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UMC Wiring

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Unfortunately, Tesla are out-of-stock of the adapters that I need. So, I considered ordering some California Connectors but they're really expensive!

I feel a minor bodge coming along involving some 14-50 trailing sockets, off-the-shelf 5-15, 6-15 & 10-30 cords and careful use of the VDS current limit.
 
This weekend I made up some cables (would have preferred the Tesla ones but, still). I made 5-15/20, 6-15/20, 10-30 & duplex 5-15 to go with the existing 14-50/30 (a trick donauker taught me).

I tried the diode and found:

wht-green open: 12,15A
Diode (cathode to wht): 16A
Diode (cathode to grn): 24A
wht-green closed: 40A

I used this in my adapters to limit max current though I did see that voltage doesn't play a part and that the UMC was happy to set up 120V@40A. I tried a couple of zener diodes, 5.1V and 12V but they made no difference.

I also found out why the outlet on the front of my house doesn't work; they've transposed hot/ground on the 120v outlet; car reports the error. Same is true for the UMC, at 120v, the grounded wire has to go to YYY. Nice.
 
...duplex 5-15 to go with the existing 14-50/30 (a trick donauker taught me)...

Are you talking about the "find the two phases to get 240V from 2 120V outlets" trick?

Like one of these? :
220 240 Volts from 110 120 Volt Outlets - Catalog & Pricing

If so, you ought to have safety relays so that the 2nd plug can't be energized while you are fishing around for an outlet with the other phase.
(Possible safety hazard if done wrong.)
 
I also found out why the outlet on the front of my house doesn't work; they've transposed hot/ground on the 120v outlet; car reports the error. Same is true for the UMC, at 120v, the grounded wire has to go to YYY. Nice.

Do you have one of those simple outlet testers that can find mis-wired outlets easily?
41ruIGVdvcL._SL500_AA300_.jpg
 
...and does the diode need to be a Zener?

For that matter, aren't Zener diodes marked with the cathode and anode in reverse? Typically, a Zener allows current to flow in both directions, but the reverse breakdown voltage is the one that's used for effect, while the normal forward voltage is not utilized.

I would suspect that a simple diode is what is needed in the charging cables, and you may just be lucky that a Zener works at all. i.e. a really low voltage Zener might confuse the charger because it allows current in both directions. Anyway, Don already mentioned that some of the chargers use a regular non-zener diode.