2 x 120 -> 240 done right

[Nicholas Sayer]

I see this come up once in a while as people want a cheap way to charge at L2 without calling an electrician.

There are a lot of caveats, but I think it can be done with a reasonable level of safety, IF it's done correctly.

http://www.kfu.com/~nsayer/hydra/220_adapter.png

The left two relays in the schematic are 120v coils, and the third is a 208/240 volt coil. DigiKey sells 120v and 208/240v coil versions of the exact same DPST relays the Hydra reference design uses for just under $15 each that are good for 30A.

Not shown in the schematic are two neon indicator lights across the two 120v relay coils and one across the two outlet hot lines. When all 3 are lit, you're good to go.

I believe that circuit is the minimum required to do this safely. It insures that there's 120v between each hot and its incoming neutral AND that there's 208/240 between the two hot lines before anything is connected to the outlet. It guards against asymmetric hots, same-phase connection, hot-neutral swaps and energized plugs when disconnected.

You must insure that the two outlets you pick are on different breakers sourced from opposite phases (there is no danger in getting it wrong - you just won't get the third light and it won't work). You must also insure that there are no GFIs present as this concept is actually guaranteed to make them trip (but again - if one trips there's no danger, it would just stop operating). You must also insure that you draw no more power than the *smaller* of the two breakers involved allows.

The circuit doesn't check that the ground is good - but OpenEVSE's own ground test would fail if it wasn't (assuming that's why you're using it).

We drive our Volt down to my parents' house in San Diego a few times a year and I've been charging with my portable OpenEVSE at L1, but I think for our next trip I will try to make one of these to see if we can't go at L2. Since it's a Volt, 16A L2 would max out it's charging speed, so there's no need to do any better than this (I might back it down to 12A just to be absolutely sure, though).

[Nicholas Sayer]

I brought it down for the holiday. It was a pain finding a pair of outlets that worked, but I did find a pair. They're not very practical, but for temporary use, it works.

[gnuarm.del...@gmail.com]

Hi Nick,

I see this is an old thread so you might not still be around.  I haven't seen enough recent posts to get to know people here yet, so maybe I've already seen your recent posts but don't recall.  lol

I'm wondering what the function of all three relays is and why they are seem to have the same part number.  I see in your description the third relay is a 240 volt coil.   So the schematic just doesn't indicate this? 

The two relays on the input seem to be checking for voltage on each input separately, but won't connect the input through unless both relays see 120 volts. 

The final relay I assume is checking that the two inputs have opposite polarity and so provide a significant voltage to the output. 

I guess this circuit uses an abundance of caution.  It seems to me the two input relays could be wired to the third relay coil only and the third relay would prevent all current from reaching the output until both inputs have voltage of opposite polarity.  Then the two input relays can be smaller, lower cost units that only need to switch the current to the third relay. 

Am I missing some important protection your relay arrangement provides? If an outlet the device is plugged into is wired wrong, the final relay coil shouldn't have enough voltage to trip and connect the output, right? 

  Rick C.

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  - Tesla referral code - https://ts.la/richard11209

[Nick Sayer]

For the full details as to the function of each of the relays, I recommend reading the functional description on the project page at Hackaday.  https://hackaday.io/project/4818-a-quick-220-clone

Sent from my iPhone

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[gnuarm.del...@gmail.com]

Great!  I found the issue. 

The purpose of these two relays is to insure that nothing is connected to the output unless both source circuits are energized. One of the hazards of this concept is that if you yank one of the 120 volt plugs of a naively designed version of this box, a high voltage would be present on the prong of the plug. This is because a circuit would exist from the opposite phase's hot line through the target device and out to the plug. The primary relays prevent this sort of thing from happening.

But you still have this protection with the simpler relay circuit I describe.  Both relays are in the circuit that activates the 240 volt relay coil.  Pull either plug and the exposed plug pin is disconnected from the circuit because the relay drops out.  The redundancy of each relay also isolating the opposite relay that is superfluous.  Once power is lost to the main relay all connection to the load is cut and the exposed pins are isolated by each primary relay. 

Am I missing something or is this design a bit over complicated?  The only advantage I can see is that with the two pole relays a single stuck contact won't be dangerous.  In a one pole approach it can pass a dangerous voltage to the input plug pin.  But you can hear relays working so a stuck relay can be detected.  Also it has indicators on the inputs so you can tell the relay isn't clicking because of a failure rather than simply no voltage.  So this risk seems minimal.

  Rick C.

  + Get 6 months of free supercharging
  + Tesla referral code - https://ts.la/richard11209

[Nick Sayer]

I went for “belt and suspenders” design since the whole concept is almost inherently unsafe. It’s been a while since I’ve thought about the design, but there was some corner case I was worried about. It might have been just verifying that the neutral was correct, though that doesn’t wind up affecting the output any. 

Sent from my iPhone

[Brendon Whateley]

Doesn't your proposed plan cause the output to energize with 120V if both inputs are on the same branch/phase of the power?

Joining 2 phases like this is really risky in a lot of different ways, which is why the design is so careful to cover the oddball cases.

- Brendon

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