compressor left on.

Just as I was getting ready to leave tonight, I heard the compressor in the metals room kick on…
I had not turned it on, So it must have been left on from earlier in the day or yesterday…

Please remember to check/turn it off after 10:30pm or when you are leaving the hive. This is necessary to help maintain good relations with the tenant in the upstairs appartment.


It would be nice if we could set up a light outside the metal room door to indicate it is on or off.

Yes we talked about making some kind or timer to turn on/off the compressor at the appropriate time. (Arduino powered servo to flip the switch).
I’m betting there are industrial timers that exist just for this purpose. I’ll do some digging.

Found a few on Amazon:

Here is one that just goes into the plug. Not sure of the Watts/Amps for the compressor. About $25:

Here is a more industrial one that would need to be wired into the circuit. About $50:

Let’s just get one.


Yes, finally, let’s do it!

We have a 5 HP motor, you’re going to want something rated >30 Amps. The 2nd, heavier duty switch looks good – I checked their datasheet on their website too. Honestly, anything not rated for 5HP or 40 amps might weld its own relay contacts together (fun, but not useful)…


I agree on the industrial one.
I’ll put it on the warden sheet since it is in the price range.

Wait, Lorin, genuinely curious here… if the motor is 5HP I’m assuming the amp rating on the side of the motor is 25 or so? Do you really need gear that’s rated for 40 amps to handle that? …Asking for a friend with a 5HP motor that just bought a lot of expensive 30-amp rated electrical stuff…

The 40 amp capability is probably a bit more than needed, but for only $50 I’m not sure it is worth trying to save money.
If it was $500 I would dig deeper.

Isn’t it bad if the timer shuts off the compressor while it is running? Is there a way to prevent that?

People should still check and shut the compressor off and they leave at the end of their making session. The timer should be the safety backup, right?


The timer should be able to handle it.
Switching under load is part of the design.


In practice, when dealing with reactive loads (motors are inductive), it’s best to use higher power handling parts. Notice that the rating on the relay is for “resistive load”. A motor is an inductive load, not a resistive load. A simple resistive load simply dissipates energy into heat at a steady rate. An inductive load STORES energy in a magnetic field. In a motor, this stored magnetism in a stator winding (inductor) is then opposed by the opposite field polarity of the rotor winding – the magnetic energy is now “spent” as kinetic energy, spinning the rotor until the two electromagnets are in phase. So, an inductor stores and releases energy as magnetic reluctance. A resistor simply releases energy.

All of this happens over time. The Henry, which is the SI unit of inductance, is defined as 1 henry = 1 ampere of change per second producing 1 volt. The key is the change / ampere per second thing. Time. The electrical characteristics of an inductor are highly dependent upon time. Simple example is, our 60 cycle AC power applies x amperes in 1/60th of a second and results in an AC motor turning at the equivalent rate.

The problem comes when you electrically disconnect an inductor. If you suddenly disconnect an inductive load, like a motor, the electrical current is almost instantaneously removed, but the stored magnetic field is not. In your 25 amp 5HP motor, 25 amperes charging the inductor at 1/60th of a second now becomes something like 25 amperes in 1/100,000 second. The power stored in the inductor is now sort of “whip-lashed” into dissipating very rapidly. The rotor can’t absorb it, because it can’t do a sudden full turn in 1/100,000 of a second. So, pressure builds up in the inductor. In electricity pressure = voltage (electromotive force).

The pressure keeps building and building faster than the motor can handle, and can become millions of volts over a matter of milliseconds. This can be a problem in a lot of different ways, but in this case: the voltage generated is high enough to arc across the relay contacts which are disconnecting and dump all of the energy in 1uS to reach ground. If the relay is a solid state relay (mosfets or igbts), it’s a pretty spectacular explosion of blue smoke and shrapnel. In this case, though, the relay is mechanical, so there is an arc of electricity jumping across a gap of air or gas. Put simply, this is an arc welder. Bang, your relay is now welded together.

TL;DR – Suddenly disconnecting an inductive load causes an instantaneous release of energy from the collapsing magnetic field. This could arc weld the relay contacts together as the energy dissipates to ground. In reality, the pressure switch mechanism and a snubber circuit dissipate this energy somewhat, but the very brief power across the relay is HIGH, and will exceed the steady state power. So, get a relay that is rated higher. It will also have larger contacts that conduct this instantaneous current rather than heating and melting.

Professor Parker

P.S. Any physics nerds feel free to correct any oversimplifications or errors I may have made in explaining this.


Although I love doing classroom demonstrations where sparks explode through relays, Brad is mostly right. But, because the timer is rated for resistive load, it does not account for dv/dt and L(di/dt). The practical answer is that a relay with 40 amp handling (150%) should be able to handle this burst of power simply due to beefy-ness.

Also, pressure switches in the compressor are generally designed to absorb some of this, otherwise we’d hear a “zap” or see a spark when we turn off the compressor with the manual switch.

If it’s a problem, and we hear arcing on the relay, the simple solution is to put a simple filter between the timer and the compressor. I think we have some ac filters just lying around (I know I do at home too).

No big deal, but don’t get the smaller one.


Here’s the visual demo:



You are the man! Thanks for that explanation and link, I appreciate it. Er…my, uh, friend appreciates it :slight_smile:

I’ve always regretted not taking a second semester of physics in college (E & M).


Just a suggestion as I have played around with my compressor from the shop. Can we just install a low amperage relay inline with the signal wire to the conactor assembly that kicks the unit on? Im pretty sure ours has a contactor that is triggered from the tank pressure safety/ pressure on switch? I just about have the car back together and the new calipers and rotors that have been backordered are supposed to be in tomorrow :slight_smile: .

I broke a caliper bracket in the rav4 and tue factory small brakes for it were 4 times the price of throwing a big brake kit on it. Vehicle manufactres have to quit using 6 different people to make things to save money on the same part, as you have to get specifics for these cars or else. More and more I really want to get an old El camino lol and slap an aftermarket basic hybrid system on it :slight_smile: * Or another ranger of course Lorin, as I know you have love for that model as well as I.

I would only replace my late 1990’s ford ranger with a late 1990’s ford ranger. I haven’t found a better compact pickup made after 2000…

Or, pop a new engine in a vintage 60 or 70’s pickup. Or same with an El Camino – that would require custom airbrush paint job too…


Regarding compressor:

Just get the timer. End of discussion. Too many solutions and analyses is what killed this discussion 2 years ago (vaguely remember suggestions of arduino and espi smart devices that could send out tweets and emails for the compressor).

Kevin, I’m not saying your suggestion is a bad one, it’s not. I’m just saying that sometimes, time is more valuable than money, and any creative, custom-engineered solutions will inevitably be reinventing the light switch. I think we need to prioritize better hacks.


Lorin, I am completely with you on just getting the timer and I likely did not phrase things well as usual lol. I quickly skimmed through previous post and arrived at the just get the timer as well. I should have stated would it be bettter to run the timmer up on the compressor head or go with a inline mount on power in. Given the compressor being off by news curfew having to be stated all the time yet we never pulled rge trigger is much like… wait look a squirrel!!! What were we saying about tge fab lab lol :wink:

I hope to make it down and see everyone Tuesday. My next acl and mcl surgery is the 19th so I will be out of comission for a few days. I am trying to get all car parts needed for the forseable future and plan on spending a lot of time at the hive to get out of the house. I have been really really wanting to build my delta kit still down there however dates of opps kept being moved and or rushed. I hope all is well with everyone! - ps I can start a seperate thread however did we ever come to a vote on media blasting cabinet that can do metal, it is something that would be extremely usefull for casting and forging stuff I would like to setup
For days out on the dock… no hijack intent on the thread :slight_smile: