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I have found a halogen lamp that only works when plugged to the wall socket the right way. I am baffled by this behaviour.

The lamp is similar to this model, and uses a bulb similar to this model. The bulb is probably not important but the lamp may be if some internal electronics are the cause of this behaviour.

The plug is a Type C plug, no earth connection, just two symmetrical metal rods. The socket is European: AC, 220-240V, 50Hz.

As far as I can understand, it should not matter which way the plug is inserted into the socket.

  • If the lamp uses AC, then both rods will be alternating polarisation and both orientations should work exactly the same.
  • If the lamp works on DC, it will have some internal rectifier / regulator electronics inside that will transform the AC in DC. These circuits, by design, should work regardless of the orientation of the plug.

What am I missing? Which electronic design may result in a lamp only working if it is plugged in the right orientation and what would be the rationale to add such circuitry to the lamp in the first place?

Extra information:

  • Other appliances (such as laptop chargers) seem to work as one would expect in that socket.
  • The lamp exhibits the same behaviour in other sockets.
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    \$\begingroup\$ I have checked the site and found two related questions, but none seem to explain the behaviour I am observing. (1) electronics.stackexchange.com/questions/134869/… (2) electronics.stackexchange.com/questions/15228/… \$\endgroup\$
    – sergut
    Commented Jan 22, 2019 at 18:50
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    \$\begingroup\$ maybe it has a DC circuit in it. That’s my best guess. \$\endgroup\$
    – Scientist Smith YT
    Commented Jan 22, 2019 at 18:52
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    \$\begingroup\$ Have you tried 1) the problem lamp in a different socket 2) a known good lamp - one without the problem - in the problem socket? \$\endgroup\$
    – mike65535
    Commented Jan 22, 2019 at 18:54
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    \$\begingroup\$ I used to buy these types of lamps of 10% of this cost. My weak guess is they added an auto-reset GFCI and the stray leakage is higher in one direction. \$\endgroup\$
    – D.A.S.
    Commented Jan 22, 2019 at 18:57
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    \$\begingroup\$ Try several different sockets. I'd suspect something mechanical like slightly bent pins combining with slightly irregular contacts in the socket, that simply don't quite touch one way round. \$\endgroup\$
    – user16324
    Commented Jan 22, 2019 at 21:39

2 Answers 2

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It must be a matter of stray leakage.

Either intentionally (e.g., GFCI circuitry) or unintentionally (e.g., stray voltages removing bias from a node) the circuit stops working.

Turning the plug around changes the relationship of any reference node to the stray capacitance to “the world” through the lamp’s body. This leads to more or less stray currents in different nodes in the circuitry.

If the lamp has a Triac driving circuit for a dimmer, this stray current might be enough to cause this circuit to stop operating.

Measure the AC voltage from the lamp housing to ground. You might be able to see a difference depending on the orientation of the plug.

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Most of these halogen torchiere lamps have a small size, high-power dimmer located in the pole. There is a large aluminum heatsink that is wrapped with a cardboard insulator. There could be some leakage between the heatsink and the pole, or between a conductor and the pole.
Take it apart and repair any poor insulation points. Even though it has a 2-prong plug, I would still open it up for safety's sake. It is used and you don't know its provenance.

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