Wednesday, April 8, 2015

OpenEVSE II / Hydra developments

It looks like the pilot generator that I've been using for OpenEVSE II and the Hydra isn't always 100% reliable. My guess is that the biasing I am doing is too near the edge of the tolerances of the parts and some transistors work and some don't.

To fix this, I'm going to change out some of the parts. To follow along, look at sheet 2 of whatever schematic you like.

The pilot generator has two pairs of transistors, an output pair that switch +12 or -12 to the output, and an input pair that switch the output pair on and off depending on the state of the logic level output from the controller.

The resistors from the emitter to base of each of the output pair will change from 1k to 47k. The resistors from the base of the output pair to the collector of the input pair will remain at 10k. The resistors from the base of the input pair to the pull-up will change to 10k, and the pull-up will change to 1k.

This will result in a decrease in the collector current of the input pair coupled with an increase in the base current. The hoped-for result will be a more solid, assured entry into saturation for the input pair. My thinking is that the problems some folks are having are centered around incomplete turn-on of the input pair, which results in nothing on the output pair for either + or - 12, which winds up causing diode check errors or invalid states, or cars being upset with the pilot or what-not.

Additionally, I have a new design which will replace the four transistors and their bias resistors with two pre-biased complementary pair transistor modules. The whole thing will shrink down to 2 TSSOP-6 packages and a single pull-up resistor.

Also, another revision I've decided to make is to go back to the 20k (instead of 16k) resistor value for the low side of the voltage divider for the GFI comparator. But I'm also going to add a 10k bleed resistor in parallel with the peak-hold cap so that the self-test settles much faster after completion. I'm going to remove the peak-hold cap entirely. Since the GFI signal simply interrupts the controller anyway, there's really no reason to hold onto the peak value. The downside is additional interrupts, but those extra interrupts effectively do no harm, since all they'll do is turn the relays off again. I'm also removing the diode and pull-down from the output of the comparator. Since the LM358 is being fed with the 5 volt (and ground) supply, there's no need to protect the controller from negative excursions.

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