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Intuitively, the hair dryer sucks power out of the wires, causing a voltage drop, causing the lights to dim. But the opposite happens: the lights get brighter. These are LED bulbs. It is the heating element that does this, not the motor (running the motor with no heat has no effect on the bulbs). The lights are on a dimmer circuit.

What causes this effect? I am assuming it has something to do with the LED power regulator or the dimmer, but am curious to know the details.

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  • \$\begingroup\$ That's... really weird lol. I wonder if it's something like a loose neutral wire in the receptacle or the dimmer switch... Might need to call an electrician. \$\endgroup\$
    – user103380
    Mar 3 '18 at 20:37
  • \$\begingroup\$ My guess is that the hair dryer has a universal motor and noise from the commutator is interfering with the dimmer or power regulator. \$\endgroup\$ Mar 3 '18 at 20:42
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I didn't think of this but adding to Spehro's addition to KingDuken's comment this might help. The socket might be on L1 and the LED light on L2.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. A North American split-phase domestic supply.

  • Let's say L1 is 120 V and your hairdryer is about 10 Ω so 10 A will flow.
  • Let's say there is a (very high) neutral resistance of 1 Ω then there will be a 10 V drop across R2.
  • Now L1-N = 110 V and L2-N = 130 V so the lights will get brighter.

You should be able to prove this to yourself by monitoring the lighting circuit voltage with a multimeter while switching the hairdryer on and off.

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    \$\begingroup\$ That might be a possibility, but the high resistance would need to be between the distribution panel and the distribution transformer. Individual 120 V circuits don't normally a share neutral conductors between the circuit components and the distribution box. I believe that is permitted only if the breakers for the two circuits are tied together to form a 2-pole breaker. It is also possible that there is a wiring error or code violation. \$\endgroup\$ Mar 3 '18 at 22:46
  • \$\begingroup\$ Good point. I'm on the other side of the Atlantic so my understanding is at schematic level rather than practical wiring. We don't do split-phase here as L-N is 230 V already. (I know you know this.) \$\endgroup\$
    – Transistor
    Mar 3 '18 at 23:07
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Let's assume your hair dryer (=its heater) causes substantial AC voltage drop. If the led bulb happens to have a "go where the fence is lowest" -designed controller, the brightening is understandable:

enter image description here

Controller X turns T1 ON => Led current IL starts to increase gradually. X turns T1 OFF when the current IL has reached the allowed maximum (=Voltage over Rs reaches the cut-off limit)

IL continues through D1, but decays gradually. When IL is assumed to be low enough, T1 is turned ON again and the leds get a new pulse.

The ON state of T1 gets longer, if AC voltage drops, because current IL grows slower when inductance voltage is lower. The OFF-state of T1 has fixed length. => The average IL grows when AC voltage has dropped.

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A high resistance neutral connection will cause more voltage to show up on the other side of the 120:120 circuit, assuming North American mains configuration, just as @KingDuken mentions in a comment.

It could be a faulty connection or just a long length of minimum gauge wire.

Edit: Charles' comment is 100% correct that wiring to code (North America) will run the conductors back to the circuit breaker panel and the wire resistance on the other side of the circuit breakers should not be an factor (assuming it's wired to our code). So that kind of leaves a bad neutral connection, or the weird possibility of a light that gets brighter as the voltage decreases. That's impossible with incandescent bulbs, but with LED bulbs it's not inconceivable.

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    \$\begingroup\$ See my comment to @Transistor. \$\endgroup\$ Mar 3 '18 at 22:48
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I would suspect the dimmer which would be causing the lights to get brighter. The dimmer may be using triac and a zero cross detection circuit to do the dimming and the lower voltage should be changing the zero crossing point (must be detected earlier than usual). Hence the triac is open for a little longer when the voltage is reduced and thus increases the overall open time, which in turn increases the brightness.

Theory The hair dryer will be drawing much current, reducing the overall voltage. Reducing the voltage inturn provide less voltage to the zero cross sensing opto coupler and the signal crossing point (pulse approaching zero) will be sensed sooner due to lower source voltage.

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  • \$\begingroup\$ Reducing the voltage shouldn't change the zero-crossing point. \$\endgroup\$ Mar 3 '18 at 20:53
  • \$\begingroup\$ @TomCarpenter Reducing the voltage inturn provide less voltage to the zero cross sensing opto coupler and the voltage crossing point (pulse approaching zero) will be sensed sooner due to lower source voltage. \$\endgroup\$
    – Zac
    Mar 4 '18 at 7:01

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