“J1772” charger - “pilot” signal sequencing wit

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Nov 16, 2017

Some background: Many electric cars are designed to use a "J1772" charging point. The main signal in the J1772 specification is called the pilot.

Long story short, the car is expected to place a resistor and diode in series across the pilot and ground. A resistor value of 2.74k indicates that the car is present, and a value of 882 ohms indicates that the car wishes to have power.

The spec requires that you make a stopover in state B (2.74k) before engaging state C (882). It does not say how long that stopover must be.

There is a second signal in the specification called "Proximity." It is a signal from the J1772 handle to the car. It is wired to the release button on the handle. When the user is pushing on that button, it is a 450 ohm resistance to ground and when the button is released, it is a 150 ohm resistance.

My Device

I am making a device that has a J1772 inlet and derives its power from the charger to which it is connected. Prior to it placing the 882 ohm resistor value in place, it has no power of its own (unlike most electric cars).

The natural thing to do would be to make a circuit such that when that resistance is 150 ohms, the extra 1.3 kOhm resistor between the pilot and ground is in place. When the resistance is, oh, greater than 300 ohms, that resistor is not in place.

The challenge is... how can one do that with merely passive components?

In short, I want to detect whether a particular pin has a resistance of less than 300 ohms to ground, and if it does, I want to change the resistance between another pin and ground from 2.74k to 882 ohms (or put another way, add another 1.3 kohms in parallel). And I would strongly prefer to do this without a power source.

The left half of the diagram below is the proximity circuit that's part of the J1772 charging handle. It cannot be changed. The right hand side is my circuit. The question, again, is when the proximity resistance is less than 300 ohms, I want to close the "mystery" switch.


simulate this circuit – Schematic created using CircuitLab

If I were going to make something active, this is what it would be:


simulate this circuit

This may be laughable overkill. I'm not sure. But it doesn't solve the power problem. It requires either a set of AA batteries (3 would last around 100 days if my calculations are correct), or a "START" button across the transistor (http://www.kynix.com/Product/Cate/90.html) to "bootstrap" the system. Both of those solutions, in my view, are lame.
First question: Why? What is said device's intent? (Monitor charging state? You could easily do that with a coil around one of the wires?)

As you have surmised: It is impossible to measure resistance without any current (funny how that V=IR thing works eh). You could always use a small rechargeable battery with a solar cell or two provided the circuit lives outdoors or in a lighted area. Or, since you do have 120V available, you could use a small cell phone charger brick providing a nice regulated 5V out.
Not sure (haven't analyzed all the voltages involved and only briefly looked at J1772 before) but maybe there'd be a way to "charge" an RC circuit that could activate an NFET or NPN to engage the other resistor when a certain amount of time elapses?

edit: I think your 2nd example works better. Consider J1772 is designed to charge something that already has power, e.g. a 12V battery.
I may not be fully understand what you are trying to accomplish and I'm not that overly familiar with the the J1772 spec but why couldn't you use the pilot signal for your power source? As an alternate if you do decide to add a power source you could make it rechargeable such that when the EVSE is providing power to whatever it is you are powering you could also recharge the power source for your circuit. Out of curiosity what are you building?