Saturday, January 3, 2015

OpenEVSE II design progress

The good news is that the design of the logic/display board has not really changed significantly since July. I have moved a couple of components around very slightly, but in general the separation of functions has really been quite a boon for the design.

When I last checked in, I had decided that the relay test functionality wasn't worth the trouble, so I converted it into a ground impedance test - which I knew was required by UL. I also replaced the original OpenEVSE L1/L2 test system (which also performed the ground test and the stuck relay test) with what I hope to be a reasonable isolated voltmeter.

Chris H corrected me, however. I thought a stuck relay test was unnecessary, but he said that UL actually requires one. So it's back in play.

What I've come up with is that the ground test circuitry is going to be replicated for a relay test line. The difference is that, while the ground test circuit has two diodes from each hot line and runs to ground, the relay test will simply run between the two hot lines on the load side, much like the voltmeter does on the line side of the relays/contactor.

OpenEVSE II relay board. This board works on both 120v and 208/240v, but is only rated for up to 24A charging.

OpenEVSE II contactor board. This board uses an external AC line powered contactor. The only limit on charge current is the contactor, but contactors generally only work at either L1 or L2 (not both).

Both variants have an isolated AC/DC power supply to produce 12 volts for the entire system. The relay variant has a 10 watt module and the contactor version has a 3 watt module. The difference is that the two relays draw 2 watts from the 12 VDC supply when energized, while the contactor is the equivalent of just an LED on the low voltage side.

Both the ground test and relay test hardware is nothing more than a comparator that feeds into a very basic peak-hold circuit so that the AC zero-crossing intervals don't count as "failures." The ground test is designed to insure that current will flow from at least one hot line to ground (for hot-neutral systems one of the hot lines will actually be a neutral line, so current flow will not be expected in that case). The test is a little trickier than it sounds. You can't naively put a pair of optoisolators in place - one for each hot line - from the hot line to ground, since a circuit path would exist from one hot line through its optoisolator to ground, and from there through the second optoisolator to the opposite hot line. That test would "pass" regardless of whether the ground impedance is high or low. You can, however, use a pair of (beefy) diodes so that, effectively, only one hot line at a time is presented with an opportunity to conduct through the optoisolator to ground.

The optoisolators are LTV-844S, which have AC emitters - each of the 4 independent optoisolators has a pair of diodes in inverted parallel (this is not shown in the schematic, which shows the unidirectional LTV-846S instead). Effectively, this means that the transistors see light proportional to the absolute value of the input voltage. The series resistors chosen are intended to try and keep the optoisolators in their linear range (if possible), and are also sized based on the power equation: P=E^2/R. Note that for the ground-fault system, the math is deceptive. The actual voltage is never more than 120 volts because the measurements are always relative to ground. In addition, each resistor only gets a 50% duty cycle, as the negative half of the cycle is blocked by the diode. For the 91k resistors, the worst case is 240 volts, which comes out to just under 2/3 watt. Lastly, all four resistors are flame-proof, so that they act like fuses and burn themselves out harmlessly if stressed.

The output side of each optoisolator is set up, more or less, with a classic voltage divider configuration. For the ground test and relay test systems, the result is simply compared to 1 volt by a comparator and peak-hold circuit. For the voltmeter, the result is fed into a non-inverting amplifier. As supplied, the amplifier is configured as a voltage-following buffer (unity gain), but gain can be added by altering the components without re-spinning the board.

The relay board has a simple common emitter switch with a flyback diode to switch the relay coils on and off. The contactor board has an optoisolated triac that's used to drive a (slightly) larger triac to switch the contactor on and off. Both the main triac and the driver triac are provisioned with snubber circuits which serve the same purpose as the flyback diode on the relay board: they provide a path for the coil collapse voltage to go when the contactor is switched off.

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