Saturday, April 26, 2014

Contactor driver boards now available

For those wanting to build OpenEVSE or Hydras that need to charge at higher than 30A, the lack of 12 VDC coil relays at elevated contact current specs has been a problem. Typically at higher power contactors are the norm (a contactor is just a relay with an AC line-powered coil rather than a low voltage DC coil). In the past, most folks have used pilot relays to drive their contactors, but I've got a better solution.

Schematic of the Contactor Adapter board

It's a simple opto-isolated triac driver. Those of you paying attention may notice startling similarities to the Toast-R-Reflow power board. In fact, it's the exact same thing, but with surface mount components and a much, much less beefy triac.

Instead of being rated for 800 watts per channel with two channels, this board is only rated to switch 100 mA of current, but that's plenty for any reasonable contactor coil. Note that the snubber circuit is a requirement because the contactor coil is a decidedly inductive load. In this case, the snubber cap is rated at 1 kV and the resistor at 1/3W.

The input is designed to accept 12 VDC. The board is a drop-in replacement for a pilot relay, except for one detail - the input is polarized - you need to connect the high side of the relay coil output to the positive input terminal. For OpenEVSE, this is the common terminal of the relay block. For Hydras, it's the "top" terminal (furthest from the pilot block) for each car.

These are available in the store.

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