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Pilot relay on HVAC control board

Discussion in 'Electronic Design' started by [email protected], Mar 3, 2007.

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  1. Guest

    I have a Viconics HP727 controller board in a swimming pool heatpump.
    The circuit I'm concerned with is the one that controls the 'pilot
    relay' for the compressor contactor.
    The pilot relay uses a 12vdc coil. It's contacts are 15amp / 24vac.

    This board monitors thermostat, pool temp, water flow sensor and
    compressor high/low sensors. IF the water is flowing AND the
    thermostat is calling for heat AND the freon sensors are reporting OK,
    THEN the board will energize the 'pilot relay' coil. When the coil
    energizes, it closes the load-side contacts which are 24 VAC and those
    in turn energize the contactor coil and the compressor starts.

    This board has gone through two pilot relays in a month. I replaced
    the original with the exact same part. I've seen other threads on
    this board talking about driver circuits of these miniature relays.
    It looks like there's more to the driver circuit design than you'd
    think. I'm not sure if a capacitor is used to assist the circuit to
    provide enough power to energize the relay's coil or not. But
    something on that board has caused the coils of two relays to run hot
    enough to open up. Both relay coils were open when tested with a

    I don't have the schematic of this board. Should I just chock it up to
    'something failed' and buy a new board?

    Sorry if this is off-topic. I'm a novice (can you tell?).
  2. What does the coil voltage measure when it is energized (just a quick
    measurement on a cheap digital meter is close enough)?

    Is the environment where the relays are located particularly hot?

    What is the relay part number and manufacturer?

    Best regards,
    Spehro Pefhany
  3. Fred Bloggs

    Fred Bloggs Guest

    Snip aggravating extraneous crap...

    Open coil usually is due to overvoltage for the DC relay. This implies
    that you want to measure the so-called 12VDC ...
  4. Rich Grise

    Rich Grise Guest

    Well, even though it's apparently out of warranty (it must be or you'd
    have already taken it to the store (or got a service call)) it should
    still do no harm to ask the dealer about it.

    Good Luck!
  5. Guest

    My relay is a Hasco Relay part number = KLT1C12DC12

    The voltage going to the relay was 13.20 volts.
    I soldered an LED across the coil connections to verify when I had
    power. I also soldered a switch across the load-side contacts. When
    the LED illuminated, I turned the switch on. The compressor started
    and I noticed the voltage to the coil at that time was 12.72.

    One more thing. The only resistor that I could see that was wired in
    series to the coil was one with Brown/Black/Brown - Gold stripes
    (100ohm). I check it. It measured 100ohms.
  6. ehsjr

    ehsjr Guest

    Change the resistor to 220 ohms, 1/2 watt. That may give
    you time, once the replacement relay is installed, to
    find the real problem. The 100 ohm resistor is intended
    to drop about 3 volts, implying a 15 volt supply. (The
    computed value comes out to 15.24 volts, with the resistor
    dropping 2.54 volts.) The 220 ohm resistor will drop about
    5.4 volts leaving about 9.84 volts for the relay, whose
    specs state that it must operate at 9 volts.

    12.7 volts is high, but should not cook a 12 volt relay in
    a month, so I suspect something else is wrong. Look for an
    electrolytic capacitor connected to the same line that
    goes to the relay coil - the other side of the capacitor
    will be connected to ground. The capacitor will be marked
    with xx uF where xx is the capacitance, and it will also
    be marked with the voltage. Betcha the &(^$*# thing is
    marked with 16 volts. If it is, stop the repair action,
    take time out to curse the manufacturer of the circuit,
    and replace it with a cap of the same xx uF but at 35 volts.
    Pay attention to the polarity marking on the cap.

    I'm taking a guess that the cap cooked and is allowing
    ripple voltage to smack the relay. You can use your DMM
    to see if that is the case. Put it on an AC scale, connect
    one lead to the relay coil and the other lead to a .1 uf
    capacitor. (The actual value of the cap doesn't matter -
    anything from .001 uf to .47 uf will do.) The other side
    of the capacitor goes to the other side of the relay coil.
    Whatever voltage your meter shows is added to the 12.7 volts.
    Your meter can't see that extra voltage when it is on the DC
    scale. If you see "big" voltage (like 15 volts) the diode
    that produces DC for the relay is shorted.

    You also mentioned a 13.2 volt measurement (which makes sense
    with respect to the 12.7 when the compressor is running) which
    means the relay is energized at a higher voltage than I used
    in the computations (I used 12.7).

    Finally, put a 1K resistor in series with the LED you added,
    if you don't have a resistor in there now. A typical red LED
    will be damaged by 12 volts with only 100 ohms to limit the

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