A power supply that will be powering both an Arduino and 288 Addressable LEDs. How do I switch the circuit on and off? First I looked at relays. The only relays that I could find that support a 15A load are ones that use High Voltages around 250V. I think they had a minimum recommended load voltage of 12V. I also looked at mosfets, but I found that many would burn up if I put 15A of current through them.

I have also considered just using a switch, as everything is at the same voltage. In that case, I would need to make sure the switch I use can handle the 15 A of current. Hopefully a switch like that wouldn't be too bulky.

Basically, My load voltage is too low to use a relay, but I'm afraid the current is too high to use a mosfet or normal switch. How should I turn this thing on and off?

• Your more likely to find a relay to handle 15 Amps VDC than you are a manual switch. And at that current youd likely wouldnt want to pass that through a user touchable part. Theres plenty of relays that list contact ratings of 15 amps or more at 30V DC and a 5V coil voltage. Where are you looking? The 250V ones are likely AC ratings. You need DC ratings. – Passerby Jul 18 at 2:53
• Secondly, are you sure you want toswitch the DC side off? That leaves the AC side connected and leaves the supply powered. Minimal energy draw but still. Or even switch it at all? The microcontroller can go into a low power mode, and all the addressable LEDs can be turned off via command. Practically the same in this instance as the current draw with everything off is a fraction of the 15 amp supply. – Passerby Jul 18 at 2:56
• Finally for the mosfets, look for power mosfets and likely put some in parallel to get the needed current. – Passerby Jul 18 at 2:58
• @Passerby I was looking on Mouser.com, but I may have mixed up the search filters and got bad search results. For your second point, Your saying if I left the Arduino on in a low power state and signaled the LEDs to turn off, the current they draw would be insignificant compared to what the power supply already draws at idle? Should I be concerned about wearing out the Arduino or LED strips? – The Mungler Jul 18 at 4:42
• Not knowing what smart led you are using, the WS2812b has a 0.4 to 1 mA quiescent current when off. And the Arduino is probably up to 20mA when in low power mode due to the leds and regulator. A bare ATMega is under 1mA deep sleep. So at worst, say 300mA when off, or 0.3A out of 15A, or 1.5W. Per standby.lbl.gov/data/summary-table a laptop charger left plugged in without a laptop, is between 4.42 to 0.15 Watts standby. So yes, you are drawing less on average than the supply by itself. As for the wearing the parts out, you need a fortune teller. With the MCU sleeping/LEDs off, I say no – Passerby Jul 18 at 5:27

You can use a simple p-channel MOSFET as a high-side switch.

simulate this circuit – Schematic created using CircuitLab

The particular part listed above is cheap and has an Rds(on) of less than 2.5m$$\\Omega\$$ @25°C with 5V drive, so it will drop less than 38mV at 15A, so it will dissipate about 0.8W when hot, requiring a few square cm of copper to run reasonably cool. A slightly more expensive part (still less than \$1.50) is more like 2m$$\\Omega\$$ @25°C so it will run cooler.

• Is there a reason I should use a p-channel instead of a n-Chanel? It seems counter intuitive to apply a gate voltage to open the circuit. – The Mungler Jul 18 at 6:22
• Good question! Often high side switching avoids problems with inputs and outputs. Otherwise sneak paths may energize outputs and cause unwanted and sometimes damaging current to flow into inputs. If the input is 0V then no current flows with a high side switch. A low side switch may require the input to be held at 5V. – Spehro Pefhany Jul 18 at 6:25

You should be able to use a simple relay, like this one:

https://ar.mouser.com/datasheet/2/307/en-g2rl-1670825.pdf

Don’t confuse the coil voltage with the max voltage the relay can handle. The 250V refers to the maximum voltage the relay can switch on/off because of the electric arc that's formed during commutation.

It depends if you are looking to control the system manually (ie flip the switch by hand) or make another electronic system to flip the switch for you. In the latter case, I would look for an automotive N-channel mosfet to install on the low side of your circuitry. The mosfet would also allow you to use a much smaller switch, if you are going for the manual solution.

There are plenty to handle 5V 15A. For instance IRLZ34NPBF (the first to appear in my google search).

A relay would also work, but I would not recommend using one. You should use a mosfet since you are dealing with direct current. Relays are more expensive and less reliable.

Be careful using relays intended for AC loads. These are designed so that any any arcing that occurs as the contacts open is quenched by the AC signal becoming zero 120 times per second.

An automotive relay would be a better choice, although its coil may require 9 V to operate.

• Good point, I didn't know that. But wouldn't arcing not be a problem if I'm only using 5 V? – The Mungler Jul 18 at 6:24
• Arcing is a long term reliability issue. It's worse at high currents (not specifically high voltages). If you only have 5V available, an automotive relay won't work for you -- you're better off using FETs. In case there are noise or other issued between the Arduino and the LEDs, you might consider using 2 separate FETs -- one for the Arduino, the other for the LEDs. – jp314 Jul 18 at 22:14

So lots of these answers detail very well that you can use a MOSFET or a relay or even a switch. All the mechanical solutions (relay and switch) will have the problem of arcing at these relatively high currents (more on that in a bit).

But they all miss an easier solution. You say these are addressable LEDs. So before you go whacking the power off to a 15A load, first command them off using the arduino (which is presumably generating the commands to the LEDs). Bam. Your load just went from 15amps to like 10mA (arduino current). Now if you want to kill power using an external switch it becomes trivial.

Now as to why the mechanical solutions can potentially have arcing issues: while the system voltage is indeed 5v, the moment you try to pull away the pieces of metal conducting those 5v @ 15amps there is a rather large change in current per unit time. Any inductance in the circuit (either stray or intended) will resist that rapid change in current and cause a voltage spike that will likely arc per V = L*di/dt.

Mechanical switch/relay designers know all this, the plating on the switches and relays are designed to handle this presuming you stay within the DC limits of the switch/relay current. Not doing so will eat away at the lifetime of the switch/relay and would require you re-rate the switch lifetime (re-calculate the expected life of the switch based on the new info). Like somebody else said, automative relays are a great source for higher-current relays, and plenty, plenty of MOSFETs are more than capable of handling 15amps.