Landscape lights transformer relay

Discussion in 'General Electronics Chat' started by Dave Turner, Nov 19, 2011.

  1. Dave Turner

    Thread Starter New Member

    Nov 19, 2011
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    Hi Everyone,

    New member here with long-time interest in electronics, but not much experience. My power supply for my low voltage landscape lights stopped working (Nitelight 1200 watt multi-tap transformer about 8 yrs old). After disassembling it, I found that is contains a small printed circuit board that among other things holds the SPST relay (T9AS1D22-12) that connects the 120v AC power to the massive toroidal transformer. A portion of this relay case is burnt and partially melted where it abuts two large (2" long) white rectangular ceramic resistors that are cracked and need replacement also. These are not marked and I measure infinite resistance across them on my cheap multi-meter.

    These are connected across the relay switch in parallel. I'm not sure of their purpose. Might they be there to help prevent arcing at the relay contacts? I would assume their resistance would have to be relatively high to keep any significant current from flowing in the transformer primary winding when the lights are off. I was wondering if anyone knew what these resistors are for and what value might be appropriate for a replacement.

    I also found it interesting that they have an elaborate circuit to activate the relay when the power is turned on. The 120V is converted to 12V DC and also to 5V DC. The 5V powers an op-amp that in turn controls the base of a Darlington transistor. This allows the 12V to flow through the relay coil and activate the relay. Seems like overkill to me, but I'm sure there must be a purpose for it.

    Thanks for any input.

    Dave
     
  2. Dave Turner

    Thread Starter New Member

    Nov 19, 2011
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    0
    Here is a schematic of the circuit board after examining it carefully. I left out one capacitor on the output of the voltage regulator.
     
  3. Dave Turner

    Thread Starter New Member

    Nov 19, 2011
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    0
    [Sound of hand hitting forehead] I finally figured it out! The circuit I have is a soft-start circuit to limit the current flow into the large transformer when the power is first turned on. This inrush current can be 10 to 20 times the normal current for the transformer and will resolve within 100ms. The circuit acts as a timer, delaying the relay activation for a fraction of a second. During that time, the current must pass through the large ceramic resistors, which limits the current flow during the critical inrush period. Once the relay kicks in, the resistors are bypassed allowing normal current flow into the transformer. I suspect that my relay failed. This caused the current to constantly be routed through the ceramic resistors. Since they were not designed to carry this amount of power for more than a short time, they overheated and also failed. This caused the burn marks noted on the adjacent relay housing. I found a web site on constructing your own soft-start circuit. He suggests using three 150 ohm 5W ceramic resistors in parallel to limit the current flow. I guess now I need to buy a replacement relay and some ceramic resistors.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I saw your post last night - but for some reason I didn't get a chance to reply to it.

    You can likely estimate the original resistance of those resistors if you can get a measurement from one of the exposed windings to one end; and then measure how long the resistor is from that point to the end, and calculate the difference from the original length. The resistors will have burned open in only one place; most likely either right near the middle, or at one of the ends if the spot weld wasn't so good.

    And yes, you seem to have the theory of operation down pat. ;)
     
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