I have an application where I have a +12VDC circuit that I need to use as a trigger for a latching relay.
So, when +12VDC is present, the relay would SET.
When +12VDC is not present, the relay would RESET.
However, I need to have no power consumption when the system is quiescent (i.e. power draw during switching is fine, but there can't be a power draw during non-switching times).
I've tried a few things, including wiring the latching relay coil through its own output contacts (so that the latching side only received a ground when the relay was in the unlatched state), however this doesn't provide enough time to latch the relay.
Any ideas would be greatly appreciated.
P.S. I'm using an Omron MY2K DC12, but happy to change to something different.
Thanks.
Best Answer
You need a capacitor large enough to hold enough energy to reset the relay when the +12 disappears. Minimum pulse width is stated as 30ms. Assuming a 1V drop would be okay and allowing for say 45ms time, you'd need a capacitor C = \$\frac{t \cdot I}{\Delta V}\$ or 0.092A * 0.045s/1V = 4,000uF. So maybe 4700uF/16V.
Then you need a control circuit to detect the power loss and pulse the set and reset coils. It may be possible to do an adequate job in your case with a few passive parts and a 2N6028 but if you need reliable operation in more pathological situations such as slow brown-out of the 12V line you might want to consider more sophisticated circuitry or even a microcontroller.