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Topic Title: Unexpected dimming of lights.
Topic Summary: Why is it happening when the load is removed?
Created On: 22 September 2017 09:43 AM
Status: Read Only
Linear : Threading : Single : Branch
 Unexpected dimming of lights.   - potential - 22 September 2017 09:43 AM  
 Unexpected dimming of lights.   - kellyselectric - 22 September 2017 12:14 PM  
 Unexpected dimming of lights.   - nad - 22 September 2017 01:00 PM  
 Unexpected dimming of lights.   - mapj1 - 23 September 2017 12:15 AM  
 Unexpected dimming of lights.   - potential - 23 September 2017 10:16 AM  
 Unexpected dimming of lights.   - mapj1 - 23 September 2017 12:32 PM  
 Unexpected dimming of lights.   - OlympusMons - 24 September 2017 02:12 PM  
 Unexpected dimming of lights.   - potential - 24 September 2017 05:41 PM  
 Unexpected dimming of lights.   - potential - 24 September 2017 05:44 PM  
 Unexpected dimming of lights.   - MWalker86 - 24 September 2017 11:10 PM  
 Unexpected dimming of lights.   - kellyselectric - 26 September 2017 12:48 PM  
 Unexpected dimming of lights.   - kellyselectric - 05 October 2017 11:12 AM  
 Unexpected dimming of lights.   - alancapon - 24 September 2017 06:11 PM  
 Unexpected dimming of lights.   - kellyselectric - 24 September 2017 07:52 PM  
 Unexpected dimming of lights.   - potential - 05 October 2017 01:57 PM  
 Unexpected dimming of lights.   - potential - 05 October 2017 02:01 PM  
 Unexpected dimming of lights.   - kellyselectric - 05 October 2017 04:41 PM  
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 23 September 2017 12:15 AM
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mapj1

Posts: 9692
Joined: 22 July 2004

This is the sort of thing best tracked down with an oscilloscope, or a low voltage transformer into the audio jack of a PC as a poor mains waveform monitor.
Firstly be aware that the eye is not good at distinguishing dimming and brightening in rapid succession, from the reverse, so it may not be quite as you imagine.
There are a couple of possibles. Iff the switch is bouncing, then as the motor spins down after connection it will act as a generator, but one that may be either in or out of phase with the mains when reconnected briefly, depending on the switch bounce period. This could either boost or reduce the voltage for a part cycle.
I have also had a an arcing double pole switch that placed a partial short across the mains as the L-N contact arcs briefly met, though it must be said, I have only ever seen that once, and it had a short and brutal life.
Then there are L-C transient effects, which become significant if the cable supply route is unusually inductive, this will be a function of substation rating and distance, type of electricity meter you have, and sometimes really bad because L and N travel different routes (an extreme case is a ring with a broken core, such that either neutral or live goes round the long way, while the other goes short .) In any case when a load goes off, which may be resistive, the change in current may lead to a change in voltage that overshoots. However this us usually in the tens of micro henry region, so only noticeable (i.e a few volts) with large loads and very fast switching - rate of change amps per micro-second.

-------------------------
regards Mike
 23 September 2017 10:16 AM
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potential

Posts: 1642
Joined: 01 February 2007

Thanks for the replies.
I've experimented a little more with the delayed dimming after switch off.
With the blower running, if the plug is pulled out of the external socket on the house the delayed dimming still occurs, so it won't be switch bounce or arc-over.
The delay period is quite substantial, not just a couple of cycles of the mains..... about the same length of time it takes to notice the dimming and then say a single syllable word.
I reckon it is about half a second.... and that is a lot of cycles.
The dimming is quite long too, about the same length of time.

It is as if there is something mechanical somewhere deliberately causing the drop in voltage.

Also remember that the dimming after switch off did not occur when the leaf blower was run from the socket on the shed.
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