There are two variants of the Hydra. One is the "splitter" variant. It has a J1772 inlet and two plug/cable sets. It was the original version of the Hydra. The other variant is the "EVSE" variant. It trades the J1772 inlet for a simple AC power cable. To be compliant with the standards, the EVSE variant has a GFCI for safety (the splitter doesn't because the host EVSE is assumed to have one). The EVSE variant also has a clock/timer chip and the firmware has an event system to facilitate allowing EV charging to happen only when electric rates are lowest.

Both variants offer two different operating modes. In "shared" mode, the rules are that if both cars are requesting power, each will be allotted half of the power available (the available power is indicated by the host EVSE for the splitter variant, and is a user configuration parameter for the EVSE variant). When one car finishes (or if only one car is connected), then that car is allowed full power. In "sequential" mode, only one car is allowed to charge at a time. When the charging car finishes, the opportunity to charge will be offered to the other car. The benefit of the Hydra is that the plug doesn't have to be manually moved from one car to the other (potentially at 2 AM for ToU charging).

Sensing the amount of available power (for the splitter variant) and informing each car of the available power is done via a signal wire within the J1772 cable called the "pilot." The EVSE indicates its availability by presenting a +/- 12 volt 1 kHz square wave on the pilot line with a 1 k-ohm impedance. The duty cycle of the square wave is the indication of the ampacity of the EVSE. The car accepts this offer by loading the pilot line with a particular resistance value in series with a diode to ground. The two most common resistor values are 2.7 k-ohm and 882 k-ohm (or 1.3 k-ohm in parallel with the 2.7k-ohm). The 2.7 k value indicates the presence of the vehicle. It will bring the 12 volt pilot down to 9 volts (due to the 1 k source impedance). The diode will prevent the resistor from impacting the negative portion of the square wave, which will remain at -12 volts. This is an important safety feature that allows the EVSE to distinguish between a vehicle and a bucket of salt water that has JUST the right resistance. Adding 1.3 k-ohms in parallel with the 2.7k will drop the voltage down to 6 volts. The host EVSE monitors the voltage of the pilot output to detect these state changes from the vehicle and turn the power on and off.

The UL specifications for charging stations require a number of safety systems, and the Hydra makes every attempt to include them all. These include:

The instructions here are for both the version 5.x Hydra EVSE and splitter variants.

Theory of operation

Follow along with the schematics in order to see what's going on. Each section will reference a different page of the schematics

HV board

The HV board has 4 jobs:

  1. Provide 5 VDC power for the logic/display board
  2. Perform a GCM (ground continuity monitor) test to insure the path to ground is functional.
  3. Perform a relay test to insure that there is power on the load side of each contactor if and only if that contactor is engaged by the firmware.
  4. Switch the contactor power on and off for each car.

The 5 VDC power is provided by an isolated power supply module. It can provide up to 5 watts, which is plenty.

Most of the remaining functions of the HV board are centered around an LTV-827S dual opto-isolator. One section is for each car. The relay test and the GCM functions are performed by the same circuit independently for each vehicle. The load-side switch terminals of each contactor are brought to their respective "relay test" terminals on the board. Each of those lines passes through a diode and the two cathodes for each vehicle are tied together. This insures that both lines are tested for voltage. Any high voltage relative to ground on either will be detected and trigger a relay failure when the contactor is not supposed to be switched on. From the common cathode, there is a series resistor to reduce the voltage to the working voltage of the LM334 current limiter. This resistor must be 150k for North America and 300k for places with 220-240 volt hot-neutral (as opposed to North American split-phase) service. This is because in North America, each hot line is 120 volts away from ground instead of 240 volts. The LM334 current limiter is configured to pass a maximum of 1 mA. This is intended both as a safety precaution (in case the chassis is not grounded and a ground path established through a person) and to avoid tripping any upstream GFCIs. From there, it passes through the optoisolator's LED and to ground.

The secondary side of each opto-isolator is configured to translate the transferred current into a voltage, which is then fed into an LM339 dual comparator. Each section compares the input to about 1 volt. The output of each comparator is an open-collector, which shorts to ground when AC is detected. Connected to the output is an RC network designed to charge a cap and retain that charge for at least 1/50th of a second. The comparator will short out the cap any time it detects AC current flow, and as long as that happens at least once per 50 Hz cycle, the output will read low. If this doesn't take place, it will rise up and read high. This allows the controller to be able to sample a continuous output voltage and depend on it being consistent.

Each contactor is driven by a solid-state relay. Both the SSRs are provisioned with snubber RC circuits to protect them from coil collapse voltage when the contactor is switched off. In addition, each contactor coil has an MOV in parallel that will carry most of the current during the coil collapse, leaving the snubbers to only reduce the initial dv/dt at commutation.

Logic/display page 1: power, GFI and inlet handling

The first page of the logic/display schematic differs depending on which variant you have, but the top half is the same for both - two DC/DC converters. The left one is a boost converter to make 12 volts from the 5 volt input. It's based around an AP3012 controller. The other one is a charge pump inverter that generates -12 volts from the 12 volt supply for the pilot generators.

There's also a MAX40200 ideal diode that's used to break a smaller power domain off of the 5V bus for programming the controller. It's important to isolate the ISP header and the microcontroller for programming so that the programmer is not asked to power the entire circuit. The ideal diode will prevent the controller Vcc section from back-feeding power to the rest of the 5v bus, but will allow the 5v bus to power the controller during normal operation.

The bottom section for the EVSE variant is a GFI circuit. Its design is copied from OpenEVSE, and is largely inspired by a CR Magnetics application note on the subject. A current transformer is a coil that outputs a current proportional to the current flowing through the conductor that runs through its core. For ground fault detection, both hot lines are fed through the coil, which means the CT will only measure any residual current (the currents entering via one hot line and exiting via the other will cancel each other out). Across the CT is a burden resistor. The burden resistor's purpose is to change the current into a voltage (using Ohm's law). The GFI threshold is around 17 mA RMS, or a peak of around 24 mA. The Te of 1011, which means the actual current threshold will be 23.7 µA. With a burden of 330Ω, that's 7.821 mV. Immediately after the burden resistor is a voltage clamp made from back-to-back zener diodes. They'll limit the excursions to no more than 2.5 volts, just in case a massive ground fault causes a spike. The LM358 first stage is an amplified rectifier with a gain of 100 and a 120 Hz roll-off to get rid of high frequency noise. The result of that will be a threshold of 782.1 mV. The output of that is fed into a comparator that will provide high logic-level output pulses during any faults. These pulses will cause an interrupt on the controller, which will quickly turn the contactors off. Since an LM358 is not a perfect comparator, the output has a pull-down resistor, which insures the output falls to ground. The GFI test circuit is simply a logic output from the controller and a current limiting resistor. The GFI test line is intended to be connected to a wire that is passed through the center of the GFI coil 3 or 4 times and then connected to ground. The GFI test routine will pulse this line at 60 Hz and look to see a ground fault interrupt. 3 or 4 wraps should result in a perceived current of well over 50 mA, which should be more than enough to trigger the GFI threshold.

The bottom section for the splitter variant is an inlet handling system. It consists of two sides of an LM339 comparator each fed from the inlet pilot and proximity lines. The proximity line is compared to 2.5 volts and will trigger a high output when it rises above that, indicating to the controller that someone has pushed the proximity latch button on the plug connected to the inlet. The controller will relay this transition to the vehicles to insure they quickly shut down. The pilot line is fed to a 2.7K resistor and then a diode to ground. This insures that the host EVSE will see state B. Additionally, a second resistor - this one 1.3K - is in parallel with the first to bring the host into state C. But this resistor is switched into place in one of two ways. Either the INT_PWR jumper can be closed, which will permanently lock in state C, which will provide power to the controller immediately when the host EVSE is connected. If INT_PWR is opened, then the assumption is that the HV board's AC IN terminals are connected to a secondary power source. In this configuration, a transistor will pull in the 1.3K resistor when either of the relay outputs are high. This will allow the host EVSE to be turned on and off only when one or both of the cars wants power. This is a useful configuration for home users who have a hard-wired EVSE that they do not want to disturb. If you go for this option, then you can power the hydra AC board with either 120V or 240V power, but you must insure that the contactors have the same voltage rating.

Logic/display page 2: Car signaling

There are two ammeter circuits and two pilot generator circuits on this page, one each for each vehicle.

Each vehicle has it's own ammeter CT. These are similar to the GFCI CT, but only one hot line passes through the core so that the complete current flow is measured. The ammeter CT has one of its leads anchored with a voltage divider to 2.5 volts. This insures that the controller's A/D converter sees the entire peak-to-peak voltage range exist within its 0-5 volt range. The anchored leg also has a capacitor to ground to reduce noise. The other leg has a burden resistor and then a zener diode clamp circuit to protect the controller from any excessive voltage excursions. The output from that is fed into the controller.

The pilot generator is a pair of complimentary MOSFET arrays in a push-pull arrangement from the +12 and -12 volt power supplies. Each of the primary FETs is switched on by a secondary FET controlled by the TTL logic output from the controller. For the +12 side, its P channel gate is pulled down towards ground by an N channel FET, but only when the TTL signal is high. For the -12 side, its N channel gate is pulled upwards towards +5 volts by a P channel FET, but only when the TTL signal is low. A pull-up resistor on the TTL input insures that when the pin is in a high-impedance state (as when the controller is being programmed or is held in RESET or if there is a firmware malfunction) that a definite state is selected. If the input is left to float, then both transistors would be turned on, shorting +12 and -12 volt together. Each primary FET has a "pull-open" resistor from source to gate. There's a second resistor from each gate to the secondary output. The gate bias resistor is substantially higher than the source-to-gate resistance. This insures that turning the FET off happens faster than turning it on. This reduces current spikes that would happen if both primary FETs were on at the same time.

Logic/display page 3: controller and display

The controller is an ATMega328PB clocked at 16 MHz with a crystal. The crystal's purpose is to insure that the pilot frequency is within the accuracy limits in the J1772 specification. 16 MHz was chosen merely because of OpenEVSE's Arduino legacy.

The display design is largely taken from the OpenEVSE display backpack, which was in turn inspired by the AdaFruit i2c RGB LCD display shield. It consists of an MC23017 i2c GPIO chip with its pins wired up to a 16x2 RGB backlit LCD display module. The extra pins of the GPIO chip not used for the display are configured for button inputs. The current firmware uses only one button. The BL_COM 2-way solder jumper selects whether the backlight common pin is tied high or low. This differs depending on whether or not the common backlight pin is a common anode or common cathode. By default, the modules are common anode. If you instead obtain a common cathode module, you must reverse the jumper (be careful you don't inadvertently close all 3 pads - that would short out the +5 volt supply) and change one line of the firmware so that the backlight lines are inverted.

In addition, the EVSE variant has a PT7C4311WEX real-time clock chip, crystal and backup battery. It shares the i2c bus with the display GPIO chip.

There's also an FTDI serial port connector on the board. This can be used for diagnostic or logging purposes with the default firmware. The DTR_RESET jumper will cause DTR transitions on the serial port to reset the controller. This is not normally recommended, but it is available if desired.

Lastly, there's an ISP header to facilitate programming the controller and installing firmware updates. It's the standard AVR ISP 6 pin layout.