Comparator/Zener Absolute on/off @ Specified Voltage

Discussion in 'The Projects Forum' started by Steveab63, May 28, 2010.

  1. Steveab63

    Thread Starter New Member

    May 28, 2010

    Hello everyone.

    I apologize in advance if this is elsewhere, but I couldn't find quite what I need.
    I have a solar water heater that had a DC pump to circulate the water which was constant trouble. I replaced it twice in 18 months for a couple of hundred a pop. I bought a 120VAC circulator pump to go in its place for $80. It works terrific, but it needs a "brain". There is a 20 watt PV cell on the roof that makes up to 19 VDC that originally powered the DC pump.

    I tried connecting a 15 volt zener in series with a SSR, however it allows some "in between" on/off for the SSR which sends improper VAC to the motor, which will likely fry it.

    I tried a few plain old 12VDC relays with some series resistors but the voltage swing was to great; if I get it to go on at 16VDC, it stays on until 6VDC.
    I tried a comparator circuit, but it lights my test LED below the threshold I want. It gets really bright past the reference voltage, but I can't have the "kind of on" scenario. I tried several comparators thinking some might have more of a "spike" between transitions: no luck.

    I need absolute on/off and I'd like it to turn on/off around 16VDC, so that anything over 16 turns on the pump. On second thought, a little bit of a "deadband" range might be good to keep it from "chattering" (think motor starter lingo), like on at 17, off at 14 or so.

    Seems like the comparator idea would give me the flexibility to choose voltages that work well for pump operation, but I can't have the LED partially lit until it hits the threshold. (I tried a pull down resistor, no luck).

    I took some electronics courses over a decade ago, obviously I'm over my head, and know just enough to get myself in trouble. Does anyone have a simple way to accomplish what I need to do? Maybe a pre-designed IC (lazy way out), different SSR, IO module.

    Thanks for your patience reading all of this, but I figure you need to know what I'm doing to help.

    Thanks very much,
  2. Bychon


    Mar 12, 2010
    Here's an idea.
  3. tkng211

    Well-Known Member

    Jan 4, 2008