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This assembly is the motor connector for a Grundfos U15-38SPA circulating pump, which interfaces to the capacitor (2uF) and power supply (220VAC).

enter image description here

Between the two brown wires leading to the cap is a black plastic holder with IDC spikes to connect a 1N4007 rectifier and unknown disk of 166 ohms resistance in series across the leads. (The leads have been removed from the spikes, and the holder top is set aside.)

enter image description here

What is this disk all about?

Best I can figure is some sort of thermal device. Perhaps its resistance goes up (or down) after current flows for a bit.

Finally, note that the black plastic carrier is a stand-alone unit that can be snapped in place over any leaded capacitor.

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  • \$\begingroup\$ It may be a surge supressor (often called MOVs), a brick of semiconducting grit (sometimes zinc oxide) which will absorb any high voltage spikes which might damage something else, but the inclusion of the diode is intriguing, there's no fancy electronic controller for the pump is there? Something which would cause one half of the AC cycle to be different from the other? \$\endgroup\$ – Sam Apr 30 '16 at 0:54
  • \$\begingroup\$ @tom Thanks for the idea. I checked the capacitance, and the disk has under 50pF (the limit of my multi-meter). The GM32A v1.2 transistor checker (Arduino based component identifier) says "resistance only". Would a MOV have that little capacitance? \$\endgroup\$ – HiTechHiTouch May 1 '16 at 3:40
  • \$\begingroup\$ probably, it's basically a bunch of zener diodes. Each grain in a mov is a diode, it has some resistance (because all the grains are just mashed together) and probably low capacitance (which is related to surface area and spacing), the magic happens when the voltage across it reaches its breakdown threshold (couple hundred volts or so), then it becomes more like a diode in that current can go up, but the voltage across it stays the same. its basically two rugged high energy zener diodes back to back. \$\endgroup\$ – Sam May 1 '16 at 22:54
  • \$\begingroup\$ @tom I understand MOVs. I don't see why one would be across the motor capacitor, as opposed to the supply. \$\endgroup\$ – HiTechHiTouch May 7 '16 at 1:37
  • \$\begingroup\$ I checked some data sheets, and most MOVs are described in nF. Only one went as low as 30uF. \$\endgroup\$ – HiTechHiTouch May 7 '16 at 1:45
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The disc is a resister that limits any prospective surge currents in the diode .It may be a PTC for more protection .It is definately a safety component which means that you should replace it with the same type unless you really know what you are doing .

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  • \$\begingroup\$ Why a (variable) resistor and a single diode across a AC motor capacitor? It effectively reduces the capacitance for only half the cycle. Then there's the problem of the (assumedly) thermal disk. \$\endgroup\$ – HiTechHiTouch May 7 '16 at 1:41

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