I am trying to understand the schematics for the GA Industries Pump Director 7600 (pages 9-11).

As far as I can tell, this unit operates completely on 120VAC. Why then are they using diodes with the relay coils? Is the supply being rectified somewhere that I'm missing?

Example: enter image description here

My overarching goal is to figure out where I can tap into the circuit to get an indicator of the HOA switch position (shown on page 9 bottom right, and page 10 bottom middle). I need some sort of output which is energized when the HOA switch is in the Auto position and not energized in the Off position, which my PLC can then read. Note that my PLC can accept 120VAC and 24VDC inputs. I'm also open to using a relay to isolate/convert the signal if that could work.

  • \$\begingroup\$ Those diodes implement a form of LOGIC. Monitoring the existence of all the phases? \$\endgroup\$ Commented Oct 26, 2017 at 16:21

1 Answer 1


Yes, the supply is being rectified just before each relay.

Notice there is a diode in line with the controlling signal that is reversed to the flyback diode across each relay.

As the schematic on page 9 is part of a multiple page design, it is difficult to suggest anything other then to test the leads of the connector to see what type of signal is present when the switch in question is activated.

Based on your description, it may be that these signals are unfiltered rectified signals. But this is a guess and should be tested.


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