user81721: I have an air compressor that I often give difficult tasks, where the compressor needs to run longer than it was designed for.  So it gets close to overheating, which requires periodically giving the compressor some "cool down" time, turned off.
Rather than walk into my garage every couple of minutes to turn the compressor on or off, I'd like to automate the process with a little arduino board.
The problem is the compressor is run by a 25A 12V DC line.  Most relays I can find that are Arduino compatible (3.3v DC control) max out at 10A.
I'm curious if there is a smart way to do this efficiently.  Two solutions that come to mind are:

Control 3 or 4 10A relays with the Arduino, and parallelize the current heading to the compressor across multiple relays.

Use one relay controlled by the Arduino, and use that relay to control a secondary big relay, that could control the 25A line.


Neither seems like a very efficient approach to the problem.  I don't know for certain but I worry that perhaps with (1), one relay might be a better conductor than the others, so perhaps one might eventually see a cascade of overheating relays.
Perhaps someone here that's more familiar with circuit design could suggest a better approach.
edit: After a little digging it looks like option (2) is the standard route most often traveled.

This is an OMRON relay that is powered by a 12VDC supply (0.167A), and can switch a 28VDC 30A line.   So I will double relay it, using an Adafruit featherwing to switch the 12VDC supply, which will then switch this big OMRON relay.   In my pile of old AC/DC converters I found a 1A 12V supply that's sufficient to run the OMRON relay.  Provided I haven't overlooked anything this is likely a decent solution.

user68729: Do not double relay it (at least not parallel aka Option 1).  Relays have a tendancy to bounce or take time.  This exposes you to a potential for only one relay to be connected for a time.  Worse, if someone decides to use the relays as high, low instead of closed, open it will short high to low for a time.
The series relay solution is sound but not efficient.  Relays are almost never the efficient approach (there are cases where infrequent switching occurs where a relay simply holding a position without power for long durations can be efficient).  A relay module is essentially a mosfet and a relay.  Mosfet controls the coil, and if you will have one of those, might as well go for a 12V mosfet of appropriate amperage.  I used to have some arduino compatible 3D printer heat bed mosfets that went up to 25A (on paper) so I can't imagine they're impossible to obtain.
You also mention digging power supplies for your relay.  If you are going so far as to need a 12V power supply, is there one providing for the compressor itself?  If so, could you just control the wall outlet AC that powers the supply?  Amperage would be a lot lower and AC can allow use of some rather nifty "Solid State Relay" devices (as well as relay modules, smart outlets, etc).

81721: <p>I have an air compressor that I often give difficult tasks, where the compressor needs to run longer than it was designed for.  So it gets close to overheating, which requires periodically giving the compressor some &quot;cool down&quot; time, turned off.</p>
<p>Rather than walk into my garage every couple of minutes to turn the compressor on or off, I'd like to automate the process with a little arduino board.</p>
<p>The problem is the compressor is run by a 25A 12V DC line.  Most relays I can find that are Arduino compatible (3.3v DC control) max out at 10A.</p>
<p>I'm curious if there is a smart way to do this efficiently.  Two solutions that come to mind are:</p>
<ol>
<li><p>Control 3 or 4 10A relays with the Arduino, and parallelize the current heading to the compressor across multiple relays.</p>
</li>
<li><p>Use one relay controlled by the Arduino, and use that relay to control a secondary big relay, that could control the 25A line.</p>
</li>
</ol>
<p>Neither seems like a very efficient approach to the problem.  I don't know for certain but I worry that perhaps with (1), one relay might be a better conductor than the others, so perhaps one might eventually see a cascade of overheating relays.</p>
<p>Perhaps someone here that's more familiar with circuit design could suggest a better approach.</p>
<p>edit: After a little digging it looks like option (2) is the standard route most often traveled.</p>
<p><a href="https://i.stack.imgur.com/FglvL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FglvL.png" alt="enter image description here" /></a></p>
<p>This is an OMRON relay that is powered by a 12VDC supply (0.167A), and can switch a 28VDC 30A line.   So I will double relay it, using an Adafruit featherwing to switch the 12VDC supply, which will then switch this big OMRON relay.   In my pile of old AC/DC converters I found a 1A 12V supply that's sufficient to run the OMRON relay.  Provided I haven't overlooked anything this is likely a decent solution.</p>
 68729: <p>Do not double relay it (at least not parallel aka Option 1).  Relays have a tendancy to bounce or take time.  This exposes you to a potential for only one relay to be connected for a time.  Worse, if someone decides to use the relays as high, low instead of closed, open it will short high to low for a time.</p>
<p>The series relay solution is sound but not efficient.  Relays are almost never the efficient approach (there are cases where infrequent switching occurs where a relay simply holding a position without power for long durations can be efficient).  A relay module is essentially a mosfet and a relay.  Mosfet controls the coil, and if you will have one of those, might as well go for a 12V mosfet of appropriate amperage.  I used to have some arduino compatible 3D printer heat bed mosfets that went up to 25A (on paper) so I can't imagine they're impossible to obtain.</p>
<p>You also mention digging power supplies for your relay.  If you are going so far as to need a 12V power supply, is there one providing for the compressor itself?  If so, could you just control the wall outlet AC that powers the supply?  Amperage would be a lot lower and AC can allow use of some rather nifty &quot;Solid State Relay&quot; devices (as well as relay modules, smart outlets, etc).</p>
