Construction and circuit information for the DIY Open EVSE V4.23 electric vehicle charging system.
The DIY Open EVSE circuit board Version 4.23 is used for electric vehicle charging using the J1772 standard. It is an Open-source hardware project which uses the GNU General Public License and is based on the original designs by Chris Howell and Lincomatic.
For the history of the DIY Open EVSE Project, and board changes from the original V1. See my previous post - The DIY Open EVSE Project
The DIY Open EVSE version 4.23 is my latest thru-hole board version of the Open EVSE charging system. It incorporates the current safety features of the surface mount component versions of the board. It is powered by a 12 volt 5 watt DC power supply, and generates and regulates +5, +/-15, and +/-12 volts DC on the board. The microprocessor IC1, is an 8‑bit ATMEGA 328‑P.
There are 6 main sections of the circuit:
- Safety checks
- Relay control
- Auxiliary circuits
The Power section consists of a 5 volt buck switching regulator, with a linear regulator option footprint, an isolated +15/-15 volt DC-DC converter, and +12/-12 volt linear regulator for the Pilot supply.
The 5 volt buck switching regulator is based on a MAX5033‑5 high-efficiency, step-down DC-DC converter. This pulse-width modulated (PWM) converter operates at a fixed 125 kHz switching frequency at heavy loads, and automatically switches to pulseskipping mode to provide low quiescent current and high efficiency at light loads, and delivers up to 500mA output current.
A cost-saving option for the 5 volt section is to replace the MAX5033‑5 with a 78L05Z linear regulator (IC9 optional) and only populate (C27 optional), C16, and C17 in this section. If the MAX5033‑5 is used, do not populate IC9 or C27.
The isolated +15/-15 DC-DC converter is a Murata 1 watt MEV1D1215SC in a SIP 7 package with 3kVDC galvanic isolation. This supplies power for the DG419CJ switch IC2 and the +12/-12 volt linear regulators IC3/IC8. The outputs of the +15/-15 isolated supply are filtered using the manufacturer recommended LC filter of 47 uH (L1, L2) and 1 uF (C6, C7) and also include a minimum load resistor of 3.74K ohms (R3, R4).
The +12/-12 regulators IC3/IC8 supply stable power to the Pilot switch IC2 and are 78L12Z positive and 79L12 negative linear regulators, both in a TO92 package.
The microprocessor is a Microchip (Atmel) 8‑bit AVR RISC-based microcontroller with 32KB ISP flash memory, 1024B EEPROM, and 2KB SRAM. It features 23 general purpose I/O lines, internal and external interrupts, serial programmable USART, a byte-oriented 2‑wire serial interface, SPI serial port, and a 6‑channel 10-bit A/D converter. The board uses the 28 pin DIP package of the IC. All pins on the IC are used with the exception of ADC3 which is connected to the optional SPI interface header pin SPI.
Analog circuit voltages for AMP_READ, PP_READ, and PilotVoltage are read by the 10-bit ADC on ADC0 thru 2. The GFC (Ground Fault Circuit) is sensed on INT0, and GFCI_TEST on AIN1 supplies current for the GFC test circuit. DC and AC relays are controlled by outputs AIN1, ICP, and OC1A. The 4 pin display header labeled LCD using an I2C interface has Ground, 5VDC, and microprocessor SCL, SDA connections. Output OC1B supplies the 1,000 Hz PWM signal to IC2 pin 6 for Pilot signal control.
The Pilot circuit is used to coordinate charging level between the car and the charger as well as other information. The pilot signal generated by the EVSE is a 1 kHz square wave at ±12 volts and is used to detect the presence of the vehicle, communicate the maximum allowable charging current, and control charging.
The Pilot circuit is comprised of a Maxim DG419CJ+ (IC2), TVS protection device, limiting resistor R1 1K ohm 1%, voltage divider and biasing resistors R5, R6, R7, and decoupling capacitors. The DG419CJ+ is a monolithic analog single pole/double-throw (SPDT) switch with one normally closed switch and one normally open switch. Switching times are less than 175ns max for tON and less than 145ns max for tOFF. The divider and bias resistor circuit scales the Pilot signal to 0–5 volts and is read by the microprocessor ADC1 input.
The GFC (Ground Fault Circuit) is comprised of a LM358 op-amp IC4, an external current sense coil, burden resistor R17, and additional resistors capacitors, and diodes to make a peak detector and comparator circuit. The output GFCI_INT is connected to microprocessor pin INT0 and goes high when a Ground Fault is detected. Resistor R24 connects to pin 3 of the GFCI connector and limits the current supplied by microprocessor pin AIN0(PD6), it is used for testing the Ground Fault circuit. GFCI connector pin 3 connects externally with a multi-turn (5 turns) coil wrapped around the current sense coil that returns to ground.
The L1/L2 sense circuit is used to determine the presence of AC voltages on the relay output terminals. It is comprised of two (IC5, IC6) MID400 optically isolated AC line-to-logic interface devices for monitoring ON or OFF status of an AC input. Resistors R12, R13 are for input current limiting and keep the MID400 in saturated mode when voltage is present. The firmware in the microprocessor performs a safety check on startup by closing the relay/relays and determines whether 120 or 240 volt power is being supplied to the EVSE using the L1/L2 sense circuit, and then opens the relay for a stuck relay check and verifies no voltage present.
The Relay Output circuits include 2 transistor driven DC 12 volt outputs and 1 solid state relay driven AC output circuit. DC_RLY1 is controlled by microprocessor output AIN1(PD7), and DC_RLY2 by output ICP(PB0). The microprocessor outputs, each drive a 2N2222A transistor thru a 330 ohm current limiting resistor. Both DC circuits include a 1N4148 diode for reverse EMF protection from the relay coil and typically are capable of 500 mA loads.
The AC output is controlled by microprocessor pin OC1A(PB1) thru 330 ohm resistor R8. Depending on the SSR part number installed, the 8 pin dip version will typically support up to 600 VAC and 0.6 to 1.2 amp loads. It is recommended to install a surge absorption circuit (Snubber circuit) at the AC relay coil, based on the manufactures recommendation.
Auxiliary circuits include a current sensor circuit and Proximity pilot (PP) circuit.
The current sensor uses an external current transformer (CR8450-1K-T7QC) rated at 100 amps. Resistor R27 is the burden resistor that converts the current from the CT to a voltage supplied to the microprocessor analog input ADC0(PC0). Resistors R25, R26 and capacitor C28 form a voltage divider to provide 2.5 volts DC to bias the positive and negative AC signal from the CT, to a positive range that the ADC can measure.
The PP_READ circuit is for detecting proximity pilot (plug present), and connects to the microprocessor pin ADC2(PC2).
Sample DIY Open EVSE 4.2X Single Relay connection diagramBOM for version 4.2 several small changes between 4.2 and 4.23 include the AC relay, current input, PP_READ, L1/L2 sense, 2nd DC relay output, and GFCI test.
Eagle CAD files for DIY Open EVSE 4.23
OSH-Park DIY Open EVSE 4.23 Project page.