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+[[!meta title="Building an EVSE test box"]]
+[[!meta author="Daniel Silverstone"]]
+[[!tag draft]]
+
+A while ago now, I was having "fun" learning about [SAE J1772][] signalling and
+started to examine the complexities in [ISO 15118][].  Realising that the
+documention available on the Internet on ISO 15118 was less than edifying, I
+contacted my brother whose company is involved with electric vehicles in the
+vain hope that he might have a copy of it which I could read.  Some hours later
+we'd had quite a discussion which ended up with me agreeing to create a
+prototype test box for J1772 signalled chargers as an urgent activity.
+
+I sketched out some schematics, picked components, ordered some stuff I was
+missing, and on a bright Saturday morning, I began to assemble things.  By
+Sunday evening (I'm very slow) I'd built and tested something I was quite proud
+of.  I thought I'd share the journey with you, so you can share my pain :-)
+
+SAE J1772 Control Pilot
+-----------------------
+
+Before we begin with the tester box, let's first think about the control pilot
+signalling.  This signal is a bidirectional negotiation between the EVSE and
+the EVCC and allows the EVSE to communicate the amount of current available,
+and the EVCC to communicate when to provide power and if the EV requires that
+the EVSE ventilate the area (e.g. if it's underground and the battery tech
+gives off hydrogen when charging).
+
+The control pilot is a +/- 12v signal with a moderate amount of current behind
+it (maybe a few hundred milliamps).  It starts life as a DC +12v signal and
+J1772 refers to this as state 'A'.  This is the "not connected" state.  When
+the charge cable is plugged into the EV, the EVCC adds a resistor between the
+control pilot and ground, which brings the 12v signal down to 9v.  The EVSE
+detects this and transitions to state 'B' (vehicle attached).
+
+When it enters state 'B', the EVSE switches from a DC signal to a square wave
+between +12v and -12v.  Naturally, since the EVCC has added the resistor to
+bring us into state 'B', the signal is +9v not +12v.  Naïvely we might expect
+the signal to be +/- 9v, but as the first major safety check, it turns out that
+the EVCC has a diode in series with the control pilot meaning that the signal
+is +9v / -12v.  If the EVSE notices that the negative half of the wave doesn't
+go properly to -12v then this is considered a 'diode failure' and it should
+enter an error state and refuse to charge.
+
+This particular safety feature is designed to cope with situations where the
+charge plug might be dropped into a bucket of water.  You don't want the EVSE
+to be confused into turning the power rails on in this situation, so the diode
+means that the EVSE is able to detect the difference between a serendipitously
+resistive bucket of water, and an EV.
+
+Assuming the diode test is passed, then the EVCC looks at the duty cycle of the
+control pilot wave and uses that to determine how much current it is permitted
+to draw.  Assuming the EVSE is offering enough power, and the EVCC wishes to
+charge, then it switches another resistor into play which brings the wave to
++6v/-12v.  At that point the EVSE switches in the power and the EVCC can charge
+the battery.  This is called state 'C'.
+
+There's more to it, but that's the essentials.
+
+Testing an EVSE
+---------------
+
+The vast majority of the behaviour and safety features of an EVSE can be tested
+by manipulating the control pilot.  In order to test the EVSE you need to be
+able to pretend to be a car, (to transition from state A to state B), request
+charge (transition from state B to state C), request ventilation (B/C to D),
+and then you need to be able to trigger the various safety checks.  To do this
+I put together a test box which has a number of switches, resistors, etc,
+and looks like this:
+
+TODO: picture of final product.
+
+The circuit looks something like this:
+
+
+
+[SAE J1772]: https://en.wikipedia.org/wiki/SAE_J1772#Signaling
+[ISO 15118]: https://en.wikipedia.org/wiki/ISO_15118