ROBERT via EV wrote:

> Again I must correct myself.  I looked back at the J1772 standard and some
> past work that I did.  An EVSE outputs a 1KHz square wave.  The duty cycle
> of this wave informs the the vehicle of the maximum amperage that the EVSE
> can supply (rated current).  The vehicle is responsible for not drawings a
> current greater than the EVSE rating.

Yes.

> All lower amperage adapters must
> limit this duty cycle to the rating of the adapter.  As an example.  If
> the EVSE is rated at 50 amps and the adapter is only rated for 20A, the
> adapter must adjust the duty cycle.

This is where you lose me.  If the EVSE is rated for 50A, then it generates a 
pilot signal that informs the vehicle/charger that it can draw up to this 
amount.  There is no need or purpose for an adapter between the EVSE and a 
vehicle unless the vehicle does not have a J1772 inlet.  If the vehicle doesn't 
have a J1772 inlet, then it also doesn't need (or understand) the J1772 pilot 
signal and so there is no need for an adapter to generate a pilot signal 
limiting the vehicle to a lower current than the EVSE is capable of providing.

> It is possible to design a 20A
> adapter with a switch and a resistor that does not modify the duty cycle
> from the EVSE.

Yes, and this adapter is used to convert a J1772 EVSE *into* a NEMA receptacle 
so that a non-J1772 compliant EV/charger may be plugged into a public charging 
station.

This is exactly the opposite of what would be required or used if NEMA 
receptacles were provided at each charging stall.  Most of us with conversion 
EVs have chargers that can plug directly into a NEMA outlet, and would not need 
an adapter more sophisticated than an extension cord.

Those of us with production EVs (other than Teslas, perhaps) will have J1772 
inlets and will need to use our J1772 "opportunity charge" cords; these are 
effectively "smart" extension cords with a NEMA plug on the wall end, a J1772 
charge paddle on the vehicle end, and a small EVSE in between that provides the 
appropriate pilot signal for the vehicle/charger.  If the NEMA plug on the wall 
end of the opportunity charge cord is a NEMA 5-15P, then the pilot signal 
should (must!) tell the charger not to draw more than 12A.  If the cord has a 
NEMA 5-20P plug, then the pilot signal can allow the charger to draw up to 16A 
(since this plug can only mate with a receptacle on a 20A rated circuit).

Even if the J1772 opportunity charge cord were to tell the charger it is OK to 
draw more current than the NEMA receptacle and circuit are rated for, there 
should be no safety issue: the NEMA receptacle and its associated supply wiring 
will be protected by an appropriately rated breaker and the breaker will simply 
open.

Cheers,

Roger.

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