Control Pilot

Every Type 1 and Type 2 plug, and every CCS plug, carries a Control Pilot wire. The charger drives a ±12 V square wave down it through a 1 kΩ resistor; the car answers by loading it with a diode and resistors. The voltage the charger reads back is the state of the session, and the width of the pulses is the current on offer. Plug in, close the car's switch, change the duty cycle and short things out to see what each side measures.

IEC 61851-1
SAE J1772
IEC 62196-2

Walk through a session

Charging

The car closes S2, putting 1.3 kΩ in parallel with 2.74 kΩ. The high level drops to about 6 V: state C2. The EVSE closes its contactor (J1772 allows it 3 s) and mains power reaches the car.

A+12 V
Not connected
B+9 V
Connected
C2+6 V
Charging
D+3 V
Charging, ventilation required
E0 V
Error
F−12 V
EVSE not available

The circuit

IEC 61851-1 Annex A / SAE J1772

Scroll sideways to see the whole circuit. Moving dashes show where current flows while the oscillator is at +12 V. On the −12 V half, a healthy diode blocks it.

On the wire

What a scope on CP shows

V_high +5.99 VV_low −12.00 Vf 1.000 kHzduty 53.3 %

EVSE measures

Positive half+5.99 V

Negative half−12.00 V

StateC2 · Charging

Diode checkpass

Vehicle reads

Signal53.3 % PWM

Offer32 A per phase

Its load882 Ω

Contactor

Closed

The vehicle may draw up to 32 A per phase.

≈ 7.4 kW at 230 V · 7.7 kW at 240 V · 22.1 kW three-phase 400 V

Controls

EVSE output

Duty cycle53.3 %

EVSE can ventilate

Vehicle plugged in

Vehicle switch S2

Inject a fault

Duty cycle → current

Click or drag to change the offer

Not allowedDigital communication (5 %)No chargingCurrent offerThese are the bands a vehicle reads, widened for tolerance. An EVSE only ever sends exactly 5 %, or a value from 10 % to 96 %.

The arithmetic

On the positive half, the oscillator, R1 and the vehicle form a voltage divider with a diode in it. With a vehicle load R and a diode drop of 0.7 V, the current is (12 − 0.7) / (1000 + R) and the EVSE reads 12 V minus that current times 1 kΩ. On the negative half the diode blocks, nothing flows, and the line sits at −12 V whatever the car is doing. That is why a missing −12 V means something other than a car is across the pins.

State	Vehicle load	Computed	J1772 worst case
A Not connected	nothing	+12.00 V	+11.40 V to +12.60 V
B Connected	2.74 kΩ	+8.98 V	+8.36 V to +9.56 V
C Charging	2.74 kΩ ∥ 1.3 kΩ = 882 Ω	+5.99 V	+5.48 V to +6.49 V
D Charging, ventilation required	2.74 kΩ ∥ 270 Ω = 246 Ω	+2.93 V	+2.62 V to +3.25 V
E Error	short to PE	+0.00 V	—
F EVSE not available	any (EVSE sends −12 V)	−12.00 V	−12.60 V to −11.40 V

The worst-case column stacks every tolerance: the oscillator's ±0.6 V, R1's ±3 %, the vehicle's resistors and a diode drop anywhere from 0.55 to 0.85 V. The standards leave the exact detection thresholds to the EVSE designer; this page splits the difference between neighbouring levels.

Proximity Pilot

The second signal pin, PP, answers a different question: is a plug really there, and what can it carry? Type 2 and Type 1 answer it differently.

Type 2: the cable says what it can carry

A detachable Type 2 cable has a resistor, Rc, between PP and PE inside each plug. Its value codes the cable's current rating, and the EVSE never offers more than the cable can take.

Cable

EVSE rated for

Rc: 220 Ω
Offer: 32 A
Duty cycle: 53.3 %

The cable can carry the EVSE's full 32 A.

Type 1: the latch button warns the car

On a J1772 plug, the thumb latch works a switch, S3. The car reads PP through its own divider and can tell unplugged, latched and “about to be pulled” apart.

Plug

PP to ground: 142 Ω
Vehicle reads: 1.51 V

S3 is closed and shorts R7, so only R6 (150 Ω) appears across the vehicle's 2.7 kΩ.

Sources

IEC 61851-1 Electric vehicle conductive charging system — Part 1: General requirements. Ed. 3.0, 2017.
- Table A.2 (EVSE pilot parameters) and Table A.3 (vehicle pilot parameters)
- Table A.4, system states detected by the EVSE, including the x1/x2 sub-states
- Table A.7 (duty cycle sent by the EVSE) and Table A.8 (how the EV reads it)
- Annex B, Table B.2: cable current coding with Rc
SAE J1772 SAE Electric Vehicle and Plug in Hybrid Electric Vehicle Conductive Charge Coupler. 2010 and 2017 revisions; table numbers as noted.
- 2010 rev. Table 4 (generator: ±12 V ±0.6 V, 1 kHz ±2 %, R1 1 kΩ ±3 %, edges ≤ 2 µs) and Table 5 (vehicle loads, diode drop 0.55–0.85 V)
- 2010 rev. Table A1, worst-case pilot voltages per state
- 2010 rev. Tables 6A and 6B, and clause 5.5 (5 %: digital communication required, mandatory for DC)
- 2010 rev. Table 7 and Figure 8: R4–R7, switch S3, ±10 % tolerance
- 2017 rev. Table 14: the vehicle stops charging within 100 ms of S3 opening
IEC 62196-2 Plugs, socket-outlets, vehicle connectors and vehicle inlets — Part 2: AC pin and contact-tube accessories. Ed. 2.0, 2016.
- Proximity contact and the resistor coding of cable assemblies
ISO 15118-3 Road vehicles — Vehicle to grid communication interface — Part 3: Physical and data link layer requirements. 2015.
- The 5 % pilot and the HomePlug Green PHY link that carries digital communication over CP
EVerest EVerest open-source charging stack (Linux Foundation Energy): bring-up guide, IEC 61851 checks, and the EvseManager DC sequence (cable check voltage, insulation limits, pre-charge current). documentation, 2026.
- Bring-up checks for pilot timing, Type 2 Rc ranges and diode faults (used to cross-check IEC values)
OpenEVSE OpenEVSE open-source charging station firmware and hardware (public reference implementation of the J1772 pilot). open-evse firmware, GitHub.
- A working pilot implementation: generator, state thresholds and diode check

Values are paraphrased from the standards, not quoted. Voltages and currents on this page are computed from the nominal component values; real hardware sits anywhere inside the tolerances.